29 Human Origins Life at the Top of the Tree Bernard Wood Paul Constantino

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517

This chapter describes the relationships and recent evolutionary

history of Homo sapiens, or modern humans. By relationships,

we mean the details of how modern humans are related

to the other great apes, the living animals closest to modern

humans. By recent, we mean the part of our evolutionary

history that postdates our most recent common ancestor with

one of the other living great apes.

Modern humans are singular in some important ways,

yet in others we closely resemble the other great apes. Three

of our singularities are noteworthy. First, our habitat is more

extensive and varied than that of any other contemporary

vertebrate, let alone any other large-bodied primate. Second,

the size of the modern human population exceeds that

of any other large undomesticated mammal, and we outnumber

all the other great apes by many, many orders of

magnitude. With respect to behavior, we are not unique in

possessing culture (Whiten et al. 1999), but we are unique

in terms of the complexity of that culture. As for our commonalties

with higher primates and with other mammals,

one of the triumphs of molecular biology has been the ways

it is helping us document the details of our relatedness to

the rest of the living world. The extent to which we share

DNA with chimpanzees (~95–99% depending on how it is

measured) is well known, but it is less known yet no less

significant that it is estimated that we share 40% of our DNA

with a banana. The magnitude of this molecular conservatism

serves to emphasize that whatever we discover to be

the genetic basis of the unique aspects of modern human

behavior (be they differences in the genes themselves, or

in the intensity of their expression; e.g., Enard et al. 2002),

the genetic differences between modern humans and the

other great apes are quantitatively trivial compared with the

overwhelming majority of our genome that we share with

other life on Earth.

Terminology

In this chapter, we have tried to avoid using technical terms,

but some are necessary. For reasons given below, we treat

modern humans as one of the “great apes,” the others being

the two African higher primates, the chimpanzee (Pan) and

the gorilla (Gorilla), and the orangutan (Pongo) from Asia.

Linnaean taxonomic categories immediately above the level

of the genus, that is, the family and the tribe, have vernacular

equivalents that end in “id” and “in,” respectively. Thus,

members of the Hominidae, the family to which modern

humans belong, are called “hominids” and members of the

Hominini, the tribe that includes modern humans, are called

“hominins.”

Paleoanthropologists have differed, and still do differ,

in the way they use the family and tribe categories with

respect to the classification of the higher primates. In the

past, Homo sapiens has been considered to be distinct

enough to be placed in its own family, Hominidae, with all

the other great apes grouped together in another family,

518 The Relationships of Animals: Deuterostomes

Pongidae. Thus, we and our close fossil relatives were referred

to as hominids and the other great apes and their

close fossil relatives were referred to as pongids (table 29.1).

As we show below, this scheme is inconsistent with morphological

and genetic evidence suggesting that one of the

living pongids, the chimpanzee, is more closely related to

modern humans (the only living “old-style” hominid) than

it is to any other pongid (table 29.2).

In response to these developments, some researchers

have advocated combining modern humans and chimps in

the same genus (e.g., Page and Goodman 2001, Wildman

et al. 2003). According to the rules of zoological nomenclature,

the name for such a genus must be Homo. In this

contribution we adopt a less radical solution: we lump all

the great apes into the family Hominidae; within that grouping,

we recognizes three living subfamilies, the Ponginae (or

“pongines”) for the orangutans, the Gorillinae (or “gorillines”)

for the gorillas, and the Homininae (or “hominines”)

for both modern humans and chimpanzees. Within the

latter subfamily we recognize two tribes, the Panini (or

“panins”) for the chimpanzees and the Hominini (or “hominins”)

for modern humans. The latter is further broken

down into two subtribes, one for all the extinct-only hominin

genera (Australopithecina) and the other (Hominina)

for the genus Homo, which includes the only living hominin

taxon, Homo sapiens. Thus, in order of decreasing inclusivity,

modern humans are hominids (family), hominines

(subfamily), and then hominins (tribe and subtribe). Therefore,

in the terminology used hereafter modern humans and

all the fossil taxa judged to be more closely related to modern

humans than to chimpanzees are referred to as

hominins, with the chimpanzee equivalent being panin. We

use the informal term “australopith” for members of the

subtribe Australopithecina.

A Different Scale

Compared with other chapters in this book, we will deal with

evolutionary history at a unique level of taxonomic detail,

that of the species and genus. The species category is the

lowest taxonomic level commonly used, and genera are composed

of one, or more, species. For a group to qualify for the

rank of genus, the taxa within it are generally taken to be both

adaptively homogeneous and members of the same clade. To

comply with the latter requirement, the genus must contain

all the descendants of a common ancestor and its members

must be confined to that clade. Species that are “adaptively

similar” but belong to different clades do not qualify for the

rank of genus.

At this level of taxonomic detail, differences in taxonomic

philosophy (see below) significantly affect the way researchers

of human evolution interpret the fossil evidence. These

differences most importantly affect decisions about the numbers

of species that are recognized in the human fossil record.

Thus, this contribution considers nuances of taxonomy that

would simply not be noticed in other chapters devoted to

larger and more diverse sections of the Tree of Life.

Close Relatives

For much of the last century, the data available for reconstructing

the phylogeny of the higher primates were effectively

restricted to gross observations of the phenotype.

Table 29.1

A Traditional “Premolecular” Taxonomy of the Living

Higher Primates (Boldface Indicates Extinct Taxa).

Superfamily Hominoidea (hominoids)

Family Hylobatidae

Genus Hylobates

Family Pongidae (pongids)

Genus Pongo

Genus Gorilla

Genus Pan

Family Hominidae (hominids)

Subfamily Australopithecinae (“australopithecines”)

Genus Ardipithecus

Genus Australopithecus

Genus Kenyanthropus

Genus Orrorin

Genus Paranthropus

Genus Sahelanthropus

Subfamily Homininae (hominines)

Genus Homo

Table 29.2

A Taxonomy of the Living Higher Primates that

Recognizes the Close Genetic Links Between Pan

and Homo (Boldface Indicates Fossil-Only

Hominin Taxa).

Superfamily Hominoidea (hominoids)

Family Hylobatidae

Genus Hylobates

Family Hominidae (hominids)

Subfamily Ponginae

Genus Pongo (pongines)

Subfamily Gorillinae

Genus Gorilla (gorillines)

Subfamily Homininae (hominines)

Tribe Panini

Genus Pan (panins)

Tribe Hominini (hominins)

Subtribe Australopithecina (australopiths)

Genus Ardipithecus

Genus Australopithecus

Genus Kenyanthropus

Genus Orrorin

Genus Paranthropus

Genus Sahelanthropus

Subtribe Hominina (hominians)

Genus Homo

Human Origins 519

Numerically, these data were either dominated by, or confined

to, observations made from the “hard tissues,” that is,

from the skeleton and dentition. In the older literature, the

phenotypic and behavioral differences among the higher

primates were interpreted as indicating a substantial gap, if

not a gulf, between modern humans and the nonhuman

higher primates. For close to 150 years (Huxley 1863), some

researchers have suggested that modern humans are more

closely related to the African apes as (Homo (Pan, Gorilla))

than they are to the orangutan. However, these researchers

generally insisted on putting a respectful distance between

modern humans and the last common ancestor we shared

with the African apes. It is only relatively recently that data

sets dominated by gross morphological observations of hard

tissues have been interpreted as favoring a particularly close

link between modern humans and chimpanzees [i.e., ((Homo,

Pan) Gorilla); Groves 1986, Groves and Paterson 1991,

Shoshani et al. 1996]. Soft-tissue data also support a (Homo,

Pan) clade, but these data are presently dominated by observations

about the gross anatomy of the limbs, especially

information about muscles (Gibbs et al. 2000, 2002).

Developments in biochemistry and immunology during

the first half of the 20th century allowed the focus of the search

for better evidence about the nature of the relationships between

humans and the apes to be shifted from traditional gross

morphology to the morphology of molecules. The earliest attempts

to use molecular morphology to determine the relationships

among the higher primates used proteins such as

albumin and hemoglobin. Proteins are made up of a string of

amino acids. In many instances, one amino acid may be substituted

for another without affecting the primary function of

a protein, but the substitution can be detected by appropriate

methods. Zuckerkandl (1963) used enzymes to break up the

hemoglobin protein into its component peptides and then

separated the components using a method called starch gel

electrophoresis. The patterns made by the hemoglobins of

modern humans, gorilla, and chimpanzee were indistinguishable

(Zuckerkandl 1963). Morris Goodman (1963) used sensitive

immunological techniques to investigate the affinities of

the albumin protein of higher primates and showed that, with

respect to this molecule, modern human and chimpanzee albumins

were again indiscernible (Goodman 1963). In the

1970s Vince Sarich and Alan Wilson continued the exploitation

of minor variations in protein structure, and they, too,

concluded that modern humans and African apes were very

closely related (Sarich and Wilson 1966, 1967).

The discovery of the genetic code by James Watson and

Francis Crick demonstrated that the sequence of bases in the

DNA molecule specifies the genes that determine the nature

of the proteins manufactured within a cell. This meant that

the affinities between organisms could be pursued at the level

of the genome, thus potentially eliminating the need to rely

on morphological “proxies” (be they traditional hard- and/or

soft-tissue anatomy or the morphology of proteins) for information

about relatedness. The DNA within the cell is located

either within the nucleus as nuclear DNA, or within the mitochondria

as mitochondrial DNA (mtDNA). Comparisons

among the DNA of organisms can be made using two methods.

In DNA hybridization, the entire DNA is compared but

at a relatively crude level. In DNA sequencing, the base sequences

of comparable sections of DNA are determined and

then compared. In brief, DNA hybridization tells you “a little

about a lot” of DNA, whereas, before the sequencing of whole

genomes, the sequencing method told you “a lot about a little”

of DNA. The results of both hybridization (e.g., Caccone and

Powell 1989) and sequencing (e.g., Bailey et al. 1992, Horai

et al. 1995; see reviews by Gagneux and Varki 2001, Wildman

et al. 2002) studies of both nuclear DNA and mtDNA suggest

that modern humans and chimpanzees are more closely related

to each other than either is to the gorilla. When researchers

calibrate these differences using paleontological evidence

such as the split between the apes and the Old World monkeys

or the split between the orangutans and the African great

apes, then the neutral mutation theory suggests that the hypothetical

ancestor of modern humans and the chimpanzee

lived between about 5 and 8 Mya (million years ago, e.g., Shi

et al. 2003). Other researchers using a different calibration

point favor a substantially earlier date (10–14 Mya) for the Pan/

Homo split (Arnason and Janke 2002).

Ancestral Differences

Although there are an impressive number of contrasts between

the gross morphology of living chimpanzees and

modern humans, differences between the earliest hominins

and the ancestors of the chimpanzee are likely to have been

more subtle. Some of the features that distinguish modern

humans and chimpanzees, such as those linked to upright

posture and bipedalism, can be traced far back into human

prehistory. Other features and distinctive behaviors of modern

humans, such as our relatively diminutive jaws and chewing

teeth and complex language, were acquired more recently

and thus cannot be used to identify early hominins, even if

we had a reliable hard tissue marker that allowed researchers

to identify a behavior such as language in the fossil record.

At least two early hominin genera, Australopithecus and

Paranthropus, had absolutely and relatively larger chewing

teeth compared with later Homo. This “megadontia” of the

premolars and molars may have been an important derived

feature of early hominins, but it has apparently been reversed

in later hominins. We do not know whether megadontia

evolved just once, or in more than one clade, nor can we be

sure it is confined to hominins. For example, a very preliminary

analysis of extinct ape taxa (P. Andrews and B. Wood,

pers. comm.) suggests that some of these taxa also have relatively

enlarged chewing teeth. How, then, are we to tell an

early hominin from the ancestors of the chimpanzees, or from

the lineage that provided the common ancestor of chimpanzees

and modern humans?

520 The Relationships of Animals: Deuterostomes

The conventional presumption is that both the common

ancestor and panin taxa would have had a locomotor system

adapted for life in the trees with the trunk held either

horizontal or upright and with the forelimbs adapted for

knuckle-walking on large branches or on the ground. This

would have been combined with projecting faces accommodating

elongated jaws bearing relatively small chewing teeth

and large, sexually dimorphic canine teeth that are honed

against the lower premolars. Early hominins, on the other

hand, would have been distinguished by at least some skeletal

and other adaptations for an upright posture and bipedal

walking and running, linked with a masticatory apparatus

that combined relatively larger chewing teeth and more

modest-sized canines that do not project as far above the rest

of the teeth.

A Third Way?

These proposed distinctions between hominins, panins, and

their hypothetical common ancestor are working hypotheses

that need to be reviewed and if necessary revised as the relevant

fossil evidence is uncovered. Evidence of only one of

the presumed distinguishing features of the hominins and

panins may not be sufficient to identify a fossil as being in

either the hominin or panin lineage. This is because there is

evidence that the higher primates, like many other groups

of mammals, are prone to homoplasy, which is the independent

acquisition of morphological characters. This means that

we cannot exclude the possibility that some of what many

have come to regard as the “key adaptations” of the hominins

(e.g., bipedalism) as well as those of the other great ape lineages

may have arisen in more than one clade and more than

once in the same clade (see below). If so, it would be very

difficult on the basis of the inevitably fragmentary fossil

record to distinguish the earliest members of the hominin

and panin lineages between 5 and 10 Mya.

Lastly, if only for the historical reasons given below, we

need to acknowledge the likelihood that a 5–10 Myr-old fossil

ape taxon may be neither a hominin nor a panin. For example,

for many years fossil great ape taxa known from African

sites were interpreted as being ancestral to either the

gorilla or the chimpanzee. Cladistic analysis has since shown

that most of these taxa display derived morphology that probably

precludes them from being a member of the extant African

ape clades (Stewart and Disotell 1998). Thus, instead

of assuming that a 5–10 Myr-old fossil taxon must be either

an ancestral hominin, an ancestral panin, or their common

ancestor, we need to entertain the possibility that it may

belong to a hitherto unknown hominin or panin subclade

or to an extinct sister group of the Pan/Homo clade. Colleagues

must also realize that morphology that is primitive

compared with later, undisputed, hominins can only make

a taxon a candidate for the common ancestry of the hominin

clade; it cannot be used to prove it is the common ancestor.

It is also very likely that 5–10 Myr-old fossil ape taxa are part

of an adaptive radiation for which we have no satisfactory

extant model. We should be prepared to find fossil apes in

this and even later time ranges that display novel combinations

of familiar features, as well as evidence of novel morphological

features.

How Many Species of Fossil Hominin Should

We Recognize in the Human Fossil Record?

It is easy to forget that statements about how many species are

sampled in the hominin fossil record are hypotheses. There is

lively debate about the definition of living species, so it is not

surprising there is a spectrum of opinion about how the species

category should be interpreted in the paleontological context.

All species are individuals in the sense that they have a

history. They have a beginning (the result of a speciation event),

a middle that lasts as long as the species persists, and an end,

which is either extinction or participation in another speciation

event. Living species are caught in geological terms at an

instant in their history, much as a single still photograph of a

running race is only a partial record of that race. In the hominin

fossil record that, albeit imperfectly, samples hundreds of thousands

of years of time, the same species may be sampled several

times. So to return to our metaphor, the hominin fossil

record may be providing us with more than one photograph

of the same running race.

Paleoanthropologists must devise strategies to ensure that

the number of species they recognize in the fossil record is

neither a gross underestimate nor an extravagant overestimate

of the actual number. They must also take into account

that they are working with fossil evidence that is largely confined

to the remains of the hard tissues that make up the

bones and teeth. We know from living animals that many

“good” species are osteologically and dentally very difficult

to distinguish (e.g., Cercopithecus species). Thus, there are

good logical reasons to suspect that a hard tissue-bound fossil

record will always underestimate the number of species.

When this attitude to estimating the likely number of

species in the fossil record is combined with a “punctuated

equilibrium” and cladogenetic interpretation of evolution

then a researcher is liable to interpret the fossil record as

containing more rather than fewer species (table 29.3A,

fig. 29.1). Conversely, researchers who favor a more gradualistic,

or anagenetic, interpretation of evolution that emphasizes

morphological continuity rather than morphological discontinuity,

and who see species as individuals that are longer lived

and more prone to substantial changes in morphology through

time, will tend to resolve the fossil record into fewer species

(table 29.3B). For the reasons given above the taxonomic hypothesis

favored in this contribution is one that recognizes

more rather than fewer species.

Human Origins 521

erature. As recommended by the International Code of Zoological

Nomenclature (ICZN; Ride et al. 1985), when a taxon

has been moved from its initial genus, the original reference

is given in parentheses, followed by the revising reference.

Further details about most of the taxa and a more extensive

bibliography can be found in Wood and Richmond (2000).

Recent relevant reviews are also contained in Hartwig (2002).

Primitive Hominins

This group includes one taxon, Ardipithecus ramidus, that is

probably a member of the hominin clade and two taxa,

Orrorin tugenensis and Sahelanthropus tchadensis, which may

be hominins. There are too few fossils as yet to be sure that

the three taxa should be in different genera (or perhaps even

different species), but until we have more evidence, the original

genus designations have been retained.

Ardipithecus ramidus (White et al. 1994)

White et al. (1995)

Type specimen. ARA-VP-6/1—associated upper and lower

dentition, Aramis, Middle Awash, Ethiopia 1993.

Approximate time range. ~4.5–5.7 Myr.

History and context. The initial evidence for this taxon

was in the form of approximately 4.5–Myr-old fossils recovered

from late 1992 onward at a site called Aramis in the

Middle Awash region of Ethiopia. A second suite of fossils,

including a mandible, teeth, and postcranial bones, was recovered

in 1997 from five different localities in the Middle

Awash that range in age from 5.2 to >5.7 Myr (Haile-Selassie

2001). One of the new localities is in the Aramis region, the

other four are several kilometers to the west in exposures

lying against the western margin of the East African Rift. With

hindsight the remains from Aramis may not be the first evidence

of this species to be found for the 5 Myr mandibular

fragment (KNM-LT 329) from Lothagam, Kenya, may also

belong to A. ramidus.

Characteristics and inferred behavior. The remains attributed

to A. ramidus have some features in common with living

species of Pan, others that are shared with the African

apes in general, and, crucially, several dental and cranial features

that are shared only with later hominins such as

Australopithecus afarensis. Thus, the discoverers have suggested

that the material belongs to a hominin species. They

initially allocated the new species to Australopithecus (White

et al. 1994), but subsequently the same researchers assigned

it to a new genus, Ardipithecus (White et al. 1995), which they

suggest is significantly more primitive than Australopithecus.

The case White and his colleagues set forward to justify

their initial taxonomic judgment centered on the cranial evidence,

whereas Haile-Selassie (2001) focused on two features

of the dentition and one of the postcranial skeleton. The

former researchers claim that compared with A. afarensis, A.

ramidus has relatively larger canines, first deciduous man-

Table 29.3

Alternate Hominin Taxonomies.

A. A more speciose (or more taxic) hominin taxonomy.

Primitive Hominins

Genus Ardipithecus

Ardipithecus ramidus

Genus Orrorin

Orrorin tugenensis

Genus Sahelanthropus

Sahelanthropus tchadensis

Australopiths

Genus Australopithecus

Australopithecus africanus

Australopithecus afarensis

Australopithecus bahrelghazali

Australopithecus anamensis

Australopithecus garhi

Genus Paranthropus

Paranthropus robustus

Paranthropus boisei

Paranthropus aethiopicus

Genus Kenyanthropus

Kenyanthropus platyops

Homo

Genus Homo

Homo sapiens

Homo neanderthalensis

Homo erectus

Homo heidelbergensis

Homo habilis

Homo rudolfensis

Homo antecessor

B. A less speciose hominin toxonomy.

Primitive hominins

Genus Ardipithecus

Ardipithecus ramidus

Australopiths

Genus Australopithecus

Australopithecus africanus

Australopithecus afarensis

Australopithecus garhi

Genus Paranthropus

Paranthropus robustus

Paranthropus boisei

Homo

Genus Homo

Homo sapiens

Homo erectus

Homo habilis

Inventory of Fossil Hominin Taxa

In this section, we summarize the main taxa researchers have

recognized in the hominin fossil record. Some researchers

think a list this long recognizes too many species (see above).

In this inventory the taxa are presented in three groups: taxa

that are (or may be) primitive hominins, australopiths, and

taxa that are conventionally included in the genus Homo.

Within each of the three groups, the taxa are considered in

the order of their formal introduction into the scientific lit522

The Relationships of Animals: Deuterostomes

dibular molars with less complex crowns, upper and lower

premolar crowns that are more asymmetric (and thus more

apelike), thinner enamel, and a flatter articular eminence. The

researchers suggest that A. ramidus should be excluded from

the apes because its upper central incisors are relatively small,

its canine honing mechanism is poorly developed, the mandibular

permanent molar crowns are too broad, the first

deciduous mandibular molars have more complex crowns

than those of Pan, and the foramen magnum is more anteriorly

situated than it is in the apes. Haile-Selassie (2001) suggests

that the relatively incisiform lower canines together with

the dorsal orientation of the proximal joint surface of the

proximal fourth pedal phalanx are further evidence of A.

ramidus having more affinities with later hominins than with

Pan.

Judging from the size of the shoulder joint A. ramidus

weighed about 40 kg. Its chewing teeth were relatively small,

and the position of the foramen magnum suggests that the

posture and gait of A. ramidus were respectively more upright

and bipedal than is the case in the living apes. The thin

enamel covering on the teeth suggests that the diet of A.

ramidus may have been closer to that of the chimpanzee than

is the case for later hominins. The paleohabitat of both subsets

of the A. ramidus hypodigm has been interpreted as predominantly

woodland or grassy woodland (Woldegabriel

et al. 2001). As yet we have no information about the size of

the brain and only scant direct evidence from the limbs about

the posture and locomotion (see above) of A. ramidus. The

remains of a skeleton likely to belong to A. ramidus have been

found at Aramis, and details are eagerly awaited.

Controversy. Although the evidence is far from conclusive,

it is reasonable to regard A. ramidus as a primitive

hominin until additional data suggest otherwise.

Orrorin tugenensis (Senut et al. 2001)

Type specimen. BAR 1000’00—fragmentary mandible,

Kapsomin, Lukeino Formation, Tugen Hills, Baringo, Kenya

2000.

Approximate time range. ~6.0 Myr date is constrained by

a 6.2 Myr underlying trachyte and a 5.6 Myr overlying sill.

History and context. The relevant remains come from

four localities in the Lukeino Formation, Tugen Hills, Kenya.

One of the 13 specimens recovered, a lower molar tooth

crown, was discovered in 1974; the remaining 12 specimens

were recovered in 2000.

Characteristics and inferred behavior. The Lukeino molar

tooth has long been regarded as displaying a mixture of Pan

and hominin morphology, but the researchers who recovered

the more recent evidence claim that the BAR 1002’00

femur shows that O. tugenensis was “already adapted to

habitual or perhaps even obligate bipedalism” (Senut et al.

2001). However, the grounds for interpreting its morphology

as that of an obligate biped (presumably the shape and

size of the head of the femur and the presence of a crestlike

linea aspera on the posterior aspect of the shaft) are far from

conclusive. A more detailed analysis of the external and internal

morphology of three femora attributed to O. tugenensis

(Pickford et al. 2002) is interpreted by the authors as confirming

the locomotor mode as obligate bipedalism, but the

computer-assisted tomographic scans of the femoral neck

instead point to a more Pan-like regime of weight transmission.

Otherwise, its discoverers admit that much of the critical

dental morphology is “apelike” (Senut et al. 2001).

Controversy. In order to use the small size of the molar

crowns of Orrorin as evidence of the latter’s close link with

Homo, parsimony dictates that all megadont early hominin

Figure 29.1. A proposed

speciose taxonomy of hominins

along with a depiction of their

morphological–functional grade

through time. See text for

details.

H. sapiens Chimps

K. platyops

4

3

2

1

0

H. ergaster

H. erectus

Ar. ramidus

Au. anamensis

Au. bahrelghazali Au. afarensis

Au. africanus

P. aethiopicus

P. robustus

P. boisei

Au. habilis

Au. rudolfensis

H. antecessor

Au. garhi

6

5

7

8

O. tugenensis

H. heidelbergensis

H. neanderthalensis

LARGE BRAIN, SMALL TEETH, OBLIGATE BIPEDALISM

SMALL BRAIN, VERY LARGE TEETH, FACULTATIVE BIPEDALISM

SMALL BRAIN, LARGE TEETH, FACULTATIVE BIPEDALISM

SMALL BRAIN, SMALL TEETH, QUADRUPEDALISM

Sahelanthropus tchadensis

INSUFFICIENT EVIDENCE

Human Origins 523

fossil evidence must be placed in a large australopith subclade

that is more distantly related to modern humans than is O.

tugenensis. However, instead of belonging in the hominin

clade, O. tugenensis may prove to belong to another part of

the adaptive radiation that included the common ancestor

of panins and hominins.

Sahelanthropus tchadensis Brunet et al. 2002

Type specimen. TM266-01-060–1—an adult cranium,

Anthracotheriid Unit, Toros-Menalla, Chad 2001.

Approximate time range. ~6–7 Myr.

History and context. The hypodigm was discovered during

a survey of likely fossiliferous localities beyond the Koro

Toro region in Chad. All the original specimens are from a

single locality (Brunet et al. 2002). The dating is based on

the match between the fauna in the Anthracotheriid Unit and

the faunas known from Lukeino and from the Nawata Formation

at Lothagam (Vignaud et al. 2002).

Characteristics and inferred behavior. The cranium of S.

tchadensis is chimp sized and displays a novel combination

of primitive and derived features. Much about the cranial base

and neurocranium is chimplike with the notable exception

that the foramen magnum lies more anteriorly than is generally

the case in chimps. Yet the presence of a supraorbital

torus, relatively flat lateral facial profile, small, apically worn

canines, low, rounded, molar cusps, relatively thick enamel,

and relatively thick mandibular corpus are all features that

would exclude S. tchadensis from any close relationship with

the Pan clade and would place it in, or close to, the hominin

clade. However, given the perils of inferring the characteristic

morphology of a taxon from the evidence of a single individual,

or even several individuals, these differences should

be seen as indicative and not the final word about the taxonomy

of this undoubtedly important late Miocene evidence

(Wood 2002).

Australopiths

This group includes the fossil evidence assigned to all of the

remaining hominin taxa that are not conventionally included

in the genus Homo. As it is used in this and many other taxonomies,

Australopithecus is almost certainly paraphyletic, but

until we have more confidence that we can identify species

from fragmentary hard tissue evidence and recover a reliable

phylogeny from an incomplete fossil record there is little

point in revising the generic terminology. In order to avoid

more confusion than already exists, we have (with two exceptions)

retained the original genus names. The exceptions

are that Zinjanthropus and Paraustralopithecus are subsumed

within the genus Paranthropus.

Australopithecus africanus Dart 1925

Type specimen. Taung 1—a juvenile skull with partial

endocast, Taung, now in South Africa 1924.

Approximate time range. ~2.4–3 Myr.

History and context. An early hominin child’s skull found

among the contents of a small cave exposed during mining

at the Buxton Limeworks at Taungs (the name later changed

to Taung) in southern Africa was referred by Raymond Dart

to a new genus and species, Australopithecus africanus, which

means literally, the “southern ape” of Africa. No other hominins

have been recovered from the Buxton deposits.

Remains of hominins we now classify as A. africanus have

been found at three other cave sites in southern Africa:

Makapansgat, well to the northeast of Johannesburg, and at

Sterkfontein and Gladysvale in the Blauuwbank Valley, close

to Johannesburg. At these sites, as at Taung, early hominin

fossils are mixed in with other animal bones in hardened rock

and bone-laden cave fillings, or breccias. The cave sites in

southern Africa can, at present, only be dated by relatively

imprecise absolute physicochemical methods. More often,

they have been dated by comparing the remains of the mammals

found in the caves with mammalian fossils found at sites

in East Africa that were dated using more precise and reliable

absolute methods. In this and in other ways, the age of

the A. africanus-bearing Sterkfontein Member 4 breccia has

been estimated to be between 2.5 and 3 Myr. A hominin

skeleton, StW 573, from Member 2 deep in the Sterkfontein

cave may be somewhat older, ~4 Myr (Partridge et al. 2003),

but it is too early to tell whether it belongs to A. africanus

(Clarke 1998, 1999, 2002a). It has recently been suggested

(Berger et al. 2002) that the Sterkfontein dates may be too

old with 2.5 Myr being the upper and not the lower age limit

of Member 4, but this reinterpretation has been contested

(Clarke 2002b, Partridge 2002). The bones of the medium

and large mammals found in the breccias of all the southern

African hominin cave sites, as well as the hominins themselves,

either were accumulated by predators or are there

because the animals fell into and were then trapped in the

caves. The other animal fossils and the plant remains found

with A. africanus suggest that the immediate habitat was

woodland with grassland beyond.

The first hominin to be recovered at Sterkfontein, TM

1511, was given the name Australopithecus transvaalensis Broom

1936 but was later transferred to a new genus, Plesianthropus

transvaalensis (Broom 1936) Broom 1938. Raymond Dart allocated

the Makapansgat fossil hominins to a new species,

Australopithecus prometheus Dart 1948. However, after 1955 it

became conventional to refer all the australopiths from southern

Africa to a single genus, Australopithecus, and soon researchers

and commentators subsumed both A. transvaalensis and

A. prometheus into the species of Australopithecus with taxonomic

priority, namely A. africanus Dart 1925.

Characteristics and inferred behavior. The picture emerging

from morphological and functional analyses suggests that,

although A. africanus was capable of walking bipedally, it was

probably not an obligate biped. It had relatively large chewing

teeth, and apart from the reduced canines, the skull is

relatively apelike. Its mean endocranial volume, a reasonable

proxy for brain size, is ~450 cm3. The Sterkfontein evidence

524 The Relationships of Animals: Deuterostomes

suggests that males and females of A. africanus differed substantially

in body size, but probably not to the degree they

did in A. afarensis (see below).

Controversy. Some researchers have suggested that the

A. africanus fossils recovered from Sterkfontein may sample

more than one hominin species, but the case is not currently

convincing enough (e.g., Lockwood and Tobias 1999) to

abandon the existing single-species hypothesis as an explanation

for the variation in that sample.

Paranthropus robustus Broom 1938

Type specimen. TM 1511—an adult, presumably male,

cranium and associated skeleton, “Phase II Breccia,” now

member 3, Kromdraai B, South Africa 1938.

Approximate time range. ~2.0–1.5 Myr.

History and context. Evidence of Paranthropus robustus

comes from Kromdraai, Swartkrans, Drimolen, and Cooper’s

caves in the Blauuwbank Valley, near Johannesburg, South

Africa. Kromdraai and Swartkrans have been a focus of research

since 1938 and 1948, respectively, with Members 1 and 2 at

Swartkrans being the source of the main component of the P.

robustus hypodigm. Research at Drimolen was only initiated

in 1992 (Keyser et al. 2000),‘ yet already more than 80 hominin

specimens have been recovered (Keyser 2000), and it promises

to be a rich source of evidence about P. robustus.

Characteristics and inferred behavior. The brain, face, and

chewing teeth of Paranthropus robustus are larger than those

of A. africanus, yet the incisor teeth are smaller. Cranial and

dental differences between the hominins recovered from

Sterkfontein and Swartkrans have led to the suggestion that

P. robustus was more herbivorous than A. africanus. Little is

known about the postcranial skeleton of P. robustus except

that the organization of the pelvis and the hip joint is much

like that of A. africanus. It has been suggested that the thumb

of P. robustus would have been capable of the type of grip

necessary for stone tool manufacture, but this claim is not

accepted by all researchers.

Controversy. Some workers point to differences between

the hominins recovered from Swartkrans and Kromdraai and

prefer to allocate the former material to a separate species,

Paranthropus crassidens Broom 1949. However, most researchers

treat the Swartkrans and Kromdraai evidence as a

single species, and the Drimolen specimens apparently blur

the distinction between the Kromdraai and Swartkrans

hypodigms. For a time some researchers insisted that the

Australopithecus and Paranthropus remains from southern

Africa belonged to the same species, but the single species

hypothesis has long since been abandoned.

Paranthropus boisei (Leakey 1959) Robinson 1960

Type specimen. OH 5—adolescent cranium, FLK, Bed

I, Olduvai Gorge, Tanzania 1959.

Approximate time range. ~2.3–1.3 Myr.

History and context. The first evidence in East Africa of

a hominin resembling Paranthropus robustus was two teeth

found in 1955 at Olduvai Gorge. However, it was OH 5, a

magnificent undistorted subadult cranium with a well-preserved

dentition recovered by Louis and Mary Leakey in

1959, that convinced these researchers that these remains

belonged to a new and distinctive hominin taxon Zinjanthropus

boisei Leakey 1959. A fragmented cranium (OH 30) and

several isolated teeth (OH 26, 32, 38, 46, and 60) have since

been assigned to the same species. An ulna (OH 36) may also

belong to it. Further evidence of P. boisei has since been recovered

from the Peninj River on the shores of Lake Natron

in Tanzania, the Omo Shungura Formation and Konso in

Ethiopia, Chesowanja in the Chemoigut basin, at West

Turkana in Kenya, and from Melema in Malawi. However,

the site collection that has provided most of the evidence

about P. boisei is that from Koobi Fora, on the eastern shore

of Lake Turkana. The new species was initially included in a

new genus, Zinjanthropus, but the generic distinction between

Zinjanthropus and Australopithecus has long since been abandoned.

It is now usual to refer to the taxon as either

Australopithecus boisei or Paranthropus boisei (see below).

Characteristics and inferred behavior. Cranially P. boisei is

presently the only hominin to combine a massive, wide, flat,

face, massive premolars and molars, small anterior teeth, and

a modest-sized neurocranium (~450 cm3). The face of P.

boisei is larger and wider than that of P. robustus, yet their

brain volumes are similar. Cranial features of P. boisei include

the complex overlap at the parietotemporal suture and the

combination of an anteriorly situated foramen magnum and

a modest-sized brain. The mandible of P. boisei has a larger

and wider body or corpus than any other hominin (see P.

aethiopicus below). The proportions of the dentition are very

derived in that very large-crowned premolar and molar teeth

are combined with small anterior (i.e., incisor and canine)

teeth. The tooth crowns apparently grow at a faster rate than

has been recorded for any other early hominin. There is,

unfortunately, no postcranial evidence that can with certainty

be attributed to P. boisei. The fossil record of P. boisei sensu

stricto extends across about 1 Myr of time, during which there

is little evidence of any substantial change in the size or shape

of the components of the cranium, mandible, and dentition

(Wood et al. 1994).

Paranthropus aethiopicus (Arambourg and Coppens

1968) Chamberlain and Wood 1985

Type specimen. Omo 18.18 (or 18.1967.18)—an edentulous

adult mandible, locality 18, section 7, member C,

Shungura Formation, Omo region, Ethiopia 1967.

Approximate time range. ~2.5–2.3 Myr.

History and context. Some researchers have suggested

that the oldest of the East African evidence for Paranthropus

should be taxonomically distinct and that the taxon name

Paraustralopithecus aethiopicus, linked with a ~2.5–Myr-old

mandible, would be available for such a taxon. Thus, when

a distinctive 2.5–Myr-old Paranthropus cranium, KNM-WT

17000, was recovered from West Turkana, it was natural to

Human Origins 525

consider whether this new specimen should also be assigned

to the same taxon.

Characteristics and inferred behavior. The mandible and

the mandibular dentition of Paranthropus boisei sensu lato apparently

become more derived about 2.3 Mya, and that shift

forms part of the evidence for the interpretation that the

“early” and “late” stages of Paranthropus in East Africa should

be recognized taxonomically, with the former being referred

to as Paranthropus aethiopicus. Among the differences between

the two East African Paranthropus species are the more prognathic

face, the less flexed cranial base and the larger incisors

of P. aethiopicus compared with P. boisei.

Controversy. When this taxon was introduced in 1968,

it was the only megadont hominin in this time range. With

the discovery of A. garhi (see below), it is apparent that robust

mandibles with similar length premolar and molar tooth

rows are associated with what are claimed to be two distinct

forms of cranial morphology.

Australopithecus afarensis Johanson et al. 1978

Type specimen. LH 4—adult mandible, Laetolil Beds,

Laetoli, Tanzania 1974.

Approximate time range. ~3–4 Myr.

History and context. This taxon was established in

1978 for hominin fossils recovered from Laetoli in Tanzania

and from Hadar in Ethiopia. Subsequently, evidence has

come from other sites in Ethiopia, including two Middle

Awash localities, Maka and Belohdelie, the sites of Fejej and

White Sands in the Omo Region, and possibly from the

Kenyan sites of Koobi Fora, Allia Bay, West Turkana, and

Tabarin. A. afarensis is the earliest hominin to have a comprehensive

fossil record that includes a skull, fragmented

crania, many lower jaws, and sufficient limb bones to be able

to attempt an estimation of stature and body mass. The collection

includes a specimen, AL-288, that preserves just less

than half of the skeleton of an adult female.

Characteristics and inferred behavior. The range of body

mass estimates is from 25 to >50 kg. The estimated brain

volume of A. afarensis is between 400 and 500 cm3. This is

larger than the average brain size of a chimpanzee, but if the

estimates of the body size of A. afarensis are approximately

correct, then relative to estimated body mass, the brain of A.

afarensis is not substantially larger than that of Pan. It has

incisors that are much smaller than those of extant chimpanzees,

but the premolars and molars of A. afarensis are relatively

larger than those of the chimpanzee and the hind limbs

of AL-288 are substantially shorter than those of a modern

human of similar stature. Attempts to reconstruct the habitat

of A. afarensis suggest that it was living in a more open

woodland environment than that reconstructed for A.

ramidus. The appearance of the pelvis and the relatively short

lower limb suggest that, although A. afarensis was capable of

bipedal walking, it was not adapted for long-range bipedalism.

This indirect evidence for the locomotion of A. afarensis

is complemented by the discovery at Laetoli of several trails

of fossil footprints. These provide very graphic direct evidence

that a contemporary hominin, presumably A. afarensis,

was capable of bipedal locomotion. The upper limb, especially

the hand, retains morphology that most likely reflects

a significant element of arboreal locomotion. The size of the

footprints and the length of the stride are consistent with

stature estimates based on the length of the limb bones of A.

afarensis. These suggest that the standing height of adult individuals

in this early hominin species was between 1.0 and

1.5 m. Recent analyses have shown that the dental and mandibular

morphology of this taxon changed relatively little

during its ~1 Myr time range.

Controversy. When the classification of the material now

referred to as A. afarensis was first discussed it was natural

for researchers to consider its relationship to the remains of

Australopithecus africanus Dart 1925. The results of morphological

and cladistic analyses suggest that there are significant

differences between the two hypodigms and that they

are rarely sister taxa in cladistic analyses. The comparisons

also emphasize that in nearly all the cranial characters examined

A. afarensis displays a more primitive character state

than does A. africanus. Despite the substantial range of estimated

body mass and claims that the taxon subsumes a mix

of upper limb morphology, most researchers continue to

interpret this fossil evidence as representing one species.

Australopithecus bahrelghazali Brunet et al. 1996

Type specimen. KT 12/H1—anterior portion of an adult

mandible, Koro Toro, Chad 1995.

Approximate time range. ~3.0–3.5 Myr.

History and context. This taxon was established for

Pliocene hominin remains recovered in Chad, north-central

Africa.

Characteristics and inferred behavior. The published evidence,

a mandible and a maxillary premolar tooth, has been

interpreted as being sufficiently distinct from A. ramidus, A.

afarensis and A. anamensis to justify its allocation to a new

species. Its discovers claim that its thicker enamel distinguishes

the Chad remains from A. ramidus, that the more

vertical orientation and reduced buttressing of the mandibular

symphysis together with the more symmetrical crowns

of the P3 separate it from A. anamensis, and that its more

complex mandibular premolar roots distinguish it from A.

afarensis.

Controversy. Not all researchers are convinced that these

remains are sufficiently different from A. afarensis to justify

their allocation to a new species.

Australopithecus anamensis Leakey et al. 1995

Type specimen. KNM-KP 29281—an adult mandible

with complete dentition, and a temporal fragment that probably

belongs to the same individual, between the upper and

lower pumiceous tuffs of the basal fluvial complex, Kanapoi,

Kenya 1994.

Approximate time range. ~4.0–4.5 Myr.

526 The Relationships of Animals: Deuterostomes

History and context. The hypodigm of the new taxon,

Australopithecus anamensis, centers on material recovered by

Meave Leakey and her team from the site of Kanapoi, together

with material recovered earlier from Allia Bay, northern

Kenya (Leakey et al. 1995).

Characteristics and inferred behavior. The main differences

between A. anamensis and A. afarensis relate to details of the

dentition. In some respects the teeth of A. anamensis are more

primitive than those of A. afarensis (e.g., the asymmetry of

the premolar crowns and the relatively simple crowns of the

deciduous first mandibular molars), but in others (e.g., the

low cross-sectional profiles and bulging sides of the molar

crowns) they show similarities to more derived and temporally

later Paranthropus taxa (see above). The upper limb

remains are australopith-like, and a tibia attributed to A.

anamensis has features associated with bipedality (Ward

2002). A useful detailed review of the fossil evidence has

appeared recently (Ward et al. 2001).

Controversy. Some researchers interpret A. anamensis not

as a separate taxon, but as the more primitive, earlier segment

of an effectively continuous hominin lineage including

both A. anamensis and A. afarensis.

Australopithecus garhi Asfaw et al. 1999

Type specimen. BOU-VP-12/130—a cranium from the

Hata member, Bouri, Middle Awash 1997.

Approximate time range. ~2.5 Myr.

History and context. The evidence for this taxon comes

from Bouri, in the Middle Awash of Ethiopia.

Characteristics and inferred behavior. Australopithecus garhi

combines a primitive cranium with large-crowned postcanine

teeth. However, unlike Paranthropus (see above), the incisors

and canines are large and the enamel lacks the extreme thickness

seen in the latter taxon. A partial skeleton combining a

long femur with a long forearm was found nearby but is not

associated with the type cranium of A. garhi (Asfaw et al.

1999). Cut-marked animal bones found in nearby horizons

of the same age suggest that either A. garhi or another contemporary

hominin were defleshing animal bones, presumably

with stone tools.

Controversy. The discoverers of A. garhi interpret it as a

probable ancestor of Homo, but it could equally well be the

sister taxon of a Homo, Paranthropus, A. africanus clade. If

future discoveries demonstrate that the mandibles of P.

aethiopicus and A. garhi cannot be distinguished from each

other, then the name P. aethiopicus would have priority for

the hypodigm.

Kenyanthropus platyops Leakey et al. 2001

Type specimen. KNM-WT 40000—cranium, Lomekwi,

West Turkana, Kenya 1999.

Approximate time range. ~3.3–3.5 Myr.

History and context. Two specimens from West Turkana,

KNM-WT 40000, a 3.5-Myr-old cranium and KNM-WT

38350 a 3.3-Myr-old maxilla, are respectively the holotype

and the paratype of Kenyanthropus platyops (Leakey et al.

2001). The initial report lists 34 other potential members of

the same hypodigm, but at this stage the researchers are reserving

their judgment about the taxonomy of these remains,

some of which have only recently been referred to A. afarensis

(Brown et al. 2001).

Characteristics and inferred behavior. The main reasons

Leakey et al. (2001) did not assign KNM-WT 40000 and

38350 to A. afarensis are this material’s reduced subnasal

prognathism, anteriorly situated zygomatic root, flat and

vertically orientated malar region, relatively small but thickenameled

molars, and the unusually small M1 compared with

the size of the P4 and M3. Some of the morphology of the

new genus including the shape of the face is Paranthropuslike,

yet it lacks the postcanine megadontia that characterizes

Paranthropus. The authors note the face of the new

material resembles that of Homo rudolfensis, but they rightly

point out that the postcanine teeth of the latter are substantially

larger than those of KNM-WT 40000. K. platyops displays

a hitherto unique combination of facial and dental

morphology.

Controversy. White (2003) has argued (not persuasively,

in our opinion) that KNM-WT 40000 is a cranium of A.

afarensis and that its distinctive morphology is the result of

pre- and postfossilization damage involving the infiltration

of external matrix into cracks produced by weathering.

Homo

This group contains hominin taxa that are conventionally

included within the Homo clade. One of us, along with others,

have suggested that two of these taxa (H. habilis and H.

rudolfensis) may not belong in the Homo clade (Wood and

Collard 1999), but until we can generate sound phylogenetic

hypotheses about the australopiths, it is not clear what their

new generic attribution should be. Thus, for the purposes

of this review, they are retained within Homo.

Homo sapiens Linnaeus 1758

Type specimen. Linnaeus did not designate a type specimen.

Approximate time range. ~150 Kyr (thousand years) to

the present day.

History and context. An early indication that modern

humans were ancient enough to have a fossil record came

when a series of skeletal remains were discovered by workmen

at the Cro-Magnon rock shelter at Les Eyzies de Tayac,

France, in 1868. A male skeleton, Cro-Magnon 1, was initially

made the type specimen of a novel species, Homo

spelaeus Lapouge 1899, but it was soon apparent that it was

not appropriate to discriminate between this material and

modern humans. Soon, more modern humanlike fossils were

recovered from sites elsewhere in Europe, but the first African

fossil evidence of populations that are difficult to distinguish

from anatomically modern humans, from Singa in the

Human Origins 527

Sudan, did not come until 1924. Comparable evidence has

since come from north, east, and southern Africa [e.g., Ethiopia

(Dire-Dawa, 1933; Omo II, 1967; Herto, 1997); Morocco

(Dar es-Soltan, 1937–1938), and Natal—now KwaZulu Natal

(Border Cave, 1941–1942 and 1974)]. In the Near East,

comparable fossil evidence has been recovered from sites

such as Mugharet Es-Skhul (1931–1932) and Djebel Qafzeh

(1933, 1965–1975). In Asia and Australasia, anatomically

modern human fossils have been recovered from sites such as

Wadjak, Indonesia (1889–1890), the Upper Cave at Zhoukoudian,

China (1930 and thereafter), Niah Cave, Borneo

(1958), Tabon, Philippines (1962), and the Willandra Lakes,

Australia (1968 and thereafter). All this material has been

judged to be within, or close to, the range of variation of living

regional samples of modern human populations, and thus

it is not appropriate to distinguish it taxonomically from

Homo sapiens.

Characteristics and inferred behavior. Paradoxically, it is

easier to assemble information about the characteristic morphology

of extinct hominin taxa than about the only living

hominin species. For each morphological region what are the

boundaries of living H. sapiens variation? How far beyond

these boundaries, if at all, should we be prepared to go and

still refer the fossil evidence to H. sapiens? These are simple

questions to which one would have thought there would be

ready answers. However, the morphological expression of

modern humanness has proved to be complex and difficult

to express. For example, spoken language is assumed to be

a sine qua non of H. sapiens, but it is difficult if not impossible

to determine language competence (as opposed to the

potential for language) from the fossil record. It is claimed

that the distinctive form of living and fossil H. sapiens crania

can be reduced to two main influences, a retracted face and

an expanded globular braincase (Lieberman et al. 2002), and

the recently announced crania from Herto (White et al. 2003)

are consistent with this prediction.

Controversy. The origin of H. sapiens has been the subject

of considerable debate. Most analyses have pointed to

Africa ~100–200 Kyr ago as the source of modern human

genetic variation (Relethford 2002; but see also Templeton

2002). The earliest evidence of anatomically modern human

morphology in the fossil record comes from sites in Africa

(e.g., Omo II and Herto) and the Near East (e.g., Qafzeh)

listed above. It is also in Africa that there is evidence for a

likely morphological precursor of anatomically modern

human morphology. This takes the form of crania that are

generally more robust and archaic-looking than those of anatomically

modern humans yet which are not archaic enough

to justify their allocation to H. heidelbergensis, or derived

enough to be H. neanderthalensis (see below). Specimens in

this category include Jebel Irhoud (Morocco, 1961 and 1963)

from North Africa; Omo 2 (Kibish Formation) (Ethiopia,

1967); Laetoli 18 (Tanzania, 1976); Eliye Springs (KNM-ES

11693) (Kenya, 1985) and Ileret (KNM-ER 999 and 3884;

Kenya, 1971 and 1976, respectively) from East Africa; and

Florisbad (Free State, 1932) and Cave of Hearths (Northern

Province, 1947) in southern Africa. There is undoubtedly a

gradation in morphology that makes it difficult to set the

boundary between anatomically modern humans and H.

heidelbergensis. However, it is clear that unless at least one

boundary is set along this cline, morphological variation

within H. sapiens sensu lato is so great that it strains credulity.

Homo neanderthalensis King 1864

Type specimen. Neanderthal 1—adult calotte and partial

skeleton, Feldhofer Cave, Elberfield, Germany 1856.

Approximate time range. ~200–30 Kyr.

History and context. The first evidence of Neanderthals

to come to light was a child’s skull found in 1829 from a site

in Belgium called Engis. An adult cranium recovered in 1848

from Forbes’ Quarry in Gibraltar also displays the distinctive

Neanderthal morphology. However, the type specimen

of Homo neanderthalensis King 1864 consists of an adult skeleton

recovered in 1856 from the Feldhofer Cave in the

Neander Valley, in Germany. Excavations were restarted at

the Feldhofer Cave in 1997 and much of what was missing

from the original skeleton plus the remains of other individuals

have recently been recovered (Schmitz et al. 2002). After

the initial recovery of hominins from the Feldhofer Cave it

was some time before discoveries were made at other sites in

Europe [e.g., Moravia (Sipka, 1880); Belgium (Spy, 1886);

Croatia (Krapina, 1899–1906); Germany (Ehringsdorf,

1908–1925), and France (Le Moustier, 1908 and 1914; La

Chapelle-aux-Saints, 1908; La Ferrassie, 1909, 1910, and

1912)]. The first evidence of Neanderthals beyond western

Europe was recovered in 1924–26 at Kiik Koba in the Crimea.

The first of many discoveries in the Near East was at Tabun

(1929), and in 1938 the first fossils were recovered from

Central Asia at Teshik-Tash. New Neanderthal localities continue

to be discovered in Europe (e.g., St. Cesaire, 1979;

Zaffaraya, 1983 and 1992; Moula-Guercy, 1991) and western

Asia (Mezmaiskaya, 1993 and 1994). Thus, Neanderthal

remains have been found throughout Europe, with the exception

of Scandinavia, as well as in the Near East, the Levant,

and western Asia. Many elements of the characteristic

morphology of the Neanderthals can be seen in remains recovered

from sites such as Steinheim and Reilingen (Germany)

and Swanscombe (England) that date from ~200–300

Kyr. It is also said to be evident in precursor form in the remains

that have been found in the Sima de los Huesos, a cave

in the Sierra de Atapuerca, Spain (see H. heidelbergensis,

below).

Characteristics and inferred behavior. Features of the Neanderthal

cranium include thick, double-arched brow ridges,

a face that projects anteriorly in the midline, a large nasal

skeleton, laterally projecting and rounded parietal bones and

a rounded, posteriorly projecting occipital bone (i.e., an

occipital “bun”). Estimates of brain size [means: female, 1286

cc. (n = 4); male, 1575 cc. (n = 7)] suggest that Neanderthal

brains were as large, if not larger, than the brains of living

528 The Relationships of Animals: Deuterostomes

Homo sapiens, but they were perhaps slightly smaller relative

to body mass. The Neanderthals were stout with a broad rib

cage, a long clavicle, a wide pelvis, and limb bones that are

generally robust with well-developed muscle insertions. The

distal extremities tend to be short compared with most modern

H. sapiens, but Neanderthals were evidently obligate bipeds.

The generally well-marked muscle attachments and the

relative thickness of long bone shafts have been interpreted

as indicators of a strenuous lifestyle. The size and wear on

the incisors suggest that the Neanderthals regularly used their

anterior teeth as “tools” either for food preparation or to grip

hide or similar material.

It is clear that the Neanderthals possessed the cognitive

and manipulative abilities to create a sophisticated, versatile

tool kit and possibly objects of symbolic value. Whether or

not Neanderthals were capable of complex speech typical of

modern humans remains unknown, largely because the neural

adaptations that make speech possible do not preserve

in the fossil record. Some reconstructions suggest that the

Neanderthal vocal tract would have been capable of fewer

differentiable vowel sounds than that of modern humans, but

this hypothesis is difficult to test. Researchers have recently

presented compelling evidence for deliberate defleshing (i.e.,

cannibalism) on the crania of ~100–Kyr-old Neanderthals

from Moula-Guercy. Paleoenvironmental and anatomical

data indicate that Neanderthals typically occupied cold,

marginal habitats.

Controversy. In the past decade or so there has been an

increasing acceptance that the Neanderthals are morphologically

distinctive, so much so that many consider it unlikely

that such a specialized form could have given rise to the

morphology seen in modern humans. There is, however,

another school of researchers who point to, and stress, the

morphological continuity between the fossil evidence for

H. sapiens and the remains others would attribute to H.

neanderthalensis. Some have argued that morphologically

intermediate specimens are evidence of admixture between

Neanderthals and modern humans, but this interpretation

has been challenged.

Recent developments. Recently researchers have been

able to recover short fragments of mtDNA from the humerus

of the Neanderthal type specimen (Krings et al. 1997, 1999).

They were able to show that the fossil sequence falls well

outside the range of variation of a diverse sample of modern

humans, and they suggest that Neanderthals would have

been unlikely to have made any contribution to the modern

human gene pool. They conclude that this amount of difference

points to 550–690 Kyr of separation. Subsequently,

mtDNA has been recovered at two other Neanderthal sites,

from rib fragments of a child’s skeleton at Mezmaiskaya

(Ovchinnikov et al. 2000) and from Vindija (Krings et al.

2000). The differences between the mtDNA fragments studied

are similar to the differences between any three randomly

selected African modern humans. The fragments of mtDNA

that have been studied are short, but if the findings of the

three studies summarized in Krings et al. (1999) were to be

repeated for other parts of the genome, then the case for

placing Neanderthals in a separate species from modern

humans on the basis of their skeletal peculiarities would be

greatly strengthened (Knight 2003). There is disagreement

about the influence that intentional burial may have had on

the preservation of Neanderthal remains.

Homo erectus (Dubois 1892) Mayr 1944

Type specimen. Trinil 2—adult calotte, Trinil, Ngawi,

Java (now Indonesia) 1891.

Approximate time range. ~1.8 Myr to 200 Kyr.

History and context. In 1890 Eugene Dubois discovered

a mandible fragment in Java at a site called Kedung Brubus.

Less than a year later, in 1891, at excavations on the banks

of the Solo River at Trinil, workers unearthed a skullcap that

became the type specimen of a new species. Dubois initially

referred the skull cap to Anthropopithecus erectus Dubois

1892, but in 1894 he transferred the new species to Pithecanthropus

(Dubois 1894), and since then others have transferred

it to Homo (see below).

The focus for the next phase of the search for hominin

remains in Java was upstream of Trinil where the Solo River

cuts through the Plio-Pleistocene sediments of the Sangiran

Dome. In 1936 a German paleontologist, Ralph von Koenigswald,

recovered a cranium that resembled the distinctive

shape of the Trinil skullcap, but the brain size, ~750 cm3,

was even smaller than that of the Trinil calotte. In China in

the early 1920s Gunnar Andersson and Otto Zdansky excavated

for two seasons (1921 and 1923) at Locality 1 at Zhoukoudian

(formerly Choukoutien) Cave, near Beijing. They

recovered quartz artifacts, but apparently no fossil hominins.

However, Zdansky subsequently realized that two “ape” teeth

belonged to a hominin, and the next year they were assigned

to a new hominin genus and species, Sinanthropus pekinensis

Black 1927. The first cranium from Zhoukoudian was found

in 1929, and excavations continued until their interruption

by World War II. The fossils recovered from Locality 1 were

consistent in their morphology and were similar in many

ways to Pithecanthropus erectus, so much so that Ernst Mayr

formerly proposed the taxa be merged and then subsumed

into Homo as Homo erectus (Mayr 1944).

Since then, similar fossils have been found at other sites

in China (e.g., Lantian, 1963–1964); southern Africa (Swartkrans,

1949 and thereafter); East Africa (Olduvai Gorge, 1960

and thereafter; West and East Turkana, 1970 and thereafter;

Melka Kunture, 1973 and thereafter and also perhaps at Buia,

Eritrea, 1995 and 1997); and North Africa (Tighenif, 1954–

1955). Many also include the “Solo” remains from Ngandong,

Indonesia, within H. erectus. Discoveries from East African

sites have since provided crucial evidence about the postcranial

morphology of H. erectus (e.g., OH 28).

Characteristics and inferred behavior. The crania of H.

erectus have a low vault, a substantial more-or-less continuous

torus above the orbits and a sharply angulated occipital

Human Origins 529

region. The inner and outer tables of the cranial vault are

thick. Cranial capacities vary from ~725 cm3 for OH 12, to

~1250 cm3 for the Solo V calotte from Ngandong. The greatest

width of the face is in the upper part. The palate has similar

proportions to those of modern humans, but the buttressing

is more substantial. The body of the mandible is more

gracile than that of the australopiths, but more robust than

that of modern humans. The mandible lacks the well-marked

chin that is a feature of modern humans. The tooth crowns

are generally larger and the premolar roots more complicated

than those of modern humans, and the third molars are usually

smaller, or the same size, as the second molars. The dense

cortical bone of the postcranial skeleton is generally thicker

than is the case for modern humans. The limb bones are

modern humanlike in their proportions, but they have more

robust shafts, with the femoral and tibial shafts flattened from

front to back (femur) and side to side (tibia) relative to those

of modern humans.

All the dental and cranial evidence points to a modern

humanlike diet for H. erectus, and the postcranial elements are

consistent with a habitually upright posture and obligate, longrange

bipedalism. There is no fossil evidence relevant to assessing

the dexterity of H. erectus, but if H. erectus manufactured

Acheulean artifacts then some dexterity would be implicit.

Controversy. Over the years several authors have suggested

that morphological continuity between H. erectus and

later H. sapiens effectively invalidates the specific status of the

former. This has resulted in the proposition that H. erectus

be sunk into H. sapiens Linnaeus 1758. Recent advocates of

this course of action include Wolpoff et al. (1994) and Tobias

(1995).

Recent developments. If the discoveries from Dmanisi,

Georgia (Gabunia et al. 2000, Vekua et al. 2002) do prove

to belong to early African H. erectus (see below), then their

small brains and primitive cranial morphology would make

H. erectus sensu lato a substantially different taxon.

Homo heidelbergensis Schoetensack 1908

Type specimen. Mauer 1—adult mandible, Mauer,

Heidelberg, Germany 1907.

Approximate time range. ~600–100 Kyr.

History and context. The Mauer mandible was considered

distinctive because it has no chin and because the corpus is

larger than those of the mandibles of modern humans living

in Europe today. Cranial evidence from Zuttiyeh (Israel,

1925) has since been assigned to this group, as have fossils

from Greece (Petralona, 1959); France (Arago, 1964–1969;

Montmaurin, 1949); Hungary (Vйrtesszцllцs, 1965); and

Germany (Bilzingsleben, 1972–1977, 1983, and thereafter).

Researchers responsible for the discovery and analysis of the

large sample of ~400–600–Kyr-old (Bischoff and Shamp 2003)

hominins from Sima de los Huesos, Sierra de Atapuerca, Spain,

also assign that collection to H. heidelbergensis, but other researchers

are more inclined to treat this evidence as an early

form of H. neanderthalensis (see above).

The first relevant African evidence for H. heidelbergensis,

or what some call “archaic” H. sapiens, came in 1921 with

the recovery of a ~250–300 Kyr cranium from a cave in the

Broken Hill Mine at Kabwe in what is now Zambia. Other

morphologically comparable remains have been found from

the same, or an earlier, time period in southern Africa

(Hopefield/Elandsfontein, 1953 and thereafter), East Africa

(Eyasi, 1935–1938; Ndutu, 1973), and North Africa (Rabat,

1933; Jebel Irhoud, 1961 and 1963; Sale, 1971; Thomas

Quarry, 1969/72). The earliest evidence (~600 Kyr) of this

African archaic group comes from Bodo (Ethiopia, 1976).

Asian evidence for an archaic form of Homo comes from

China (e.g., Dali, 1978; Jinniushan, 1984; Xujiayao, 1976/

7, 1979; Yunxian, 1989/90) and possibly India (Hathnora,

1982). Most of these fossils are not reliably dated and their

estimated ages range from 100 to 200 Kyr.

Characteristics and inferred behavior. What sets this material

apart from H. sapiens and H. neanderthalensis is the morphology

of the cranium and the robusticity of the postcranial

skeleton. Some brain cases are as large as those of modern

humans, but they are always more robustly built with a thickened

occipital region and a projecting face and with large

separate ridges above the orbits, unlike the more continuous

brow ridge of H. erectus. Compared with H. erectus (see

above), the parietals are expanded, the occipital is more

rounded, and the frontal bone is broader. The crania of H.

heidelbergensis lack the autapomorphies of H. neanderthalensis,

such as the anteriorly projecting midface and the distinctive

swelling of the occipital region. The mean cranial

capacity for this taxon, ~1200 cc, is substantially larger than

the ~970 cc mean for H. erectus. However, the upper end

of the range of H. erectus brain size overlaps the lower end of

the range of H. heidelbergensis. H. heidelbergensis is the earliest

hominin to have a brain as large as anatomically modern

H. sapiens, and its postcranial skeleton suggests that its robust

long bones and large lower limb joints were well suited

to long-distance bipedal walking.

Controversy. There are currently different views about

the scope and phylogenetic relationships of H. heidelbergensis.

Researchers who interpret the Steinheim, Swanscombe, and

Sima de los Huesos remains as the beginnings of a distinctive

Neanderthal taxon see insufficient “morphological space”

for H. heidelbergensis and do not recognize it as a valid taxon

(e.g., Stringer 1996). Instead, they advocate sinking H.

heidelbergensis into H. neanderthalensis. Others have used an

elaborate system of grades of “archaic H. sapiens” to accommodate

the same fossil evidence or have taken to ignoring

species-level classifications in favor of recognizing a larger

number of paleo-, or p-demes (e.g., Howell 1999), which are

defined as “local populations” of species. The researchers who

do accept H. heidelbergensis as a valid taxon have different

interpretations of it. Some researchers who recognize H.

heidelbergensis interpret the taxon to include all non-Neanderthal

“archaic” Homo fossils, whereas others interpret it as

being confined to the European Middle Pleistocene. If there

530 The Relationships of Animals: Deuterostomes

is to be a single species to cover the archaic material from

Europe, Africa, and Asia, then the species name H. heidelbergensis

Schoetensack 1908 has priority. However, if there

was evidence that the non-European subset of the hypodigm

sampled an equally good species, then the species name with

priority is H. rhodesiensis Woodward 1921.

Homo habilis Leakey et al. 1964

Type specimen. OH 7—partial skull cap and hand bones,

FLKNN, Bed I, Olduvai Gorge, Tanzania 1960.

Approximate time range. ~2.4–1.6 Myr.

History and context. In 1960 Louis and Mary Leakey recovered

substantial parts of both parietal bones, six hand

bones (OH 7), and “a large part of a left foot” (OH 8) from

Bed I of Olduvai Gorge and in the next year or so further

evidence of a “nonrobust” hominin came from both Beds I

and II of Olduvai Gorge. In 1964, Leakey et al. set out the

case for recognizing a new species for the nonrobust hominin

from Olduvai and for accommodating it within the genus

Homo. In due course additional specimens from Olduvai were

added to the hypodigm of H. habilis, the most significant

being the cranium OH 24 and the associated skeleton OH

62. Evidence of fossils resembling H. habilis from Koobi Fora

includes a well-preserved skull (KNM-ER 1805), a well-preserved

cranium (KNM-ER 1813), several mandibles, and

some isolated teeth. Initially these specimens were not allocated

to a species but were given the informal name “early

Homo.” Some of the hominin fossils recovered from members

G and H of the Shungura Formation have also been

assigned to H. habilis, as has a fragmentary cranium and some

isolated teeth from member 5 at Sterkfontein, the cranium

SK 847 from member 1 at Swartkrans and a maxilla from

Hadar. Suggestions that H. habilis remains have been recovered

from sites beyond Africa are as yet unsubstantiated (but

see above the evidence recovered from Dmanisi).

Characteristics and inferred behavior. The endocranial volume

of H. habilis as originally described (H. habilis sensu

stricto) ranges from just less than 500 cm3 to about 600 cm3.

All the crania are wider at the base than across the vault, but

the face is broadest in its upper part. The only postcranial

evidence that can with confidence be assigned to H. habilis

sensu stricto are the postcranial bones associated with the type

specimen, OH 7, and the associated skeleton, OH 62. If OH

62 is representative of H. habilis sensu stricto, the skeletal

evidence suggests that its limb proportions and locomotion

were australopith-like. The curved proximal phalanges and

well-developed muscle markings on the phalanges of OH 7

also indicate the hand was used for more powerful grasping

(such as would be needed for arboreal activities) than is the

case in any other species of Homo. The inference that H. habilis

sensu stricto was capable of spoken language was based on

links between endocranial morphology and language comprehension

and production that are no longer valid.

Controversy. The case for splitting H. habilis sensu lato

(i.e., the Olduvai evidence plus crania such as KNM-ER 1470

and 1590) into two taxa, H. habilis sensu stricto (see above)

and Homo rudolfensis (see below), has attracted broad support,

but it is by no means universally accepted. As will be

apparent from inferences about its locomotion and capacity

for language set out above, in several ways H. habilis sensu

stricto is adaptively more like the australopiths than later

Homo taxa. This evidence combined with at best weak cladistic

evidence (see below) for its inclusion in the Homo clade

prompted Wood and Collard (1999) to suggest that both it

and H. rudolfensis should be removed from the genus Homo.

But what genus do those taxa properly belong to? The same

authors recommended that until the phylogenetic relationships

among the australopiths become clearer, they should

be referred to Australopithecus, but that would make that

taxon almost certainly paraphyletic. For the purposes of this

review, we retain the conventional taxonomy of both taxa,

at least until there is more consensus on this topic.

Homo ergaster Groves and Mazбk 1975

Type specimen. KNM-ER 992, Area 3, Okote member,

Koobi Fora Formation, Koobi Fora 1971.

Approximate time range. ~1.9–1.5 Myr.

History and context. This taxon was introduced in 1975

as part of a review of the taxonomy of the “early Homo” fossils

from Koobi Fora. The type specimen is KNM-ER 992 an

adult mandible that had been compared with, and by some

workers referred with, Homo erectus. The paratypes include

the skull KNM-ER 1805, but the only detailed analysis of

KNM-ER 1805 has concluded that it should be referred to

H. habilis sensu stricto. Any decision about whether Homo

ergaster is a good taxon is dependent on researchers demonstrating

that the type specimen KNM-ER 992 can be distinguished

from H. erectus (see above). Similarities between

the Koobi Fora component of the H. ergaster hypodigm and

the juvenile skeleton, KNM-WT 15000 from West Turkana

suggest that the latter should also be included in H. ergaster.

More recently, it has been claimed that there is evidence for

H. ergaster beyond Africa. Well-preserved crania and mandibles

from Dmanisi, Republic of Georgia, in the Caucasus

have been assigned to early African H. erectus (or H. ergaster)

or to a new taxon, Homo georgicus (Gabunia et al. 2000, Vekua

et al. 2002).

Characteristics and inferred behavior. The features claimed

to distinguish H. ergaster from H. erectus fall into two categories.

The first consists of the ways in which H. ergaster is

more primitive than H. erectus. The best evidence in this

category comes from details of the mandibular dentition and

in particular the mandibular premolars. The second category

consists of the ways in which H. ergaster is less specialized,

or derived, in its cranial vault and cranial base morphology

than is H. erectus. For example, it is claimed that H. ergaster

lacks some of the more derived features of H. erectus cranial

morphology such as thickened inner and outer tables and

prominent sagittal and angular tori, but other researchers

dispute the distinctiveness of this material (see below). H.

Human Origins 531

ergaster is the first large-bodied hominin taxon with a body

shape that was closer to that of modern humans than to the

australopiths (Wood and Collard 1999). It is also the first

hominin to combine modern human-sized chewing teeth

with a postcranial skeleton (e.g., long legs, large femoral head)

committed to long-range bipedalism and to lack morphological

features associated with arboreal locomotor and postural

behaviors. The small chewing teeth of H. ergaster imply either

that it was eating different food than the australopiths,

or that it was preparing the same food extra-orally, probably

by using tools and/or by cooking it.

Controversy. Many researchers do not regard the H.

ergaster hypodigm worthy of a separate species. They either

dispute there are any consistent, or significant, morphological

differences between the “early African” part of H. erectus

(i.e., H. ergaster) and the main H. erectus hypodigm, or they

acknowledge there are differences but suggest that they do

not merit recognition at the level of the species.

Homo rudolfensis (Alexeev 1986) sensu Wood 1992

Type specimen. Lectotype: KNM-ER 1470, Area 131,

Upper Burgi member, Koobi Fora Formation, Koobi Fora,

Kenya 1972.

Approximate time range. ~1.8–1.6 Myr.

History and context. In 1986 Alexeev suggested that differences

between the cranium KNM-ER 1470 from Koobi

Fora and Homo habilis sensu stricto from Olduvai Gorge justified

referring the former to a different new species he named

Pithecanthropus rudolfensis. Thus, if Homo habilis sensu lato

does subsume more variability than is consistent with it being

a single species and if KNM-ER 1470 is judged to belong

to a Homo species other than Homo habilis sensu stricto, then

Homo rudolfensis (Alexeev 1986) Wood 1992 is available as

the name of a second early Homo taxon.

Characteristics and inferred behavior. The main ways that

H. rudolfensis differs from H. habilis sensu stricto are that they

have different mixtures of primitive and derived, or specialized,

features. For example, although the absolute size of the

brain case is greater in H. rudolfensis, its face is widest in its

mid-part, whereas the face of H. habilis is widest superiorly.

Despite the absolute size of its brain (~750–800 cm3), when it

is related to estimates of body mass the brain of H. rudolfensis

is not substantially larger than those of the australopiths. The

more primitive face of H. rudolfensis is combined with a robust

mandible and mandibular postcanine teeth with larger,

broader, crowns and more complex premolar root systems

than those of H. habilis. At present no postcranial remains can

be reliably linked with H. rudolfensis. The mandible and postcanine

teeth are larger than one would predict for a generalized

hominoid of the same estimated body mass, suggesting

that its dietary niche made mechanical demands similar to

those of the megadont australopiths.

Controversy. The detailed case for dividing Homo habilis

sensu lato into two species is set out in Wood (1991, 1992).

A recent review of the cladistic and functional evidence for

H. rudolfensis (Wood and Collard 1999) has concluded that

there are few grounds for its retention in Homo and recommended

that it (along with H. habilis sensu stricto) be transferred

to Australopithecus as Australopithecus rudolfensis

(Alexeev 1986 Wood and Collard 1999.

Homo antecessor Bermudez de Castro et al. 1997

Type specimen. ATD6–5—mandible and associated

teeth, Level 6, Gran Dolina, Spain 1994.

Approximate time range. ~500–700 Kyr.

History and context. The Gran Dolina (TD) site is a cave

in the Sierra de Atapuerca that was exposed when a railway

cutting was excavated a century ago. The fossils attributed

to H. antecessor were recovered when a test excavation

reached Level 6.

Characteristics and inferred behavior. The authors of the

initial report claim the combination of a modern humanlike

facial morphology with the relatively primitive crowns and

roots of the teeth is not seen in H. heidelbergensis, nor do the

Gran Dolina remains have the derived H. neanderthalensis

traits seen in H. heidelbergensis. It is the apparent lack of these

derived features combined with differences from H. ergaster

that led the authors to propose the new hominin species.

They suggest that H. antecessor is probably the last common

ancestor of Neanderthals and H. sapiens.

Controversy. Many researchers question the grounds for

excluding this material from H. heidelbergensis.

Phylogeny

There is a wide spectrum of opinion about phylogenetic relationships

within the hominin clade. Most researchers are convinced

that the existing methods are capable of recovering

reliable phylogenetic relationships among fossil hominin taxa.

However, a minority of researchers are less confident that reliable

phylogenies can be extracted using traditional data obtained

from the existing fossil record. One faction within this

minority argues that until the selection of characters is better

integrated with information about the molecular basis of development,

character independence will never be assured

(Lovejoy et al. 2000). Another faction within the minority

suggests that even if character independence could be assured,

much of the hard-tissue evidence provided by the fossil record

may be so prone to various forms of homoplasy that the phylogenetic

signal it retains is too weak and the homoplastic noise

so strong that the former cannot be detected with any reliability

(Corruccini 1994, Collard and Wood 2000). The introduction

of new three-dimensional methods for capturing information

about shape and size may improve the likelihood that

phenetic information can be used to reconstruct phylogeny

(Lockwood et al. 2002, Guy et al. 2003).

The phylogenetic tree in figure 29.2 is a consensus of

recent attempts to recover the phylogeny of hominins. Some

taxon hypodigms are so small that any phylogenetic hypoth532

The Relationships of Animals: Deuterostomes

esis is speculative. Other hominin taxa are sufficiently well

known (e.g., P. boisei, A. afarensis, H. neanderthalensis) that

paucity of the fossil record per se is unlikely to be the reason

for any ambiguity about their phylogenetic relationships. Two

clades, later Homo and Paranthropus, are supported by nearly

all phylogenetic reconstructions (e.g., Wood 1991, Skelton

and McHenry 1992, Strait et al. 1997). Taxa that for many

years have been regarded as human ancestors (e.g., H. neanderthalensis

and late H. erectus) are almost certainly too derived

to be directly ancestral to modern humans.

Conclusions

The living and fossil taxa within the (Homo, Pan) clade can

be resolved into the four crude grades identified in figure

29.1. Many fossil taxa are excluded from this grade classification

because they lack one or more of the necessary lines

of evidence to infer brain size, relative tooth size, or locomotor

pattern. Two of the grades coincide with major

multitaxon clades and are coincident with Homo and

Paranthropus, two of the five genera recognized within the

(Homo, Pan) clade. Although the results of cladistic analyses

of the hominin fossil record differ in detail (e.g., Strait et al.

1997, Wood and Collard 1999), nearly all agree about the

robusticity of the Homo sensu stricto and Paranthropus clades.

A linear, sequential model is no longer tenable for the

post-2.5-Myr period of human evolutionary history, but influential

researchers continue to interpret the period between

5.0 and 3.0 Myr as a series of time-successive hominin

species (Asfaw et al. 1999). Thus, they view A. ramidus as the

direct ancestor of A. anamensis and the latter as the direct

ancestor of A. afarensis. This simplistic interpretation was

always likely to be challenged by fresh fossil evidence, and

this came in the form of a proposal to establish not just a new

species but a new genus for fossil hominins discovered at

West Turkana in 1998 and 1999. In that paper, Meave Leakey

et al. (2001) make the case that Kenyanthropus platyops is a

distinct taxon that shares some facial similarities with

Paranthropus taxa without sharing the latter’s distinctively

large premolars and molars and thick enamel. The newly

discovered and described Sahelanthropus tchadensis (Brunet

et al. 2002) combines facial features hitherto considered

apparently distinctive of advanced australopiths and Homo

with a chimp-sized brain and a good many other cranial features

seen only in Pan. All this suggests that the origins of

the (Homo, Pan) clade and subsequent evolution within the

hominin clade are a good deal more complex than many had

anticipated (Wood 2002). It is truly remarkable that thus far

no hominid fossil evidence in the 4–7 Myr time range has

been interpreted as being more closely related to Pan than

to Homo. Is this because none has yet been discovered? Or is

it because we are aware of it but have misinterpreted it as

belonging to the hominin and not the panin clade?

Acknowledgments

B.W. is grateful to the organizers of the Tree of Life meeting for

their invitation to place modern humans in their proper context

within the living world. The Henry R. Luce Foundation, the

National Science Foundation, and The Leverhulme Trust have

funded research by B.W. that is incorporated in this review. P.C. is

supported by an NSF-IGERT graduate fellowship. Special thanks

to Mark Collard for contributing to many of the ideas incorporated

in this review, and to Sally Gibbs for carrying out the soft-tissue

study.

Figure 29.2. A speculative

phylogeny of the hominins over

time. Solid lines indicate the

authors’ preferences. See text for

details.

K. platyops

Ar. ramidus

Au. bahrelghazali Au. afarensis

Au. africanus

P. aethiopicus

Au. rudolfensis

H. ergaster

H. erectus

Au. garhi

H. heidelbergensis

H. sapiens

0

4

3

2

1

Au. anamensis

P. robustus

P. boisei

H. antecessor Au. habilis

6

5

7

8

O. tugenensis

Sahelanthropus tchadensis

H. neanderthalensis

Human Origins 533

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Perspectives on the Tree of Life

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