Hominid phylogeny

Despite a large number of fossil hominid finds since the 1950s and increasingly illuminating

scientific advances in the field, hominid phylogeny is still far from resolved (Chamberlain and Wood 1987).

For a good part of the twentieth century a unilinear model of human evolution based on Neo- Darwinian

Synthesis was documented with morphological diversity being explained by intraspecific variation or

geographical differences(Strait et al. 2007). First conceived in the 1940’s to make sense of the increasing

inter-disciplinary application of Mendelian genetics, the Neo-Darwinin Synthesis sought to approach the

unification of biological sciences in a mathematical, quantitative way(Foley 2001). However, by the 1960’s

the unilinear view of hominid evolution was extremely difficult to champion, especially with new

discoveries such as proof that European Neanderthals were living contemporaneously with Homo sapiens

(Klein 2009). Ernest Mayr’s definition of species in a cladistic framework laid the foundation for the

organisation of hominid evolution by taxa (Mayr 1963). Mayr’s publication was at the time supported by

Dobzhansky’s understanding of evolution at a genetic level (Dobzhansky 1955)but the emphasis on

evolution from parent to offspring has since been overshadowed by the arguments surrounding the

definition of a species concept as applied to the fossil record and the semantic arguments that proliferate

(Dobzhansky 1944, Gingerich 1985, Ghiselin 1987). The speciation problem was clouded further by Mayr’s

declaration that humans had ceased to speciate due to our unique place on the evolutionary tree (Tattersall

2000).

During the 1960s a temporally linear theory was supported(Fagundes et al. 2007); at the

same time the ‘polycentric origin’ (now known as multi regional continuity theory) was being developed by

Franz Wiedenrachs (Conroy 1997)); the polycentric origin theory was subsequently incorporated into the

single species theory(Tattersall 2000). The conservative reductionist approach fell out of favour in the 1970s

as more fossils were found and the Out of Africa theory gained proponents. The discovery of

Australopithecus afarensis in 1978 (Johanson and White 1979) and subsequent finds such as The Black

Skull (KNM-WT 17000) (Lewin 1986, Strait et al. 2007) triggered discussion of hominid phylogeny in the

early 1980s and subsequently (even up to present day) the relationship between Homo sapiens, Homo

erectus and H. neanderthalensis is extensively debated (Rightmire 1998, Rightmire 2008, Hebsgaard et al.

2007, Stringer 1987, Frayer 1992, Tattersall 2006). The placing of Neanderthals on any phylogenetic tree is

extremely problematic due to the relatively unique morphology, such as seemingly prehensile toe bones

(Straus and Cave 1957) and unique thorax shape (Sawyer and Maley 2005). Any attempt at synthesis is

complicated even further by the discovery of shared genetic elements between Neanderthals and Homo

sapiens (Green et al. 2010) and the relationship between these species and others (Red Deer Cave people,

Denisovans) is unresolved.

Following the translation of Hennig’s book in the early 1960s (Hennig et al. 1999) an articulated

approach to biological classification and determination of evolution relationships called ‘phylogenetic

systematics’ began to gain favour. Mayr (Mayr 1963) r called this approach “cladistics”. Phylogeny is more

complex than simply classifying by groups however, instead it provides a basis for taxonomy and

classification as well as a bigger perspective on inherited adaptations of organisms and their relationships,

both ancestral and interspecific. Phylogeny should encompass cladogenesis; the splitting and divergence of

species from one common ancestor into two (or more) distinct species; and phyletic evolution (the

gradualistic mode of evolution through time). Differing opinions prevail as to which of these is the most

important and to what extent, and to a large degree depend on which theory of evolution one favours.

Cladists classify solely on characteristics; plesiomorphic (shared primitive), synapomorhies (shared

derived), apomorphies (uniquely derived). The relationship between species depends on the assumption

that species with synapomorphies share an evolutionary link regardless of temporal restraints. On the other

hand, phylogeny takes temporal factors into account and as such attempts to define the nature of the

evolutionary link between species. Buried in the theoretical models of phylogenetics and cladograms lies the

scenario; the scenario takes into account hypothetical, abstract and inferred forces that act on the evolution

of separate species. Both phylogeny and cladistics should help us reveal the scenario but the abstract nature

of the scenario leaves it open to argument, discussion and opinion and this is very much the case for

hominid phylogeny.

Following new fossil finds in the 1980s and the movement towards a realisation that the fossil

record only encompasses a very small amount of the actual species and/or genera represented. Eldridge and

Tattersall’s cladistic analysis in 1975 recognised 4 genera; in 2000 Tattersall describes 8 (Tattersall and

Chauhan 1995, Tattersall 2000, Hecht et al. 1975). Tattersall himself describes this original analysis as

‘clumsy’ ‘(Tattersall and Chauhan 1995), however it did highlight the difficulty with describing Homo

erectus as the intermediate species leading from Australopithecus to Homo sapiens due to the apomorphic

cranial features (low cranial vault etc). Indeed, we now recognise Australopithecus sediba as the ancestral

to Homo (Berger et al. 2010).

A.

B.

C.

D.

E.

Fig 1 A. Australopithecus afarensis ancestor theory. After Johansen and White (1979) (Johanson and White

1979)

B. Multiple lineage (early divergence) theory. After Senut and Tardieu (1985)

C. Australopithecus afarensis ancestor theory. After Skelton et al. (1986)

D. Early robust Australopithecine theory. After Delson, (1986) and Grine and Moniz 1997)

E. Ardipithecus lineage theory. After McHenry and Skelton (1992)

(Johanson and White 1979, Senut and Tardieu 1985, Skelton et al. 1986, Delson 1986, Grine and Moniz 1997,

Skelton and McHenry 1992),

It is easy to see why Tattersall calls the field of hominid taxonomy “simply arguing about the names”

(Tattersall and Chauhan 1995). However, evolutionary systematics underpins the entire field of

palaeoanthropology; how species interact with each other, how they evolve into different species and adaptive

grades; ultimately leading to our own selves. Darwin’s Tree of Life is an iconic symbol of evolution and our

understanding of systematics gives a framework which, if incorrect, compromises our entire understanding. The

fossil record illuminating the proto-chimapanzee/ proto-human spilt is extremely scarce with only the fragmentary

Orrorin tugenensis and the crania of Sahelanthropus tchadensis filling the temporal gap between 11-7 mya.

Conservative taxonomists recognise five Australopith genera, Orrorin, Sahelanthropus, Ardipithecus,

Australpithecus and Kenyanthropus, some even more (Conroy 1997, Klein 1989). What is clear is that very early

hominids are hugely diverse and show a mosaic pattern of traits (Tattersall 2000). Ardipithecus ramidus/kadabba

and Australopithecus both show unique sets of traits that differ from one another and species that follow after.

Conroy (1997) explains that “Ar. kadabba, Ar.s ramidus, A. anamensis and A. afarensis form a reasonably good

ancestor-descendant series.” Lieberman 2001 presents six alternative hypotheses about early hominin evolution. In

all six hypotheses Ar. ramidus appears close to the root of the tree and the A. ramidus - A. anamensis - A. afarensis

series is highly conserved in all six scenarios. Phylogeny of these very early hominids is still to be settled and

requires more evidence to highlight strong links between species but it is clear that there are multiple lineages of

bipedal hominids present after the ape/hominid divergence.

Consensus on the australopith lineage is also lacking; agreement on the number of species is adventurous,

although most agree on a core set of A. anamensis, A. afarensis, P. robustus, P. boisei, P. aethiopicus and A. sediba

with some disagreement about A. garhi and A. bahrelghazali (Tattersall 2000). Some researchers prefer to place

these fossils with A. afarensis. Many simply choose to classify ‘gracile’ and ‘robust’ forms as adaptive grades, with

others grouping according to dietary behaviour although many theories on Australopithecine evolutionary

relationships abound (Fig 1). P.aethiopicus was originally thought to be ancestral to P. boisei (Grine and Moniz

1997, Delson 1986). Wood (Wood 2000) demonstrates that gracile and robust Australopithecus and Paranthropus

are paraphyletic, supporting the splitting of these forms into separate genera.

Unfortunately, there is a problematic thread running through all these theories; it is very difficult to

account for a mosaic set of features and differing adaptations when a singular ‘more-modern’ trend is not

immediately obvious. Organising into ‘gracile’ and ‘robust’ forms is further complicated by Kenyanthropus

platyops which is an inconvenient outlier and some researchers have debated whether it is an

Australopithecine or not (Lieberman 2001). Nevertheless, Kenyanthropus platyops is proof of the variation

and number of diverse forms of hominid between 5 – 1.5mya.

At 1.8 mya, Homo ergaster and Homo habilis fossils are found in a wide range of areas. Whether this

is down to depositional factors or a rapid evolutionary or behavioural change is as yet to be qualified;

however it does make the grey area between Australopithecus and Homo stand out by contrast. Fossils in

the 2.5-2mya temporal range, such as Paranthropus aethiopicus show little morphological similarity to

Homo making the postulation of ancestor to Homo difficult. However, research on a newly discovered

fossil, Australopithecus sediba, (Berger et al. 2010) is proving promising. A. sediba dating is relatively

precise due to a number of coincidental factors; palaeomagnetism showed that the fossils are not older than

2mya; the discovery of fauna that was extinct by 1.5mya and the ‘normal’ magnetic polarity trace narrow the

range even further to 1.7-1.9mya. Not only is the time frame of 1.9mya a good match to Homo ancestor, A.

sediba is also found in similar locations and shows intermediate morphology between A. afarensis, A.

africanus and Homo. Cranial capacity of 430cc is in the upper range for A. africanus (Stringer 1987) and

reaching the lower levels of early Homo. Mandibular and dental features are similar to Homo, as is the hand

morphology, but A. sediba possesses a more primitive foot. The crowning of A. sediba as immediate

ancestor to Homo is not without criticism, especially from White and Leakey, who maintain that early

Homo, specifically Homo habilis, was already present in Africa and therefore co-existed with A. sediba

instead of one form leading to the other. Louis Leakey’s longstanding belief in the temporal range of early

Homo has often been criticised (Tattersall and Chauhan 1995), and helps to explain the unusual grouping of

H. habilis in the same genera as H. sapiens despite quite large morphological difference. Bernard Wood has

long held the opinion that H. habilis and H. rudolfensis should be removed from the genus Homo, based on

rigorous cladistic analyses although it is clear they do not fit the Australopithecus genus either

(Chamberlain and Wood 1987, Wood and Collard 1999, Wood 2000)

It can be no surprise that the proliferation of fossil finds between circa 1986 and 2000 have led to a

proliferation of theories(Fig 1) which both reflect the uncertainties of inferring evolutionary relationships

and/or ancestry and the unknowns of variation in species variation and plasticity. A lack of consensus over

taxonomic affinities of specimens such as Homo floresiensis further cloud synthesis as well as the creation

of such ‘umbrella’ taxon, Homo erectus, which is used as a ‘catch-all’ for suggested species like Homo

georgicus, Homo ergaster, Homo pekinensis and Homo heidelbergensis. The unique morphology of the

Dimanisi material varies widely from the African fossils and it would seem prudent to allocate a new species

on this fact. Similarly, the Homo antecessor material suggests a different morphology to Homo

heidelbergensis although whether it is a truly unique species or a local ‘tribe’ will only be settled with more

material.

In the 1990s a growing consensus that Homo habilis sensu stricto may encompass more than one

species, with some recognising Asian and African groups as separate species ("Homo erectus sensu stricto"

for Asian H. erectus, and "Homo erectus sensu lato" for both the early African populations (H. ergaster) and

Asian populations).

Molecular data has had a profound effect on most biological sciences since the 1950s and despite a

severe lack of DNA data for the fossil record, hominid phylogeny is no exception (Green et al. 2010).

Comparison between chimpanzee and human DNA reinforced our obvious similarity with great apes and

supported immunological comparisons between species. Unfortunately this led to uncouth arguments

about the taxonomical status of humans and apes; hominids or hominins? (Tattersall 2000) Most notable

molecular analysis has extracted and analysed the mtDNA of Feldhöfer Neanderthals although the

interpretation of this analysis has not been without controversy(Sawyer and Maley 2005, Tattersall 2006,

Hebsgaard et al. 2007). In addition, the intermediate features of the Swanscombe, Steinheim and Atapuerca

specimens are helping to affirm Neanderthals as, at least, a subspecies of Homo sapiens neanderthalensis, if

not simply a geographical variant of Homo sapiens. The very fact that we share some genetic material with

this contemporaneous species should be definitive if the biological species concept is upheld, but the

primitive grunting caveman as our ancestors is a difficult concept to shake off, especially in the popular

press. Neanderthals were widespread and diverse and many theories on their demise have been purported;

whatever the truth, Neanderthals are doomed to limbo as an evolutionary ‘dead-end’. Many researchers

classify Neanderthals as separate species due to some unique morphologies; however until we understand

the full range of human variation, these theories are always under the spotlight of scrutiny.

Further DNA extraction work may be necessary to fully illuminate this issue and there are many

other areas of hominid phylogeny which have the potential to be partly resolved in the next century of the

science. A better understanding of the ape/hominid divergence will give us a better idea of where humans

started. The relationship between Paranthropus and Australopithecus and the nature of gracile vs robust

forms will help our understanding of hominid phylogeny greatly. Further analysis of the A. sediba fossils

offer great potential for the exact nature of the lineage into Homo and the resolution of the conflicting ideas

on the beginnings of the Homo genus. DNA analysis and a greater synthesis of the extent of human

variation may help to resolve the Neanderthal speciation issue, as well as shedding light on the Denisovia

and Red Deer Cave people and their relationships with Homo sapiens. In evolutionary systematics, a better

understanding of the biological species concept and how it applies to the fossil record, as well as a more

consistent application across the board may help to bring about a better understanding of evolutionary

relationships and how they fit within our frameworks. Quantitative and systematic units of evolutionary

change, like Haldane’s darwin, may help to quantify in a systematic way the morphological change in

species (Haldane 1949); however this approach is somewhat artificial and does not take into account rapid

evolutionary changes or geographical interpretations.

Fig 2 temporal and taxonomic consideration for hominid species

Approx. Date

Species Discoverer/ Date

Found Alternative interpretations of classification

Alternative names

5.6m Ardipithecus kadabba

White 2004 East Africa (Ethiopia)

designated subspecies of Ardipithecus ramidus

Ardipithecus ramidus kadabba

5.2 - 4m Australopithecus anamensis

Patterson East Africa (Ethiopia,

Kenya)

Praeanthropus anamensis

4.4m Ardipithecus ramidus

White 1994 East Africa (Ethiopia)

originally classified as Australopithecus ramidus

Ardipithecus ramidus ramidus

3.9 - 2.9m

Australopithecus afarensis

Johanson 1974 East Africa (Ethiopia,

Kenya)

Praeanthropus africanus

3.6m Australopithecus bahrelghazali

Brunet 1995 Central Africa (Chad)

Australopithecus afarensis based on mandibular

morphology

Praeanthropus bahrelgazali

3.5m Kenyanthropus platyops

Leakey 1999 East Africa (Kenya)

Maybe Australopithecus afarensis with depositional

distortion

Australopithecus platyop, Kenyanthropus

rudolfensis 3.3 - 2.2m

STW575 "Little Foot" genus

unnamed

Clarke 1994 South Africa unknown species of Australopithecene, not

Australopithecus afarensis or Australopithecus africanus

3 - 2m Australopithecus africanus

Dart 1925 South Africa

2.5m Paranthropus aethiopicus

Walker 1985 East Africa (Kenya)

Australopithecus aethiopicus

2.5m Australopithecus gahri

Asfaw 1997 East Africa (Ethiopia)

Local variant of Australopithecus afarensis

2.4 - 1.4m

Homo habilis Leakey 1964 East Africa (Kenya,

Tanzania)

Individual specimens may be Homo erectus (eg. KNMER

180gf)

Homo erectus

2.3 - 1.2m

Paranthropus boisei

Leakey 1959 East Africa (Tanzania)

Originally named Zinjanthropos boisei

Zinjanthropus boisei

2 - 1.95m

Australopithecus sediba

Berger South Africa

2 - 1.2m Paranthropus robustus

Broom 1938 South Africa Subtribe of Australopithecina (Australopithecus robustus)

Australopithecus robustus, Paranthropus

crassidens 1.9m Homo rudolfensis Leakey 1972 East Africa

(Kenya) Originally named

Pithecanthropus, sometimes classed as Homo habilis

Pithecanthropus rudolfensi, Homo habilis

1.8m Homo gautengenis Curnoe 2010 South Africa Homo sapiens or 'archaic' Homo sapiens

Homo sapiens sub spp.

1.8m Homo erectus * (see also Homo

ergaster)

Leakey * see notes Homo erectus sensu stricto = Local 'tribe' in Asia, Homo erectus sensu lato = larger bodies, early populations

(Africa and Asia). Argument to include Homo floresiensis and

Homo gorgicus

Pithecathropus erectus, Tchadanthropus uxeris,

Homo erectus sensu lato, Homo erects sensu

stricto. See also Homo ergaster

1.8m Homo floresiensis Brown 1994 Java (Flores) Discussions to classify as Homo erectus or pre-Homo

Homo erectus

1.8m Homo georgicus Lordkipanidze 2007

Georgia (Dminisi)

Discussions to classify as Homo erectus

Homo erectus

1.8 - 1.4m

Homo ergaster Robinson 1949 East Africa (Kenya)

Homo erectus sensu lato See Homo erectus

1.3m - 600k

Homo heidelbergensis

Asfaw East Africa (Ethiopia,

Eritrea)

Previously called Homo rhodesiensis. Local variant of

Homo erectus

Homo erectus, Homo rhodesiensis

1.2m - 800k

Homo antecessor Carbonell 2008

Spain (Atapuerca)

Local variant of Homo heidelbergensis

Homo heidelbergensi, Homo mauritanicus

600-300k

Homo cepranensis Biddittu 1994 Italy (Frosinone)

maybe local variant of Homo erectus

500 - 350k

Homo neanderthalensis

Clark 1957 Northern Europe

Sub-species of Homo sapiens; Homo sapiens neanderthalensis

Homo sapiens, Homo sapiens neanderthalensis

160 - 150k

Homo sapiens idaltu

White 1997 Northern Europe

Previously called Homo helmei, Homo njarasensis, "archaic"

Homo sapiens

Homo helmei, Homo njarasensis, "archaic"

Homo sapiens 100k Homo sapiens Leakey 1967 Worldwide

(oldest OMO, Ethiopia)

Homo sapiens sapiens, anatomically modern humans,

Homo sapiens sapiens

40k Denisovia Russia (Siberia)

Sub-species of Homo sapiens Homo sapiens sub spp.

References

Berger, L. R., De Ruiter, D. J., Churchill, S. E., Schmid, P., Carlson, K. J., Dirks, P. H. G. M. and Kibii, J. M. (2010) 'Australopithecus sediba: A new species of Homo-like australopith from South Africa', Science, 328(5975), 195-204.

Chamberlain, A. T. and Wood, B. A. (1987) 'Early hominid phylogeny', Journal of Human Evolution, 16(1), 119-133.

Conroy, G. C. (1997) Reconstructing human origins, WW Norton.

Delson, E. (1986) 'Palaeoanthropology: Human phylogeny revised again', Nature, 322, 496-497.

Dobzhansky, T. (1944) 'On species and races of living and fossil man', American Journal of Physical Anthropology, 2(3), 251-265.

Dobzhansky, T. (1955) 'Evolution, genetics, and man', Evolution, genetics, and man.

Fagundes, N. J. R., Ray, N., Beaumont, M., Neuenschwander, S., Salzano, F. M., Bonatto, S. L. and Excoffier, L. (2007) 'Statistical evaluation of alternative models of human evolution', Proceedings of the National Academy of Sciences, 104(45), 17614-17619.

Foley, R. (2001) 'In the shadow of the modern synthesis? Alternative perspectives on the last fifty years of paleoanthropology', Evolutionary Anthropology: Issues, News, and Reviews, 10(1), 5-14.

Frayer, D. (1992) 'The persistence of Neanderthal features in post-Neanderthal Europeans', Continuity or replacement: controversies in Homo sapiens evolution. AA Balkema, Rotterdam.

Ghiselin, M. T. (1987) 'Species Concepts', eLS.

Gingerich, P. D. (1985) 'Species in the fossil record: concepts, trends, and transitions', Paleobiology, 27-41.

Green, R. E., Krause, J., Briggs, A. W., Maricic, T., Stenzel, U., Kircher, M., Patterson, N., Li, H., Zhai, W., Fritz, M. H.-Y., Hansen, N. F., Durand, E. Y., Malaspinas, A.-S., Jensen, J. D., Marques-Bonet, T., Alkan, C., Prüfer, K., Meyer, M., Burbano, H. A., Good, J. M., Schultz, R., Aximu-Petri, A., Butthof, A., Höber, B., Höffner, B., Siegemund, M., Weihmann, A., Nusbaum, C., Lander, E. S., Russ, C., Novod, N., Affourtit, J., Egholm, M., Verna, C., Rudan, P., Brajkovic, D., Kucan, Ž., Gušic, I., Doronichev, V. B., Golovanova, L. V., Lalueza-Fox, C., de la Rasilla, M., Fortea, J., Rosas, A., Schmitz, R. W., Johnson, P. L. F., Eichler, E. E., Falush, D., Birney, E., Mullikin, J. C., Slatkin, M., Nielsen, R., Kelso, J., Lachmann, M., Reich, D. and Pääbo, S. (2010) 'A Draft Sequence of the Neandertal Genome', Science, 328(5979), 710-722.

Grine, F. E. and Moniz, M. A. (1997) 'A reappraisal of early hominid phylogeny', Journal of Human Evolution, 32, 17-82.

Haldane, J. B. S. (1949) 'Suggestions as to quantitative measurement of rates of evolution', Evolution, 51-56.

Hebsgaard, M. B., Wiuf, C., Gilbert, M. T. P., Glenner, H. and Willerslev, E. (2007) 'Evaluating Neanderthal genetics and phylogeny', Journal of molecular evolution, 64(1), 50-60.

Hecht, M. K., Eldredge, N. and Gould, S. J. (1975) 'Morphological transformation, the fossil record, and the mechanisms of evolution: a debate' in Evolutionary Biology, Springer, 295-308.

Hennig, W., Davis, D. and Zangerl, R. (1999) Phylogenetic systematics, University of Illinois Press.

Johanson, D. C. and White, T. D. (1979) 'A systematic assessment of early African hominids', Science, 203(4378), 321-330.

Klein, R. G. (1989) The human career, University of Chicago Press Chicago.

Klein, R. G. (2009) 'Darwin and the recent African origin of modern humans', Proceedings of the National Academy of Sciences, 106(38), 16007-16009.

Lewin, R. (1986) 'New fossil upsets human family', Science, 233(4765), 720-721.

Lieberman, D. E. (2001) 'Another face in our family tree', Nature, 410(6827), 419-420.

Mayr, E. (1963) 'Animal species and evolution', Animal species and their evolution.

Rightmire, G. P. (1998) 'Human evolution in the Middle Pleistocene: The role of Homo heidelbergensis', Evolutionary Anthropology, 6, 218-227.

Rightmire, G. P. (2008) 'Homo in the Middle Pleistocene: Hypodigms, variation, and species recognition', Evolutionary Anthropology: Issues, News, and Reviews, 17(1), 8-21.

Sawyer, G. J. and Maley, B. (2005) 'Neanderthal reconstructed', The Anatomical Record Part B: The New Anatomist, 283(1), 23-31.

Senut, B. and Tardieu, C. (1985) 'Functional aspects of Plio-Pleistocene hominid limb bones: implications for taxonomy and phylogeny', Ancestors: the hard evidence, 193-201.

Skelton, R. R. and McHenry, H. M. (1992) 'Evolutionary relationships among early hominids', Journal of Human Evolution, 23(4), 309-349.

Skelton, R. R., McHenry, H. M., Drawhorn, G. M., Bilsborough, A., Chamberlain, A. T., Wood, B. A. and Vančata, V. (1986) 'Phylogenetic Analysis of Early Hominids [and Comments and Reply]', Current Anthropology, 21-43.

Strait, D. S., Grine, F. E. and Fleagle, J. G. (2007) 'Analyzing hominid phylogeny', Handbook of paleoanthropology, 3, 1781-1806.

Straus, W. L. and Cave, A. J. E. (1957) 'Pathology and the posture of Neanderthal man', The Quarterly Review of Biology, 32(4), 348-363.

Stringer, C. B. (1987) 'A numerical cladistic analysis for the genus< i> Homo</i>', Journal of Human Evolution, 16(1), 135-146.

Tattersall, I. (2000) 'Paleoanthropology: the last half-century', Evolutionary Anthropology Issues News and Reviews, 9(1), 2-16.

Tattersall, I. (2006) 'Neanderthal skeletal structure and the place of Homo neanderthalensis in European hominid phylogeny', Human Evolution, 21(3-4), 269-274.

Tattersall, I. and Chauhan, P. R. (1995) 'The Fossil Trail. How we know what we think we know about human evolution', New York: Oxford.

Wood, B. (2000) 'Origin and evolution of the genus Homo', Shaking the Tree: Readings from Nature in the History of Life, 371.

Wood, B. and Collard, M. (1999) 'The human genus', Science, 284, 65-71.