The History of the Genus Homo

Transcription

1 HUMAN EVOLUTION Vol N. 1-2 (3%49) B.A. Wood Department of Anthropology The George Washington University 2110 G Street, NW Washington D.C bwood@gwu.edu Key Words: Genus; Homo; Homo habilis; Homo rudolfensis; Taxonomy. The History of the Genus Homo The genus Homo was established by Carolus Linnaeus in During the course of the past 150 years, the addition of fossil species to the genus Homo has resulted in a genus that, according to the taxonomic interpretation, could span as much time as 2s Myr, and include as many as ten species. This paper reviews the fossil evidence for each of the species involved, and sets out the case for their inclusion in Homo. It suggests that while the case for the inclusion of some species in the genus (e.g. Homo erectus) is well-supported, in the case of two of the species, Homo habilis and Homo rudolfensis, the case for their inclusion is much weaker. Neither the cladistic evidence, nor evidence about adaptation suggest a particularly close relationshil~ with later Homo. Introduction When the genus Homo was introduced by Carolus Linnaeus it contained only two species, both of which were living forms (Linnaeus, 1758). One was referred to as a "nocturnal cave dweller" and given the name Homo sylvestris. The other, Homo sapiens, is the species to which modern humans belong. Since its introduction our understanding of Homo has been altered by the addition of fossil species. The incorporation of these species has nearly always involved relaxing the criteria for including taxa into the genus Homo. This review traces the history of this process of increasing inclusivity, beginning with the recognition of the Neanderthals and concluding with the latest proposals for the incorporation of the species Homo antecessor. As each species is introduced the fossil evidence that has accrued since its recognition is summarized and current taxonomic interpretations are reviewed. The year(s) given in parentheses are those in which the main fossil evidence for a species was discovered. The review concludes by surveying contemporary interpretations of the genus Homo. These range from the view that it should be a monotypic genus, with Homo sapiens as the only species, to the judgment that Homo should be interpreted as containing more, rather than fewer, species. Finally, it will consider whether the relationships and adaptations of the more 'primitive' species are such that it would be more appropriate to attribute them to a genus other than Homo. Taxonomic History Homo sapiens Linnaeus, 1758 The first indication that modern humans were ancient enough to have fossilized representatives came with the discovery, in 1868, of skeletal remains at the Cro-Magnon rock shelter at Les Eyzies de Tayac, France. A male skeleton, Cro-Magnon 1, was made the type specimen of a new species, Homo spelaeus Lapouge, 1899, but it was not long

2 40 WOOD before it was realized that it made no sense to place this material and living people in different species (Topinard, 1890; Keith, 1912). The first fossil evidence from beyond Europe of populations that cannot be distinguished from the skeletons of contemporary local human populations came in 1924, from Singa, in the Sudan (Woodward, 1938). Thereafter more evidence from Africa came from the sites of Dire-Dawa, Ethiopia (1933); Dar es-soltan, Morocco (1937-8); Border Cave, Natal, ( and 1974); Omo (Omo 1 - Kibish Formation), Ethiopia, (1967) and from Klasies River Mouth, Cape Province (1968). It is probable that none of these remains exceeds 150 Kyr, and most date from less than 100 Kyr (Brauer et al, 1997). In the Near East comparable fossil evidence has been recovered from sites such as Mugharet Es-Skhul (1931-2) and Djebel Qafzeh (1933, ). In Asia and Australasia anatomicallymodern human fossils have been recovered from sites that include Wadjak, Indonesia ( ), the Upper Cave at Zhoukoudian, China (1930), Niah Cave, Borneo (1958), Tabon, Philippines (1962) and the Willandra Lakes, Australia (1968 and thereafter). Few of these sites can be dated accurately, but new evidence suggests that anatomicallymodern H. sapiens had penetrated Australia by as early as c.70 Kyr (Thorne et al., 1999). Homo neanderthalensis King, 1864 The type specimen of Homo neanderthalensis consists of the fossil remains belonging to a single, adult, skeleton recovered from the Feldhofer Cave in the Neander Valley in Germany in With hindsight this was not the first fossil evidence of Neanderthals to come to light, for skulls of an adult and a child were found in 1829, at a site in Belgium called Engis (Schmerling, 1833), and a cranium recovered in 1848 from Forbes' Quarry in Gibraltar (Busk, 1865) also display the distinctive Neanderthal morphology. The next Neanderthal discovery in Europe was made in Moravia, at Sipka (1880). Thereafter came discoveries from Belgium at Spy (1886); Croatia, at Krapina ( ); Germany, at Ehringsdorf ( ), and in France, at Le Moustier (1908 and 1914); La Chapelleaux-Saints (1908); La Ferrassie (1909, 1910 and 1912), and at La Quina (1911), as well as in the Channel Islands, at St. Brelade (1911). It was not until the 1920s that evidence of Neanderthals was found outside of Europe at Kiik Koba in the Crimea ( ), Thereafter came discoveries at Tabun cave on Mount Carmel, in the Levant, (1929), and then in central Asia, at Teshik Tash, (1938). In the meantime two sites in Italy, Saccopastore ( ) and Guattari/Circeo (1939), had yielded the remains of Neanderthals. Further evidence was added after the war, first from Iraq (Shanidar, 1953 and ) and then from sites in Israel (Amud, 1961,1964 and thereafter; Kebara, 1964 and thereafter) and more recently from sites in France and Spain (e.g. St. Cesaire, 1979, and Zafaraya, 1983 and 1992). The latter are the geologically most recent evidence of Neanderthals, with dates suggesting that they are just less than 30 Kyr-old (Hublin et al., 1995). Thus, Neanderthal remains have been found throughout Europe, with the exception of Scandinavia and the North German Plain, as well as in the Near East, the Levant and Western Asia. At one time it was suggested that Neanderthal-like remains were also to be found in Africa and Asia. However, there proved to be no sound evidence for a geographically more dispersed 'Neanderthal Phase' (Hrdlicka, 1927). It seems, therefore, that Neanderthals were a phenomenon confined to Europe and adjacent areas. The characteristic Neanderthal morphology is seen throughout the cranial and postcranial skeletons, in the cranium it includes discrete and rounded supraorbiltal ridges, a face that projects anteriorly in the midline, laterally-projecting and rounded parietal

3 THE HISTORY OF THE GENUS HOMO 41 bones, a rounded, posteriorly-projecting, occipital bone, an additional bony crest within the mastoid process, large incisor teeth, and postcanine teeth with large root canals. The postcranial peculiarities include short limb bones with stout shafts and relatively large joint surfaces, especially well-marked areas for the attachment of a muscle that helps to control the shoulder, and features of the pubic ramus of the pelvis that are likely to be related to the way the pelvis transmits the loading which takes place during locomotion. Despite these peculiarities of the pelvis there is no indication that the Neanderthals were anything other than upright, obligate, long-range, bipeds. Their brains were as large, if not larger, than the brains of living Homo sapiens. For many years interpretations of Neanderthal physiognomy were influenced by Marcellin Boule's reconstruction of a skeleton recovered from La Chapelle-aux-Saints, in France. This is the skeleton of an aged male afflicted by osteoarthritis, and the curvature of the spine that is such a feature of the reconstruction is due to the effects of disease and is not a reliable indication of the habitus of non-pathological Neanderthal individuals. Many elements of the characteristic morphology of the Neanderthals can be seen in remains recovered from sites such as Steinheim (Germany) and Swanscombe (England) that date from c Kyr. It is also apparently evident in precursor form in the remains that have been found in the Sima de los Huesos, Atapuerca, Spain. If so, this would extend the time of origin of the Neanderthals back to c.300 Kyr. Homo heidelbergensis Schoetensack, 1908 This species was introduced for a hominin mandible found in 1907 during excavations to extract sand from a quarry at Mauer, near Heidelberg, Germany. The mandible has no chin and the body is a good deal larger than that of the mandibles of modern humans living in Europe today. The next evidence within Europe of fossil remains that showed equivalently archaic features came from Petralona (Greece), where in 1959 a cranium was recovered from a cave. Because of the lack of any sedimentary context its age of c Kyr can only be approximate. A similar date is likely for comparable evidence from Arago (France, ), whereas the more fragmentary, but similarly morphologically archaic, material from Montmaurin (France, 1949), V6rtessz6116s (Hungary, 1965) and Bilzingsleben (Germany, , 1983 and thereafter) are apparently more recent (c.250 Kyr). The first African evidence for 'archaic' Homo sapiens came in 1921, with the recovery of a c Kyr cranium from a cave at the Broken Hill Mine at Kabwe, in what is now Zambia. Other morphologically-comparable remains have been found from the same time period in East Africa at Eyasi, Tanzania (1935). The earliest geological evidence of this African 'archaic' group comes from Ethiopia, at Bodo (1976, 1981 and 1990), which is dated at c.600 Kyr (Clark et al., 1994). Specimens intermediate in age (c.400 Kyr) include crania from southern Africa at Hopefield/Elandsfontein (1953), East Africa at Ndutu, Tanzania (1973), and North Africa, in Morocco at Sale (1971) and Thomas Quarry (1969/72). The Asian evidence for an 'archaic' form of Homo that does not qualify for inclusion in Homo erectus, comes from China at Dali (1978), Mapa, (1958), and India at Hathnora (1982). These fossils apparently range in age from c Kyr. What sets this material apart from H. sapiens is the morphology of the cranium and the robusticity of the postcranial skeleton. The brain cases are often, but not always, smaller than those of modern humans, but they are always more robustly built, with large ridges above the orbits and a thickened occipital region. Postcranially the shapes of the limb bones are much like those of Homo sapiens except that the shafts of the long bones are generally thicker, with higher robusticity indices.

4 42 WOOD For many years it was conventional to label this material as 'archaic' Homo sapiens, but there is now compelling evidence that this group of specimens, in terms of its overall cranial, dental and postcranial morphology, is distinct from that of Homo sapiens. Thus, it would be reasonable to place it in a separate species. If there is to be a single species name to cover all the archaic material, then the species name with priority is Homo heidelbergensis Schoetensack, However, if there was evidence that the main geographic regions, Europe, Africa and Asia, each sampled 'good' species, then the name for the African species would be Homo rhodesiensis Woodward, Similarly, if the Ngandong material is not to be included in Homo erectus (see below) the appropriate species name for an Asian 'archaic' Homo species would be Homo soloensis Oppenoorth, Some workers have even suggested that within the African evidence there is room for a species, Homo hehnei, between the more archaic remains within H. heidelbergensis, or H. rhodesiensis, and anatomically-modern Homo sapiens (Foley and Lahr, 1992; Lahr and Foley, 1994). Specimens that would be candidates for inclusion in H. helmei include Florisbad, Orange Free State (1932); Rabat, Morocco (1933); Cave of Hearths, Transvaal (1947); Jebel lrhoud, Morocco, (1961 and 1963); Prop 2 (Kibish Formation), Ethiopia, (1967); Laetoli 18, Tanzania, (1976); Eliye Springs (KNM-ES 11693), Kenya, (1985) and lleret (KNM-ER 999 and 3884), Kenya, (1971 and 1976 respectively) (Brauer et al., 1997). There is undoubtedly a gradation in morphology that makes it difficult to set a boundary between 'archaic' Homo sapiens and Homo heidelbergensis, but it is equally clear that unless a boundary is set then morphological variation within Homo sapiens sensu lato is then so great that it strains credulity (Brauer, 1992). Homo erectus (Dubois, 1892) Mayr, 1944 In 1890 Eugene Dubois transferred his search for human fossils from Sumatra to Java, and in the same year found a mandible fragment at a site called Kedung Brubus. Less than a year later, in 1891, his workers unearthed the skull cap that was to become the type specimen of a new, and significantly more primitive, species of fossil homin]n. In the initial publication about the Trinil finds, in 1892, Dubois placed the skull cap in the genus Anthropopithecus, but two years later he changed the species designation to Pithecanthropus. What made the discovery of the Trinil braincase so significant was its small cranial capacity, c.850 cm ~, its low brain case and quite sharply-angulated occipital region. It was these features that led Dubois to think initially that the Trinil evidence may have belonged to an ape. The search for hominin remains in Java moved to Ngandong ( ), upstream of Trinil, where the Solo River cuts through the Plio-Pleistocene sediments of the Sangiran Dome. It was also in this region, in 1936, that Ralph yon Koenigswald, a German palaeontologist, discovered a cranium that resembled the Trinil skull cap, but which had a substantially smaller, c.750 cm 3, brain size than the Trinil calotte. Research in this area continued until 1941, restarted in 1951, and has been taking place at varying levels of intensity ever since. Excavations at a cave at Choukoutien, now called Zhoukoudian, near Beijing, in China, in 1921 and 1923, apparently recovered only quartz artefacts and non-hominin fossils. However, in 1926, Otto Zdansky realised that two teeth, an upper molar and a lower premolar, previously identified as ape-like were actually hominin. A year later the two teeth, together with a well-preserved left permanent first lower molar tooth (Ckn. A. 1.1) found in 1927, were assigned to Sinanthropus pekinensis (Black, 1927). Excavations at Choukoutien were resumed by Black, Weng Wanhao and Anders Bohlin in 1927, the

5 THE HISTORY OF THE GENUS HOMO 43 first skull was found in 1929, and work continued there until its interruption by WW2. Similar-looking material has been found at other sites in China (e.g. Lantian, ; Hexian, 1980), and in southern Africa at Swartkrans, Gauteng (1949 and thereafter), East Africa (Olduvai [1960 and thereafter]; East Turkana [1970 and thereafter]; West Turkana [1975 and thereafter], and at Melka Kunture [1973 and thereafter]), and in North Africa at Tighenif (1954-5). Despite the relatively large numbers of crania that had been recovered from Java, China and elsewhere, relatively little was known about the postcranial morphology of what was to become known as Homo erectus, and it was discoveries from East African sites that provided the crucial evidence. This came in the form of a pelvis and femur from Olduvai Gorge (OH 28), two fragmentary partial skeletons from East Turkana (e.g. KNM- ER 803 and 1800), and an especially rich source of evidence was the unusually wellpreserved skeleton from West Turkana (KNM-WT 15000). There are morphological differences between this material and Homo sapiens, but all the postcranial elements are consistent with a posture that is habitually upright, and with a long-range bipedal mode of locomotion. The crania of Homo erectus all have low vaults, with the greatest width being towards the base of the cranium. There is a substantial, essentially continuous, torus above the orbits, posterior to which there is usually a well-marked sulcus. There is both a sagittal torus, and an angular torus that runs towards the mastoid process. The occipital region is sharply-angulated, with a well-marked supratoral sulcus. The inner and outer tables of the cranial vault are thickened. 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 robust than that of modern humans and it lacks a well-marked chin. The tooth crowns are generally larger than those of modern humans, the third molar is usually smaller, or the same size, as the second. The roots of the premolar teeth tend to be more complicated than is usually the case in modern humans. The outer cortical bone of the postcranial skeleton is generally thick, and the limb bones have more robust shafts than is the case for modern humans. The shafts of the femur and the tibia are relatively flattened from front to back and from side to side, respectively, relative to those of modern humans; this is referred to as platymeria and platycnemia, respectively. The two main regional subsets of this material were originally attributed to at least three genera. Two, Pithecanthropus and Meganthropus, were used for the Javanese evidence, one, Sinanthropus, for the Chinese component, and a fourth genus, Atlanthropus, for the evidence from North Africa. In t943 Franz Weidenreich formally sank Sinanthropus into Pithecanthropus, and in 1944 Mayr proposed that the latter be sunk into Homo. Mayr went on to complete the process of taxonomic rationalization in 1950 by proposing that Atlanthropus be sunk into Homo. This, of course, had the effect of modifying the definition of Homo so that it could accommodate the relatively primitive remains that make up the new, combined, hypodigm of Homo erectus. In particular this meant that Homo now included a much wider range of cranial shape, brain size, and tooth and mandible shape and size than had been the case when the only fossil taxon included within the genus was Homo neanderthalensis.

6 44 WOOD Homo habilis Leakey, Tobias and Napier, 1964 A year after the discovery of a sub-adult fossil hominin cranium, OH5, the hololype of Zinjanthropus boisei, from Bed i at Olduvai Gorge, the Leakeys found evidence of an apparently more advanced hominin in the form of substantial parts of both parietal bones, much of a mandible and at least 13 hand bones of a juvenile skeleton (OH 7). The parietal bones of OH 7 did not show evidence of the bony crests that are such a distinctive feature of Zinjanthropus boisei, and the molar and premolar teeth were much smaller. In the next year, or so, further evidence of a 'non-robust' hominin was unearthed in Bed I (OH 4 and 6 - skull fragments and teeth; OH 8 - an adult foot; OH 14 - juvenile cranial fragments, and OH 16 - the fragmented cranial vault and maxillary dentition of a young adult) and Bed II (OH 13 - the incomplete skull of an adolescent) of Olduvai Gorge. In 1964 Louis Leakey, and his colleagues Phillip Tobias and John Napier, set out the case for recognizing a new species for these 'gracile' hominin remains from Olduvai, and they proposed that the new species be attributed to the genus Homo, as Homo habilis Leakey, Tobias and Napier, This decision meant that Le Gros Clark's 1955 diagnosis of the genus Homo needed to be amended. This involved relaxing some criteria, such as a brain size of 750 cm 3 so that the relatively small-brained (c cm 3) crania from Olduvai would qualify. Leakey and his colleagues claimed that other criteria, such as dexterity, an upright posture and a bipedal gait did not need to be changed because Leakey et al.'s interpretation of the functional capabilities of Homo habilis suggested that the Olduvai fossils complied with these expectations. The proposal to incorporate the new species within Homo did not go unchallenged. Some critics suggested that the new material was not sufficiently different from Australopithecus africanus to justify specific distinction. Other workers accepted the case for a new species, but challenged the view that it should be included within Homo. A third group took the view that specimens such as OH 13 were 'advanced' enough to be referred to Homo erectus, and claimed that Homo habilis was a mixture of A. africanuslike material from Bed I, and Homo erectus-like remains from Bed l[. In due course more specimens from Olduvai were added to the hypodigm, the most significant being the cranium OH 24 and the fragmentary associated skeleton, OH 62. In their different ways, these specimens have helped to clarify the nature of Homo habilis. The discovery of OH 24 was important because it resembled OH 13, but was found not in Bed Ii, but near the base of Bed I, making it the oldest of the specimens from Olduvai Gorge allocated to H. habilis. This meant that it was no longer possible to argue that there was a temporal cline in the morphology of the H. habilis remains, from the more 'primitive' specimens at the base of Bed I, to the morphologically 'more advanced' fossils in Bed 11. The implications of the OH 62 associated skeleton were rather different. It is very fragmentary, but its estimated limb proportions are said to be more primitive than those of any other Homo species (Hartwig-Scherer and Martin, 1991). If OH 62 belongs to H. habilis, then it is clear that researchers can no longer be confident that H. habilis was both habitually upright and an obligate biped. The most informative component of the tt. habilis hypodigm comes from the site of Koobi Fora, initially known as East Rudolf and now also referred to as East Turkana. These remains include a skull, KNM-ER 1805, two well-preserved crania, KNM-ER 1470 and 1813, mandibles e.g. KNM-ER 1802 and isolated teeth, all found in 1972, or soon thereafter. Initially, many of these specimens were referred to as 'early Homo'. Some of the hominin fossils recovered from Members G and H of the Shungura Formation have been assigned to H, habilis, including a fragmented cranium, two mandibles and isolated teeth.

7 THE HISTORY OF THE GENUS HOMO 45 A fragmentary cranium and some isolated teeth from Member 5 at Sterkfontein are also said to resemble H. habilis, and the same proposal has been made with respect to the socalled 'composite' cranium, SK 847, from Member 1 at Swartkrans. Suggestions that H. habilis remains have been recovered from sites beyond Africa have not received wide acceptance. Among remains that have been suggested as belonging to H. habilis are the hominin fragments from Ubeidiyah, Israel, and the material allocated to Meganthropus palaeojavanicus from Indonesia. in neither case have the researchers involved been able to make a convincing case that the remains should be allocated to H. habilis. The material allocated to what some refer to as 'early Homo', and others as Homo habilis sensu lato, has a relatively variable cranial morphology (see below). The endocranial volume ranges from just less than 500 cm 3 to C.800 cm 3, and all the crania in this group are wider at the base than across the vault. The facial morphology varies (see below), with KNM-ER 1470 having its greatest width across the mid-face and little nasal projection, compared with KNM-ER 1813 which is broadest across the upper face. The mandibles vary in size and robusticity, with those from the larger individuals having robust bodies and premolar teeth with complex crowns and roots. Our knowledge of the postcranial skeleton of H. habilis sensu lato has traditionally come from the remains from Bed I at Olduvai Gorge, but although these were allocated to Homo habilis they may equally well belong to Paranthropus boisei. The only postcranial evidence from Olduvai Gorge that can, with confidence, be allocated to Homo habilis is the associated skeleton OH 62. Very little of the morphology of this specimen is preserved, but it is possible to determine that the skeleton had longer arms, relative to leg length, than any other Homo species. Thus, if OH 62 does belong to Homo habilis, then it would mean that the postcranial skeleton of at least one species of Homo could not be distinguished from that of Australopithecus and Paranthropus. From the outset researchers have questioned the integrity of H. habilis. Initially, the main criticism was that within the linear, anagenetic model of evolution prevailing at the time of its discovery there was insufficient 'morphological space' for another taxon between Australopithecus africanus and Homo erectus. The criticism that Homo habilis was an amalgam of geologically older 'advanced' Australopithecus africanus fossils, and geologically younger 'primitive' Homo erectus remains has been countered by the demonstration that at Olduvai Gorge one of the most morphologically 'advanced' specimens, OH 24, was also geologically the oldest. Researchers have also shown that the characteristics of Homo habilis are not simply an admixture of the characteristics of Australopithecus africanus and Homo erectus, but are a distinctive combination of morphological features (Wood, 1992). The third objection to Homo habilis was that it was too variable to make a plausible species. Views on this are polarized, with some researchers supporting the retention of a single taxon, Homo habilis sensu lato, for this material, and others supporting a 'twotaxon' solution. This debate will be explored in more detail in the section on Homo rudolfensis. Homo ergaster Groves and Maz~k, 1975 This taxon was introduced as part of a taxonomic review of the material from Koobi Fora allocated to 'early Homo'. The type specimen, KNM-ER 992, is an adult mandible that some workers had referred to Homo erectus, and the paratypes include the skull KNM-ER The only detailed analysis of the latter has concluded that KNM-ER 1805 should be referred to Homo habilis sensu stricto. Thus, decisions about whether

8 46 WOOD Homo ergaster is a 'good' taxon hinge on whether researchers can demonstrate that the material referred to it can be distinguished from Homo erectus. The features that have been claimed to distinguish Homo ergaster from Homo erectus sensu stricto fall into two groups. The first group consists of the ways in which Homo ergaster is more primitive than Homo erectus sensu stricto. The best evidence comes from the mandibular dentition, and in particular the mandibular premolars. It is claimed that the crowns and the roots of these teeth in Homo ergaster are more like those of the hypothetical common ancestor of the hominins than are those of Homo erectus sensu stricto. The second group consists of the ways that Homo ergaster is less specialised, or derived, in its cranial vault and cranial base morphology, than is Homo erectus sensu stricto. For example, Homo ergaster lacks some of the more derived features of Homo erectus sensu stricto cranial morphology, such as prominent sagittal and angular tori. Although some researchers support a taxonomic distinction between 'early African H. erectus', or 1t. ergaster, and H.erectus sensu stricto, (e.g. Wood, 1992, 1994; Bermudez de Castro et al., 1997), others regard the morphological differences as warranting at best subspecific recognition (e.g. Turner and Chamberlain, 1989; Brauer and Mbua, 1992; Kramer, 1993; Brauer, 1994). Homo rudoifensis (Alexeev, 1986; Groves, 1989) As part of a comprehensive, although idiosyncratic, presentation of the evidence for human evolution, published in English in 1986, Valery Alexeev suggested that there were sufficient differences between the cranium KNM-ER 1470 and the material allocated to Homo habilis to justify referring the former to a new species, Pithecanthropus rudolfensis Alexeev, The proposal to include the new species in a genus that others had long ago sunk into Homo was an unusual one, but eccentricity is not sufficient reason to disregard Alexeev's proposal to establish a new species. Some workers have claimed that Alexeev either violated, or ignored, the rules laid down within The International Code of Zoological Nomenclature (Kennedy, 1999). However, there are no grounds for concluding that Alexeev's proposal did not comply with the rules of the Code, even if he did not follow all of its recommendations (Wood, 1999). Thus, Alexeev's species is available and if it is transferred to Homo it should be referred to as Homo rudolfensis (Alexeev, 1986), although there is evidence that Alexeev erected the taxon in a publication (Leney and Wood, in preparation). Thus, if Homo habilis sensu lato is judged to subsume more variability than is consistent with a single species, and if KNM-ER 1470 belongs to a different species group than the type specimen of Homo habilis sensu stricto, then Homo rudolfensis (Alexeev, 1986) would be available as the name of a second early Homo taxon. This scenario appears to have some support, for several studies have shown that the degree of variation within Homo habilis sensu lato is greater than that which is expected in a single species. Researchers have recommended that the material be split into two species, Homo habilis sensu stricto, whose hypodigm consists of all the material attributed to the original taxon from Olduvai Gorge, together with a subset of the material attributed to Homo habilis sensu lato from Koobi Fora. That subset does not include KNM-ER 1470, thus making that specimen available as the type specimen of the second species, Homo rudolfensis (Alexeev, 1986) (Groves, 1989). Homo rudolfensis and Homo habilis sensu stricto have different mixtures of primitive and derived, or specialised, features (Wood, 1992). For example, whereas the absolute size of the brain case is greater in Homo rudolfensis, the latter has a face that is widest in

9 THE HISTORY OF THE GENUS HOMO 47 its mid-part, compared to Homo habilis sensu stricto, that has its greatest facial width superiorly. The primitive face of Homo rudolfensis is linked with a mandible that is robust, and with postcanine teeth that have larger crowns and more complex root systems than those of Homo habilis sensu stricto. Although we have a little information about the postcranial skeleton of Homo habilis sensu stricto from the associated skeleton, OH 62, there are no postcranial remains that can be reliably linked with Homo rudolfensis. Homo antecessor Bermudez de Castro et al., 1997 The cave complex that provided the fossils from the Simo de los Huesos (see above) has also yielded remains from level 6 of the Gran Dolina (TD) site; this material is most likely at least 500 Kyr-old. The authors of the report claim that the material shows a combination of morphology not seen in any other hominin species. They contrast the remarkably modern human-like morphology of the face, with the relatively primitive crowns and roots of the teeth. The authors consider that because Homo heidelbergensis shares some derived traits with Homo neanderthalensis, and because these derived features are not seen in the Gran Dolina material, then there are good grounds for not allocating the TD collection to Homo heidelbergensis. It is the apparent lack of these derived features, combined with differences from Homo ergaster, that compelled the authors to propose that the Gran Dolina fossils should be assigned to a new species, Homo antecessor Bermudez de Castro et al., Controversies There are two major current debates about the genus Homo. The first is not concerned with the boundaries of the taxon, but with the number of species within it. One side of the debate takes the view that there was a major adaptive and morphological shift between Ardipithecus, Australopithecus, Paranthropus, on the one hand, and Homo on the other. They claim that once Homo had evolved then it is difficult, if not impossible, to identify species boundaries within Homo. The suggested solution is that there should be only one species within Homo, namely the type species, Homo sapiens Linnaeus, 1758 (e.g. Mayr, 1950; Jel-fnek, 1978; Wolpoff et al., 1994). According to the rules of zoological nomenclature all the other Homo species referred to above would be junior synonyms of Homo sapiens. The problem with this solution is that the species they propose, which we can call Homo sapiens sensu lato, would be so variable that it would not be equivalent to any other higher primate species, either fossil or living. The second controversy is about the boundaries of Homo, and concerns the trend to relax the criteria for including species within the genus, combined with the discovery that some of the species included within Homo may not have the functional capabilities that have been attributed to them. The 'problem' species are Homo habilis sensu stricto and Homo rudolfensis. In the case of the former, the original fossils attributed to Homo habilis sensu stricto were believed to provide evidence that it was an animal which was habitually upright and bipedal, and that it was capable of the type of dexterity necessary to make the Oldowan stone tools. Unfortunately, since these assumptions were made nearly all of the functional studies of the relevant fossil evidence from Olduvai Bed 1 have tended to emphasize that these fossils are best interpreted as belonging to an animal that was not a committed biped, but one in which bipedalism was combined with the ability to climb. Likewise, there is nothing about the hand morphology of Homo habilis sensu stricto that distinguishes it functionally from hand fossils attributed to

10 48 WOOD Australopithecus and Paranthropus. There are no anatomical reasons to identify it as the only possible maker of the Olduvai Bed! stone tools, and in any case stone artefacts now antedate fossil evidence of Homo habilis sensu stricto (Semaw et al., 1997). Thus, Homo habilis sensu su'icto no longer matches the functional criteria for inclusion into Homo that were suggested by Leakey, Tobias and Napier (1964). As for Homo rudolfensis, although no postcranial remains can be linked with the taxon, there is no good evidence that it shows any significant advance over Australopithecus and Paranthropus in terms of its dietary adaptations. Thus, with hindsight, it seems that Homo habilis sensu stricto and Homo rudolfensis are insufficiently advanced in terms of their adaptations to justify their inclusion in Homo. If this is the case, then the boundaries of Homo should be adjusted so that Homo ergaster is included, but Homo habilis sensu stricto and Homo rudolfensis are excluded, and either transferred to Australopithecus (Wood and Collard, 1999a), or placed in a new genus (Wood and Collard, 1999b). ACKNOWLEDGEMENTS - Research incorporated in this review was supported by The Leverhulme Trust, The Wellcome Trust and The Henry Lace Foundation. BW is presently supported by The Henry Luce Foundation. REFERENCES Alexeev, V.P The origin of the haman race. Moscow: Progress Publishers. Bermudez de Castro, J.M., Arsuaga, J.L., Carbonell, E., Rosas, A., Martinez, I,, and Mosqueria, M A hominid from the Lower Pleistocene of Atapuerea, Spain: Possible Ancestor to Neandertals and Modern Htunans. Science, 276: Black, D. t927. OIl a lower molar hominid tooth from the Chou Kou Tien deposit. Palaeontologicia Sinica, New Series D, 7:1-28. Brauer, G Africa's place in the evolution of Homo sapiens. In: Continuity and Replacement: controversies in Homo sapiens evolution (eds. Brauer, G. and Smith, F.H.), pp Balkema: Rotterdam. Brauer, G How different are Asian and African Homo erectus? In: 100 years of Pithecanthropus. The lfomo erectus problem (ed. J.L. Franzen), Courier. Forschungs-lnstitut Senckenberg, 171: Brauer, G. and Mbua, E Homo erectus features used hs cladistics and their variability in Asian and African Homhffds. Journal of Human Evolution, 22: Brauer, G., Yokoyama, Y., Falgueres, C. and Mbua, E Modern human origins backdated. Nature, 386: Busk, G On a very ancient cranium front Gibraltar. Report of the British Association Advancement of Science, (Bath, 1864): Clark, J.D., de Heinzelin, J., Schick, K.D., Hart, W.K., White, T.D., Wolde Gabriel, G., Walter, R.C., Suwa, G., Asfaw, B., Vrba, E. and H. Selassie, Y African Homo erectus: old radiometric ages and young Oldowan assemblages in the Middle Awash Valley, Ethiopia. Science, 264: Dubois, E Palaeontogische anderzoekingen op Java, in Verslag van het Mijnwezen. Batavia, 3: Foley, R.A. and Lahr, M.M Beyond "out of Africa": reassessing the origins of Homo sapiens. Journal of Human Evolution, 22: Groves, C.P. and Maz~ik, V An approach to the taxonotny of the Hontinidae." Gracile Villafrancbian hominids of Africa. Casopis Pro Mineralogii A Geologii, 20: Hartwig-Schrerer, S., and Martin, R.D Was 'Lucy' more haman than her child? Observations on early hominid postcranial spechnens. Journal of Human Evolution, 21: Hrdlicka, A The Neanderthal phase of man. Journal of the Royal Anthropological Institute, 57:

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