Knmer

Figure 1.5 ► A model of the Multiregional hypothesis, with long-existing human regional lineages shown to be established within parts of the Old World by 1.8 million years ago. From Groves (1994), p. 30.

Wolpoff et al. (1984) argued that in both Europe and Australia, peripheral groups absorbed genetic material from the main population centers of Asia and Africa. In Australia, they maintain, gene flow was mainly from the southern and eastern parts of East Asia, while Europe is thought to have been influenced more by the major centers of Africa and western Asia. Therefore, some regional continuity in fossil anatomy can be shown, especially in the peripheral regions, and anatomy is still linked to the ongoing evolution of our species by gene flow between the centers and the peripheral regions. For example, a continuation of anatomical form, they suggest, can be seen between the H. erectus populations of Java and the Pleistocene Australians. Both groups are said to have a large supraorbital torus, a flat frontal bone, a developed occipital torus, and facial prognathism. The Pleistocene Homo pekinensis populations of China are linked, they argue, to the modern populations of northeast Asia and the Americas by possession of large and shovel-shaped incisors (incisor cutting edge is curved, not straight, at the lateral margins) and other features (Wolpoff et al., 1984). Conversely, the peripheral European populations, allocated here to H. heidelbergensis, are said to maintain certain Neanderthal features, including strong mid-facial prognathism and a backward projection (bunning) of the occipital. Only Africa is said to lack any evidence for regional continuity features. Of course, multiregionalists do not recognize Homo erectus, Homo pekinensis, and Homo heidelbergensis as different species. For multiregionalists, they are all archaic versions of Homo sapiens.

Many of these "unique" features, however, appear to be no more than primitive retentions, passed on from a common ancestor, for they can be identified in numerous human populations, not just in the regions where they are claimed (rightly or wrongly) to predominate. Some of these regional "transitional" fossils are characterized by a mixture of primitive and derived features, which say little about their evolutionary past (Groves, 1989a). Other features, which may be considered regionally distinct (Neanderthal populations with large nasal cavities and sinuses), are as likely to be related to environmental conditions (part of an exaptation that enables greater warming of the freezing "Ice Age" air before it reaches the brain [see Chapter 8; also partly Coon, 1962]) as they are to regional continuity based on close evolutionary relationships. Indeed, modern Africans, who are said to lack a list of regionally unique features, have a number of derived features commonly observed in all modern human populations throughout the world, including a high forehead positioned directly above a vertical face, a chin, a rounded occipital, and a short, flexed braincase

(D.E. Lieberman, 1995). This would tend to support the idea that modern humans really did originate in Africa.

Recent studies and interpretations of fossil H. sapiens and Neanderthal mtDNA suggest to some multiregionalists that interpretations based on living human mtDNA may be oversimplifying the picture of modern human origins. It is suggested that mtDNA from a Willandra Lakes Australian fossil skeleton (Mungo 3), dating from between 40,000 and 60,000 years ago, is moderately different from mtDNA observed in living modern humans (Adcock et al., 2001). No one denies that Mungo 3 represents a modern human, so the difference in mtDNA must be the result of the "extinction" of a modern mtDNA lineage from a prehistoric modern human population. This, the describers suggest, creates a problem for previous molecular interpretations of modern human origins. For example, when examining living human mtDNA, the deepest branch is African, but when examining fossil human mtDNA (Mungo 3), the deepest branch is Australian. This does not mean that modern humans originated in Australia any more than extant mtDNA means they originated in Africa. Indeed, the difference observed between Neanderthal and modern human mtDNA (which is even more distinct) does not necessarily mean that the Neanderthals did not play a direct role in our own evolution. Rather, the absence of the modern mtDNA type, as in the case of Mungo 3, is the result of their long prehistory, and as such it has become extinct through the vagaries of time. We would argue, however, that this study when interpreted correctly actually supports the "Out of Africa" hypothesis. For example, the Mungo specimen is shown by this same analysis to be closer in its mtDNA to the modern human range than to the Neanderthal samples, which are later in time, thus confirming the distinctiveness of the Neanderthals not only from living humans, but also from earlier fossil modern human populations. This interpretation supports the "Out of Africa" model and reflects the distinctiveness of all modern humans (in time and space) compared to our near contemporaries, the Neanderthals.

Recently, Curnoe and Thorne (2003) have provided a revision of the Multiregional hypothesis based on their interpretation of extant ranges of genetic distance. They propose that the human lineage consists of one genus, Homo, spanning a period of around 6 million years. In addition to this, they suggest that only four or five species of Homo have ever existed over this long temporal span, with the last species, H. sapiens, having emerged around 2 million years ago. This is a revised version of the "Single Species Hypothesis," originally rejected in the 1970s when the fossil evidence made it clear to all that a number of different species had to be recognized, given the great degree of variability observed with the available hominid fossil samples. The continued recovery of more fossil specimens over the last 25 years or so has provided even greater evidence that a number of hominid species were contemporary in time and space. The new version of the single species hypothesis, however, ignores the fossil evidence and is based on an abstract interpretation of the available molecular data. Those of us who work with the fossil record tend to recognize that the current situation, in which there is only one species of Homo, is unique in the history of our own lineage. Cladogenesis is recognized by almost all as the mode defining evolution, and multiple species of hominids are to be expected. This is best summed up perhaps by Arsuaga (2002:36), who states that

In reality, a species' complete disappearance from the world does not necessarily have to coincide with the appearance of its descendant species in any given place. This would be a theoretical prerequisite only if one species evolved into another species throughout its entire geographical range, in a process that affected each and every one of its separate populations. In most cases though, a descendant species evolves in a specific geographical location and from a specific population of its ancestral species. Thus the two may coexist over long periods of time within different geographical ranges. ... In fact, if a descendant species extends its range to other areas still inhabited by its ancestral species, the mother and daughter species could even coexist within one geographical range. Eventually, if the two species occupy the same ecological niche, they compete with each other and the ancestral species could finally disappear.

This is demonstrated not only in the Pliocene and early Pleistocene hominid fossil record, but also in the later Pleistocene hominins. It was only around 40,000 years ago that at least three species of Homo existed, H. neanderthalensis (the famous Neanderthal people), who occupied parts of Eurasia, H. erectus in Indonesia (if we can believe the recent dates for this species), and modern H. sapiens, who had by then occupied most parts of Africa, Europe, and Asia (including Australia). Following the emergence of modern humans in Africa between 250,000 and 150,000 years ago, and their later dispersal into Europe and Asia, the more archaic human populations became extinct, not through a form of genocide, but as a result of losing in a competition for finite resources to the sapiens.

Curnoe and Thorne (2003), however, recognize only four species within Homo that tend to be time successive. The four species that they acknowledge are, starting from the earliest, Homo ramidus, H. africanus, H. habilis, and finally, modern humans, H. sapiens. They suggest that the chimpanzee should be considered a species of Homo.

They dismiss the idea that the pygmy chimpanzee is a distinct species, Pan paniscus, and recognize only one species, Homo troglodytes. Their evidence for lumping these two species together is that hybrids have been born in captivity. This is a misunderstanding of what "species" are: Ernst Mayr, the biologist who first fully articulated the so-called Biological Species Concept, was very clear that two putative species should be repro-ductively isolated in nature, and it does not matter what happens in captivity. In fact, as Common and Pygmy Chimpanzees do not overlap in the wild, there is no way of deciding whether or not they rank as distinct species under the Biological Species Concept, so primate specialists have turned to the Phylogenetic Species Concept, under which two putative species differ absolutely (no individual can ever be mistaken for the "wrong" species).

Curnoe and Thorne argue that, given that the DNA differs by 1% between Homo troglodytes and Homo sapiens, and there is a minimum genetic difference that can support a species distinction (0.25%, in their estimation), only 4 or 5 species can be supported in the human fossil record (see also Eckhardt, 2000). This is paleoanthropology by short division. Their tacit assumption seems to be that hominin evolution is based on anagenesis: Most or all fossil hominin species are directly ancestral to H. sapiens, with no contemporary speciation events and no extinctions. This would be truly remarkable for any mammal group over a 5 to 6 million year period. Second, we must accept a concept of "generic ranges of variability," a concept that no other biologist would seriously entertain.

In any case, this preoccupation with genetic variation betrays the fundamental flaw in the revised Multiregional hypothesis — they confuse species and genera. A species is a real biological unit, while a genus is merely a system of biological classification — it is not a blown-up species. There is no automatic relationship between a species and genus, except that a genus will normally consist of a number of species, but this number is not fixed or constrained by genetic variation. Mayr defined living species by their propensity to interbreed in the wild and produce offspring that can themselves reproduce; in paleontology, we cannot possibly determine whether fossil A could interbreed with fossil B, let alone produce offspring that can themselves reproduce. It is because we can rarely make this determination, even in the case of living animals, that many, perhaps most, taxonomists nowadays reject the "interbreeding" criterion altogether, and instead use the Phylogenetic Species Concept. Usually, paleontologists measure degrees of anatomical variability in living species (especially the ones that are thought to be closely related to their chosen fossil group) in order to determine whether a fossil sample can reasonably be considered to fall within or outside an acceptable range of anatomical variability. This is nothing new and has been endorsed by practicing paleontologists for the last 100 years or so.

A genus, however, is not directly related to any species concept — that is, it does not presume to define populations within given genetic bounds, or whether members can successfully reproduce together; rather, genus is part of a human-made system of classification. While the concept of the genus has biological implications, it is a category, not a real biological entity like that of a species. This important and crucial distinction appears to lie at the heart of the Curnoe/Thorne confusion: They believe that a genus has a finite number of species that it can contain; i.e., over a 6 million year period, a maximum of only four or five species can exist. This is incorrect: A genus can potentially contain 1,5,20, 30, or 50 species. In living Old World monkeys, for example, there are at least 18 well recognized species within the genus Macaca, and at least 19 in the genus Cercopithecus; these numbers do not include the fossil species of these genera (see Fleagle, 1999; Groves, 2001, has 19 species in Macaca and 24 in Cercopithecus, both probably underestimates). There are certainly rules in the formulation and recognition of genera, though they have nothing to do with concepts of anatomical or genetic variability. Genera are groups of organisms recognized as sharing an immediate common ancestor, partly defined by all species sharing a number of unique anatomical features, usually associated with specific derived adaptations which help define the group. Thus, a genus is defined by evolutionary relationships between species and is totally unrelated to concepts of fixed degrees of anatomical or molecular variability (see Figure 1.6). Attempts to objectify the concept relate to giving it a standard time depth, and have nothing to do with the number of species allowed per genus; thus, Groves (2001) urged that a genus should have separated from its sister genera by the Miocene-Pliocene boundary, and this, if adopted, would indeed make it thinkable to unite humans and chimpanzees in the same genus. Ironically, therefore, Curnoe and Thorne (2003) might be doing the right things, but for totally the wrong reason!

Even ignoring this basic flaw in their model, if Curnoe and Thorne (2003) wish to propose such a fundamental revision of the human family

-Dryopithecus

Graecopithecus — Gorilla r Pan

|— Sahelanthropus i- K. platyops L K. rudolfensis ■ H. habilis H. ergaster H. sapiens

A. africanus P. walkeri P. boisei P. robustus

L A. garhi L Praeanthropus L A. anamensis Ardipithecus

Figure 1.6 ► Unlike a species, which represents real biological entities, a genus is a unit of classification and is not defined by variability but by evolutionary relationships. It can be seen that the species within Paranthropus, Homo, and Kenyanthropus are each defined by a common ancestor, to the exclusion of all other taxa (monophyletic group). They are also defined by a number of unique adaptations. For example, Paranthropus species are defined by a large robust face, neuro-orbital disjunction (brain set back from the face with a low frontal), with large, grinding, stonelike molars, while Homo species have a large brain, marked neuro-orbital convergence (brain set above the face — high frontal), small face and dental complex, and a more efficient mode of bipedal locomotion. It can also be seen that the species usually allocated to Australopithecus (e.g., A. africanus, A. garhi, A. anamensis) do not share a common ancestor. Thus they are paraphyletic, and each can be considered as representing a distinct genus, with only the type species africanus representing a species of Australopithecus. A genus is defined by phylogenetic relationships and not by concepts of anatomical and/or genetic variability.

tree, then the onus is on them to provide anatomical definitions of their species, and this is conspicuously absent. So far, they have been working in a fossil-free zone. Species descriptions are crucial because any paleoan-thropologist who finds a new fossil needs to be able to allocate his or her

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