New Interpretations of the Genus Homo Genetics Based Interpretations

Morris Goodman and his colleagues have presented their interpretation of how hominin taxonomy should be adapted to reflect the recent molecular and genetic evidence suggesting a particularly close relationship between modern humans and chimpanzees (Goodman et al., 1998, 2001; Wildman et al., 2003). The only paleoanthropologists who have responded to these new data have been Groves (2001), Cameron and Groves (2004) (both reviewed above) and Curnoe and Thorne (2003). The conclusions of the latter study can be summarized by their statement that "the genetic proximity of our analysis of genetic distances between humans and chimpanzees has been used to suggest these species are congeneric. Our analysis of genetic distances between them is consistent with this proposal. It is time that chimpanzees, living humans and all fossil humans be classified in Homo" (Curnoe and Thorne, 2003: 201).

It is clear that taxonomic hypotheses generated by pale-oanthropologists should take cognizance of comparative molecular evidence, but it makes no sense to generate such hypotheses as if morphological evidence and functional inference have ceased to exist, or as if they had no relevance for such decisions. Molecular biologists have some excuse for being blind to such evidence; paleoanthropologists should know better. Unless we abandon any consideration of the grade of an organism when considering what genus to assign them to, then it makes no sense to include chimpanzees, bonobos and modern humans in the same genus.

Fossil Evidence-Based Interpretations

Although the last 2 decades have seen substantial heat generated by debates about how many species should be recognized among the fossil evidence for early Homo, surprisingly little attention has been paid to the definition and identification of the genus Homo.

There are two options for putting two of the principles of genus identification (i.e., monophyly and adaptive coherence) into practice. You can either start in the present, or in the past. If you start in the present, and adopt the "top down" option, you start with the type species. In the case of the genus Homo you take stock of the derived morphology and behavior of H. sapiens, decide on the cardinal features and behaviors you will use to determine the adaptive zone of H. sapiens, and then determine the characters you will use to generate phylogenetic hypotheses. Then you work backwards into the past, and apply the same two tests to each hominin taxon you encounter. Is there reliable (i.e., the same cladogram is generated even if you change details of the OTUs and the method) evidence that the taxon is in the same clade as H. sapiens? Is there reliable (i.e., reliable quantitative proxies of important behaviors) evidence that the taxon is in the same adaptive zone as H. sapiens?

If you adopt the "bottom up" approach, you have to make a subjective judgment about whereabouts in the past you should start to pick up the trail leading to Homo. You then work towards the present applying the tests set out above to the hominin taxa you encounter. The difference between this approach and the "top down" option is that the evidence is sketchier, and thus the likelihood that one can satisfy the

"reliability" criterion of the two tests, monophyly and adaptive similarity, is diminished.

Ironically, there have been very few attempts to formally assess the relationships of modern humans with respect to H. neanderthalensis and H. erectus. Eldredge and Tattersall (1975) included all three taxa in the cladogram (see Eldredge and Tattersall, 1975, Fig. 4) presented in their seminal paper that pioneered the application of cladistic methods to homi-nin relationships. However, the authors did not carry out a formal analysis of the relationships among the taxa, nor did they refer to any specific characters when considering the merits of different cladograms for expressing the relationships among the pre-modern Homo taxa within the hominin clade.

Presumably most researchers since then considered the hypothesis of monophyly of later Homo (i.e., H. sapiens, H. neanderthalensis, Homo heidelbergensis, H. erectus s.l.) to be so obvious that it did not require formal investigation. Although there are grounds for adding H. habilis sensu stricto and H. rudolfensis to the (H. sapiens, H. neandertha-lensis, H. heidelbergensis, H. erectus s.l.) clade, I think even the supporters of such an interpretation would accept that the evidence is not as strong as the evidence for including H. neanderthalensis and H. erectus s.l. within the crown group that includes modern humans.

Thus, as far as relationships are concerned there seem to be two options. You either draw the lower boundary of Homo so that it includes H. habilis sensu stricto and H. rudolfensis (Fig. 3.1, arrow A), or you draw it beneath early African H. erectus so that it excludes H. habilis sensu stricto and H. rudolfensis (Fig. 3.1, arrow B).

As far as adaptive grade is concerned, the problem is more complicated. If the criteria are restricted to what can be

Every Paraceratherium Fossil Record Ever

Fig. 3.1 A cladogram setting out one hypothesis of relationships among early hominins. The nodes A and B represent two hypotheses for the Homo clade. If Homo were to include node A, it would embrace the species presently included in early Homo (i.e., H. habilis sensu stricto and H. rudolfensis). If Homo was defined so as to exclude node A, and include just node B, then it would be confined to early African H. erectus and temporally later, more derived Homo species.

Fig. 3.1 A cladogram setting out one hypothesis of relationships among early hominins. The nodes A and B represent two hypotheses for the Homo clade. If Homo were to include node A, it would embrace the species presently included in early Homo (i.e., H. habilis sensu stricto and H. rudolfensis). If Homo was defined so as to exclude node A, and include just node B, then it would be confined to early African H. erectus and temporally later, more derived Homo species.

deduced about the adaptive grade of a taxon from its morphology, then it could be argued that if the combination of a modern human-sized brain and obligate long range bipedal-ism are the criteria, then the boundary of Homo would be set so that it includes H. heidelbergensis, but not H. erectus s.l. or H. floresiensis. If a modern human body shape and obligate bipedalism are deemed to be the criteria, then the boundary would be set so that Homo would include early African H. erectus, but not H. habilis sensu stricto and H. rudolfensis (but see Haeusler and McHenry, 2004, 2007 for an alternative interpretation). But even that solution results in a homi-nin genus that embraces a substantial range of ontogenies and life histories (Robson and Wood, 2008). If H. habilis sensu stricto and H. rudolfensis are included in Homo for relationship reasons, this poses problems for any genus definition that insists on adaptive coherence for the same genus would include taxa with a range of cranial and postcranial morphology (including very different semicircular canals) that imply different dietary and locomotor adaptations. Furthermore, the adaptive strategies of H. habilis sensu stricto and H. rudolfensis are probably closer to the adaptive strategy of the type species of the genus Australopithecus (i.e., Au. africanus) than they are to H. sapiens, the type species of Homo.

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