Do extant apes make good models for our ancestors?

There seems to be some debate in the recent literature (since the publication of Ardipithecus ramidus) about the use of non-human primates as models for hominin ancestors. The argument seems to arise from the traditional assumption of palaeoanthropologists that chimpanzees have remained "more conservative" in anatomy, behaviour and ecology than have modern humans, making them informative about the last common ancestor of the two lineages. The discoverers of Ardipithecus, in contrast, suggest that the distinctive anatomy and paleobiology of that taxon suggest that the last common ancestor (which must have lived only a short time before) could not have been troglodytian; indeed, it likely had a different diet, a novel locomotor pattern and a habitat preference very different to that of modern chimpanzees (Hanson 2009).

In the January edition of Science, there is a reply to this assertion by the Ardipithecus authors, in which several primatologists and palaeoanthropologists assert that, in fact, the end of comparative studies in the field is not nigh, and that much remains to be learned from the study of extant hominoids (Whiten et al. 2010). Some of the authors of the Ardipithecus papers then reply, to say that of course they didn't mean there was no merit in studies of extant taxa, and, in fact, strongly support that work - but not its use as the source of direct models for hominin ancestros, which must instead be studied from the perspective of "fundamental evolutionary theory" (Lovejoy et al. 2010).

I must admit to being a little confused by this exchange. I read the Ardipithecus papers, and found no suggestion that all studies of extant hominoids were necessarily redundant; similarly, I would have thought that evolutionary theory ought to have played a substantial part in palaeoanthropology even before the direct comparison of chimpanzee and last common ancestor was disputed. Jolly (2001), I think, makes a good point that valuable analogies about the evolution of key hominin features can be drawn from a wide range of primates and mammals (not limited to, but not excluding the extant hominoids). what is needed, it seems, is more awareness of how those analogies are constructed and used - researchers using analogy are not saying "we are going to assume that this extant taxon is representative of a particular ancestral taxon". That would not get past the peer review system of most journals, I suspect, as we know it is not typical of evolutionary theory. Chimpanzees are not our ancestors, but they may be more similar to them (or indeed, more different) than we are. At the same time, analogies, of the form "in a certain characteristic, X is to Y and A is to B" can be dran from any taxon - Jolly suggests Theropithecus baboons - and do not make any assumptions about either the last common ancestor (which may be extremely distant), or the rest of the characteristics of the two organisms that are being compared.

In summary, modern apes clearly can shed some light on our evolutionary history, as they are phylogenetically our closest relatives (a point to Whiten et al.) But they are not necessarily analogues of our last common ancestor in any respect, and certainly are unlikely to be so in all respects (as Lovejoy et al. and Hanson have noted). The term "analogue" implies a certain type of relationship between compared taxa, but the term "model", at least to me, does not; it is more general, and might refer either to analogue studies or to more general comparative studies. Perhaps the palaeoanthropological community simply needs to be more careful about its use of comparisons and terminology?

References

Hanson, B. 2009. Light on the Origin of Man. Science volume 326, pages 60-63.

Whiten et al. 2010. Studying Extant Species to Model our Past. Science volume 327, page 410.

Lovejoy et al. 2010. Response to Whiten et al. Science volume 327, pages 410-411.

Jolly, C.J. 2001. A Proper Study for Mankind: Analogies from the Papionin Monkeys and Their Implications for Human Evolution. Yearbook of Physical Anthropology volume 44, pages 177-204.