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Unformatted text preview: SYNTHESIS The Macroevolution of our Ancient Lineage: What We Know (or Think We Know) about Early Hominin Diversity Rebecca Rogers Ackermann Richard J. Smith Received: 17 January 2007 / Accepted: 20 March 2007 / Published online: 17 July 2007 Springer Science+Business Media, LLC 2007 Abstract Quantitative, evolutionary models that incor- porate within- and between-species variation are critical for interpreting the fossil record of human diversity, and for making taxonomic distinctions. However, small sample sizes, sexual dimorphism, temporal trends, geographic variation, and the limited number of relevant extant models have always made the consideration of variation difficult for paleoanthropologists. Here we provide a brief overview of current early hominin diversity. We then argue that for many species our limited understanding of within species variation hampers our ability to make taxonomic decisions with any level of statistical certainty. Perhaps more sig- nificantly, the underlying causes of between-species vari- ation among early hominins are poorly studied. There have been few attempts to correlate aspects of the phenotype with meaningful evidence for niche differentiation, to demonstrate the selective advantage of traits, or to provide other evidence for macroevolutionary divergence. More- over, current depictions of vast pattern (but not size) diversity are inconsistent with expectations derived from most other extant primate clades that have adaptively radiated. If indeed the early hominin record is highly speciose, the reasons for this remain unclear. Keywords Hominin evolution Interspecific and intraspecific variation Adaptive radiation Australopith Species recognition Introduction Scientists often have a naive faith that if only they could discover enough facts about a problem, these facts would somehow arrange themselves in a com- pelling and true solution. Theodosius Dobzhansky A fundamental assumption of the paleoanthropological research agenda is that the path of human evolution will be clarified by the discovery of more fossil evidence, and that major gaps in our understanding are due at least in part to gaps in the fossil record. If this is true, the extraordinary successes of the past decade should have greatly illumi- nated our understanding of hominin evolution. Certainly in many ways they havefor example, by providing firm fossil evidence of hominins before 5 million years ago. And yet, in other respects, the picture is more muddied than ever. How many of us who study and teach human evo- lution currently find ourselves at a loss to communicate a coherent picture of phylogenetic diversity prior to the evolution of our own genus? Why is this? In this review we provide an overview of our current understanding of early hominin diversity, and explain our view of why the plethora of new fossil taxa seems to have done little to improve our understanding of the human past. We suggest that it is largely because evolutionary models that incor-...
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- Spring '07