Why are Australopithecus garhi and Homo habilis important in human evolution?

The fossil record bearing on the ancestry of Pleistocene Homo is poor. However, the more we learn about early Homo, the less compelling is the case that an adaptive shift can be read from currently documented skull and skeletal anatomy as a ‘major transition’ from generalized Australopithecus precursors. Early, phylogenetically basal species of the Homo clade resemble generalized australopiths more than they do later species of the clade—as expected from a Darwinian pattern of descent with modification. These and subsequent ‘transitional’ species of the Homo lineage (e.g. H. erectus) erode the impression of distinct adaptive suites created by comparisons between terminal taxa or those separated by large temporal gaps. The epigraph heading this paper indicates that Darwin [2] himself recognized this as a probable outcome of descent with modification, given a grown fossil record. (The argument has recently been updated by Cartmill [94].) This recognition is not, however, a paean to wholesale phyletic or ‘gradual’ evolution in Homo, as was commonly asserted half a century ago. The African fossil record of Homo demonstrates diversity quite clearly between 2.0 and 1.7 Ma, and there are hints of it as far back as 2.4 Ma. It is, rather, an argument against ‘adaptive unity’ as a biologically necessary adjunct to monophyly in the definition of the genus category. Whether or not we choose to adopt the larger number of supraspecific taxonomic ranks (and their associated taxon names) dictated by a bushy hominin clade is not a pressing scientific matter in the quest to understand human origins. Of greater importance can be counted recent arguments for the use or manufacture of stone tools in time periods predating by some half a million years the earliest Homo fossils known so far [95,96], and the potential they have to shrink the adaptive space between Homo and Australopithecus still further. Indeed, the expanded brain size, human-like wrist and hand anatomy [97,98], dietary eclecticism [99] and potential tool-making capabilities of ‘generalized’ australopiths root the Homo lineage in ancient hominin adaptive trends, suggesting that the ‘transition’ from Australopithecus to Homo may not have been that much of a transition at all.

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