From Australopithecus to Homo : the transition that wasn't

Although the transition fromAustralopithecus toHomo is usually thought of as a momentous transformation, the fossil record bearing on the origin and earliest evolution ofHomo is virtually undocumented. As a result, the poles of the transition are frequently attached to taxa (e.g.A. afarensis, atca 3.0 Ma versusH. habilis orH. erectus, atca 2.0–1.7 Ma) in which substantial adaptive differences have accumulated over significant spans of independent evolution. Such comparisons, in which temporally remote and adaptively divergent species are used to identify a ‘transition’, lend credence to the idea that genera should be conceived at once as monophyletic cladesand adaptively unified grades. However, when the problem is recast in terms of lineages, rather than taxaper se , the adaptive criterion becomes a problem of subjectively privileging ‘key’ characteristics from what is typically a stepwise pattern of acquisition of novel characters beginning in the basal representatives of a clade. This is the pattern inferred for species usually included in earlyHomo , includingH. erectus , which has often been cast in the role as earliest humanlike hominin. A fresh look at brain size, hand morphology and earliest technology suggests that a number of keyHomo attributes may already be present in generalized species ofAustralopithecus , and that adaptive distinctions inHomo are simply amplifications or extensions of ancient hominin trends.This article is part of the themed issue ‘Major transitions in human evolution’.