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Review
, 371 (1698)

From Australopithecus to Homo: The Transition That Wasn't

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Review

From Australopithecus to Homo: The Transition That Wasn't

William H Kimbel et al. Philos Trans R Soc Lond B Biol Sci.

Abstract

Although the transition from Australopithecus to Homo is usually thought of as a momentous transformation, the fossil record bearing on the origin and earliest evolution of Homo is virtually undocumented. As a result, the poles of the transition are frequently attached to taxa (e.g. A. afarensis, at ca 3.0 Ma versus H. habilis or H. erectus, at ca 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 clades and adaptively unified grades. However, when the problem is recast in terms of lineages, rather than taxa per 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 early Homo, including H. 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 key Homo attributes may already be present in generalized species of Australopithecus, and that adaptive distinctions in Homo are simply amplifications or extensions of ancient hominin trends.This article is part of the themed issue 'Major transitions in human evolution'.

Keywords: Australopithecus; early Homo; transition.

Figures

Figure 1.
Figure 1.
The effect of transitional fossils on distinct adaptive suites inferred from the comparison of terminal taxa. Top left: genus α is distinguished from genus β by a suite of characters [–4] thought to establish distinct adaptive grades. (In practice, characters 1–4 might be treated as a single functionally integrated character depending on the nature of the inferred adaptation.) Top right: fossil taxa (X–Z) bearing transitional forms show that, in reality, the pattern of acquisition of the characters was stepwise, defying clear delineation of adaptive grades along the lineage. Bottom: fig. 31 from Simpson [21] shows transitional populations (added dotted oval) linking species that occupy different ‘adaptive zones’. Although Simpson depicted this transition as an example of phyletic evolution (as opposed to speciation), the pattern of character acquisition is neutral with respect to evolutionary mode.
Figure 2.
Figure 2.
In The Descent of Man, Darwin [2] presented a model, sketched here, to account for the changes that needed to be invoked to account for human divergence from great apes. Darwin postulated environmental change as the spark and natural selection for attributes enhancing skill in defence and hunting as the engine of change, and he envisioned positive feedback linking adaptive changes in technology, intelligence and anatomical structure as the key to the human species' success. He proposed this, of course, in the absence of a fossil record that could have provided a chronology of events for these changes.
Figure 3.
Figure 3.
Isaac's [52] food-sharing model of ‘proto-human’ behavioural evolution in the Plio-Pleistocene.
Figure 4.
Figure 4.
The partial occipital bone KNM-ER 2598 from the upper Burgi Member of the Koobi Fora Formation (a, posterior view; b, left lateral view). Thick bone (seen in cross section), a well-developed occipital torus (especially centrally), and the low position of the internal occipital protuberance relative to inion affiliate this specimen with the Homo erectus morphological pattern [61]. The widely divergent limbs of the lambdodial suture are also characteristic of many African and Asian H. erectus crania. Photos courtesy of Fred Spoor (scale bar, 1 cm). (Online version in colour.)
Figure 5.
Figure 5.
Early hominin endocranial volumes (ECVs) over time. The average ECV for chimpanzees is shown for comparison (based on data in [77]). A, Australopithecus (sensu lato); H, H. habilis and H. rudolfensis; E, H. erectus. Data for fossil hominins from references [6,31,32,75]. (Online version in colour.)

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