Question

What distinguished the evolution of members of Homo from the evolution of members of Australopithecus?

minimum 700 words

Answer 1

If we restrict the use of Homo sapiens in the fossil record to specimens which share a significant number of derived features in the skeleton with extant H. sapiens, the origin of our species would be placed in the African late middle Pleistocene, based on fossils such as Omo Kibish 1, Herto 1 and 2, and the Levantine material from Skhul and Qafzeh. However, genetic data suggest that we and our sister species Homo neanderthalensis shared a last common ancestor in the middle Pleistocene approximately 400–700 ka, which is at least 200 000 years earlier than the species origin indicated from the fossils already mentioned. Thus, it is likely that the African fossil record will document early members of the sapiens lineage showing only some of the derived features of late members of the lineage. On that basis, I argue that human fossils such as those from Jebel Irhoud, Florisbad, Eliye Springs and Omo Kibish 2 do represent early members of the species, but variation across the African later middle Pleistocene/early Middle Stone Age fossils shows that there was not a simple linear progression towards later sapiens morphology, and there was chronological overlap between different ‘archaic’ and ‘modern’ morphs. Even in the late Pleistocene within and outside Africa, we find H. sapiens specimens which are clearly outside the range of Holocene members of the species, showing the complexity of recent human evolution. The impact on species recognition of late Pleistocene gene flow between the lineages of modern humans, Neanderthals and Denisovans is also discussed.

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 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.

The origin and earliest evolution of the genus Homo perennially fascinate and frustrate in equal measure. Our fascination stems from the near-mythic qualities of uniqueness with which we tend to imbue the evolution of our lineage , whereas the frustration stems from critical gaps in the fossil record that actually bears on the first appearance of these features. By almost all accounts, the earliest populations of the Homo lineage emerged from a still unknown ancestral species in Africa at some point between approximately 3 and approximately 2 million years ago. This temporal interval reaches forward in time from the latest known occurrences of ‘generalized’ Australopithecus species (A. afarensis in eastern Africa, A. africanus in southern Africa) to the earliest known records of two, perhaps three, species commonly attributed to the genus Homo (H. habilis, H. rudolfensis and H. erectus). Between them lies a million years of rare, isolated or fragmentary fossils that constitute the hard evidence for the origin of Homo. That this time period also contains some of the earliest undisputed evidence for stone-tool manufacture is of no small consequence for scenarios about the origin of characteristics thought to be critical human adaptations, including proclivities for meat consumption, hunting, mobility, cooking, prosocial behaviour, etc., usually in novel ‘open’ habitats of the Early Pleistocene. It also witnessed the rise and diversification of the craniodentally specialized ‘robust’ australopiths, which means that isolated traces of behaviour and taxonomically uninformative pieces of skeletal anatomy often reside in a phylogenetic limbo between these lineages.

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 himself recognized this as a probable outcome of descent with modification, given a grown fossil record. 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 , 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 , 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.

Other fossil remains found in the same cave in 2008 were named Australopithecus sediba, which lived 1.9 million years ago. A. africanus probably evolved into A. sediba, which some scientists think may have evolved into H. erectus, though this is heavily disputed.Australopithecus afarensis, famously known as 'Lucy', is an extinct hominid that lived between 3.9 and 2.9 million years ago. Australopithecus afarensis was slenderly built, and closely related to the genus Homo, possibly as a direct ancestor or a close relative of an unknown ancestor.Australopithecus afarensis is usually considered to be a direct ancestor of humans. It is also considered to be a direct ancestor of later species of Australopithecus and all species in the Paranthropus genus.The fossil record seems to indicate that Australopithecus is ancestral to Homo and modern humans. It was once assumed that large brain size had been a precursor to bipedalism, but the discovery of Australopithecus with a small brain but developed bipedality upset this theory.

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