EvoWiki is now a project of the RationalMedia Foundation.
We are moving all content to RationalWiki.
See the EvoWiki project page for details!

Out of Africa

From EvoWiki

Jump to: navigation, search

Contents

Introduction

Among the most contentious current debates that divide paleoanthropologists is the question of the origin of anatomically modern humans. The debate actually ranges across a broad spectrum of opinion, but is generally described and discussed in terms of two opposing poles, known as the Out Of Africa (Out of Africa-2, hereafter as OOA2) hypothesis and the competing Multiregional Hypothesis. The current majority opinion supports the Out of Africa hypothesis, predominantly on the basis of the genetic evidence, but both hypotheses remain viable. The question revolves around the degree to which erectus-grade hominids outside of Africa contributed to modern human gene pools.

OOA2 generally argues that the extra-African erectus-grade hominids were sufficiently geographically separated that they were effectively reproductively isolated from each other. The Asian H. erectus went extinct, apparently without issue, and the European H. heidelbergensis evolved into the neandertals, who in turn went extinct after the arrival of anatomically and behaviorally modern humans in Europe. The OOA2 hypothesis argues that modern human anatomy evolved only in the African H. ergaster lineage, and that H. ergaster alone, is an immediate ancestor of humans, and the other branches of the Middle Pleistocene hominids went extinct without contributing to moderns (Cann et al 1987, Jacobs 2000b, Stringer 1985).

Evidence

The debate can not currently be resolved on the basis of the East and South Asian evidence, since too little is known about both the behaviors and the genetics of the Asian H. erectus populations. Although Thorne and Wolpoff (1981) argued that cranial measurements of H. erectus in South Asia bear similarities to the remains of early modern humans in Australia, the authors recognize that the cranial features they measured also appear in other populations. Until further evidence from East and South Asia is recovered, the resolution of the debate must therefore rely on the currently more thorough analyses of European and African hominids. However, the current evidence drawn from archaeology and genetics tends on the whole to favor the OOA2 model.

The Pleistocene archaeological record of Europe is divided into three distinct periods, the Lower, Middle, and Upper Paleolithic (analogous to the Early, Middle, and Late Stone Age of African Archaeology), each of which is subdivided into several distinct phases. The most important to modern human origins is the transition from the Middle to the Upper Paleolithic, which occurred approximately 40,000 years ago.

The Middle Paleolithic is characterized by flake tools that, in Europe, are commonly referred to as "Mousterian" technology, and are associated with H. neanderthalensis (sometimes called H. sapiens neanderthalensis). The Upper Paleolithic is characterized by "blade tools," which are stone tools made from flakes that are at least twice as long as they are wide, and also by tools made from bone, antler, ivory, and other materials. The Upper Paleolithic also contains a much more diverse array of tools than the Middle Paleolithic, and in deposits where Upper Paleolithic tools are associated with hominid remains, the remains are usually those of anatomically modern H. sapiens. One exception is a stone tool industry known as the Ch�telperronian (also "Castelperronian" and "Lower P�rigordian"). Harold (1989) notes that the Ch�telperronian technologies appear to be an indigenous development of the earlier Mousterian, possibly due to diffusion of ideas from contemporaneous migrations of modern humans into the area, and that both tool making styles are associated with neandertals. The Aurignacian technologies that appear in Europe at roughly the same time as the Ch�telperronian and persist even after its disappearance around 30,000 years ago are exclusively associated with modern humans. Thus the archaeological evidence points to a discontinuity between Ch�telperronian and Aurignacian, even while suggesting that neandertals did have some contact with modern humans. The archaeological evidence is supportive of the OOA2 hypothesis, but is currently tentative at best. The genetic evidence is much stronger.

In 1987, Cann, Stoneking, and Wilson published their study of mitochondrial DNA (mtDNA) of living humans, and determined that all humans share a single common ancestor who lived at some time between 100-200,000 years ago. Their article also noted that the greatest range of mitochondrial diversity today exists in central Africa, and that non-African mitochondrial lineages formed a subset or sample of the lineages that exist in Africa (c.f. Armour et al. 1996). They concluded therefore that the lineage that lead to modern humans began in Africa, and the greater diversity of African mtDNA was accounted for by a longer history of in situ evolution. More recently, Krings et al. (1997, 1999) were able to recover a sample of mtDNA from the neandertal type specimen and determined that it fell well outside the range of any anatomically modern humans. Further, the mtDNA of the neandertal type specimen was no more similar to the mtDNA of modern Europeans than to any other modern population, which would be expected if neandertals had contributed to modern human ancestry. In addition, using the rate of mutation (as did Cann et al. 1997) as a standard, Krings et al. (1997) estimated that the time of divergence of the modern and neandertal mitochondrial lineages was approximately 600,000 years ago, which is roughly the date of the earliest known H. heidelbergensis occupation of Europe. This finding confirms that neandertals were the direct descendants of H. heidelbergensis, and also suggests that the H. heidelbergensis lineage diverged without reticulation from its African and Asian contemporaries.

Neanderthal Hybrids?

The possibility that neandertals contributed to modern human DNA, and were part of our ancestry is not entirely precluded by the genetic evidence. It is possible that neandertal mitochondrial lineages were lost through drift, and that we retain neandertal nuclear DNA. Unfortunately, there is insufficient nuclear DNA present in any known neandertal remains for us to test this hypothesis. It is also possible that humans and neandertals were only partially inter-fertile, and that a female human, male neandertal hybrid was viable, but a male human, female neandertal hybrid was not. That hypothesis is equally consistent with the mtDNA evidence, and it is only Occam's Razor that causes us to favor the alternative; that humans and neandertals were distinct species, and the OOA2 hypothesis is the better explanation.

Conclusions

While the evidence remains inconclusive, and new evidence from archaeology, anatomy, and genetic analyses may alter our understanding of the relationship between humans and neandertals (and thus our resolution of the multiregional/OOA2 debate), the best current evidence favors a recent African origin of all modern humans, with the Asian and European erectus-grade hominids as evolutionary "dead ends." Neandertals, in short, were our "cousins," and not our "grandparents."

References

Armour, J.A. et al. 1996. Minisatellite diversity supports a recent African origin for modern humans. Nature Genetics 13: 154-60.

Cann, R. et al. 1987. Mitochondrial DNA and Human Evolution. Nature 325: 31-36.

Harold, F.B. 1989. Mousterian, Ch�telperronian and Early Aurignacian in Western Europe: Continuity or Discontinuity? In Mellers, P. and C. Stringer (eds) The Human Revolution: Behavioural and Biological Perspectives on the Origins of Modern Humans. Princeton, Princeton University Press.

External Links

Acknowledgements

Floyd

Personal tools
Namespaces
Variants
Actions
RWF
Navigation
Toolbox