Genetics & Genealogy: Article#20

Evolution: Out of Africa and the Eve Hypothesis

by Chris Stringer

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British paleoanthropologist Chris Stringer has taken a leading role in many of the earliest human origins debates. Educated at the University College London and Bristol University, Sringer is one of the leading proponents of the "Out of Africa" model of Homo sapiens diffusion. Stringer is currently working in the Paleontology Department at the British Natural History Museum.

Introduction

Most palaeoanthropologists recognize the existence of two human species during the last million years - Homo erectus, now extinct, and Homo sapiens, the species which includes recent or "modern" humans. In general, they believe that Homo erectus was the ancestor of Homo sapiens. How did the transition occur ?

The Multiregional Model

There are two opposing views. The multiregional model says that Homo erectus gave rise to Homo sapiens across its whole range, which, about 700,000 years ago, included Africa, China, Java (Indonesia), and, probably, Europe. Homo erectus, following an African origin about 1.7 million years ago, dispersed around the Old World, developing the regional variation that lies at the roots of modern "racial" variation. Particular features in a given region persisted in the local descendant populations of today.

For example, Chinese Homo erectus specimens had the same flat faces, with prominent cheekbones, as modern Oriental populations. Javanese Homo erectus had robustly built cheekbones and faces that jutted out from the braincase, characteristics found in modern Australian Aborigines. No definite representatives of Homo erectus have yet been discovered in Europe. Here, the fossil record does not extend back as far as those of Africa and eastern Asia, although a possible Homo erectus jawbone more than a million years old was recently excavated in Georgia.

Nevertheless, the multiregional model claims that European Homo erectus did exist, and evolved into a primitive form of Homo sapiens. Evolution in turn produced the Neanderthals: the ancestors of modern Europeans. Features of continuity in this European lineage include prominent noses and midfaces.

Genetic Continuity

The multiregional model was first described in detail by Franz Weidenreich, a German palaeoanthropologist. It was developed further by the American Carleton Coon, who tended to regard the regional lineages as genetically separate. Most recently, the model has become associated with such researchers as Milford Wolpoff (USA) and Alan Thorne (Australia), who have re-emphasized the importance of gene flow between the regional lines. In fact, they regard the continuity in time and space between the various forms of Homo erectus and their regional descendants to be so complete that they should be regarded as representing only one species - Homo sapiens.

The Opposing View

The opposing view is that Homo sapiens had a restricted origin in time and space. This is an old idea. Early in the 20th century, workers such as Marcellin Boule (France) and Arthur Keith (UK) believed that the lineage of Homo sapiens was very ancient, having developed in parallel with that of Homo erectus and the Neanderthals. However, much of the fossil evidence used to support their ideas has been re-evaluated, and few workers now accept the idea of a very ancient and separate origin for modern Homo sapiens.

The Garden of Eden

Modern proponents of this approach focus on a recent and restricted origin for modern Homo sapiens. This was dubbed the "Garden of Eden" or "Noah's Ark" model by the US anthropologist William Howells in 1976 because of the idea that all modern human variation had a localized origin from one centre. Howells did not specify the centre of origin, but research since 1976 points to Africa as especially important in modern human origins.

The consequent "Out of Africa" model claims that Homo erectus evolved into modern Homo sapiens in Africa about 100,000-150,000 years ago. Part of the African stock of early modern humans spread from the continent into adjoining regions and eventually reached Australia, Europe, and the Americas (probably by 45,000, 40,000, and 15,000 years ago respectively). Regional ("racial") variation only developed during and after the dispersal, so that there is no continuity of regional features between Homo erectus and present counterparts in the same regions.

Like the multiregional model, this view accepts that Homo erectus evolved into new forms of human in inhabited regions outside Africa, but argues that these non-African lineages became extinct without evolving into modern humans. Some, such as the Neanderthals, were displaced and then replaced by the spread of modern humans into their regions.

… and an African Eve ?

In 1987, research on the genetic material called mitochondrial DNA (mtDNA) in living humans led to the reconstruction of a hypothetical female ancestor for all present-day humanity. This "Eve" was believed to have lived in Africa about 200,000 years ago. Recent re-examination of the "Eve" research has cast doubt on this hypothesis, but further support for an "Out of Africa" model has come from genetic studies of nuclear DNA, which also point to a relatively recent African origin for present-day Homo sapiens.

Studies of fossil material of the last 50,000 years also seem to indicate that many "racial" features in the human skeleton have developed only over the last 30,000 years, in line with the "Out of Africa" model, and at odds with the million-year timespan one would expect from the multiregional model.

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