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Evolution: Out of Africa and the Eve Hypothesis
by Chris Stringer
Introduction
Most palaeoanthropologists recognize the existence of two human
species during thelast million years Homo erectus , now extinct,
and Homo sapiens , the species whichincludes recent or 'modern'
humans. In general, they believe that Homo erectus wasthe ancestor
of Homo sapiens . How did the transition occur?
The multiregional model There are two opposing views. The
multiregional model says that Homo erectus gaverise to Homo sapiens
across its whole range, which, about 700,000 years ago,
includedAfrica, China, Java (Indonesia), and, probably, Europe.
Homo erectus , following anAfrican 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, withprominent cheekbones, as modern Oriental populations.
Javanese Homo erectus hadrobustly built cheekbones and faces that
jutted out from the braincase, characteristicsfound 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 faras those of Africa and eastern Asia, although
a possible Homo erectus jawbone morethan 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 theNeanderthals: the ancestors of
modern Europeans. Features of continuity in thisEuropean 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, whotended to regard the regional lineages as
genetically separate. Most recently, the modelhas become associated
with such researchers as Milford Wolpoff (USA) and AlanThorne
(Australia), who have re-emphasized the importance of gene flow
between theregional lines. In fact, they regard the continuity in
time and space between the variousforms of Homo erectus and their
regional descendants to be so complete that theyshould 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) andArthur Keith (UK) believed that the
lineage of Homo sapiens was very ancient, havingdeveloped in
parallel with that of Homo erectus and the Neanderthals. However,
much
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of the fossil evidence used to support their ideas has been
re-evaluated, and fewworkers now accept the idea of a very ancient
and separate origin for modern Homosapiens .
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 USanthropologist William Howells in 1976 because
of the idea that all modern humanvariation 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
modernhuman origins.
The consequent 'Out of Africa' model claims that Homo erectus
evolved into modern Homo sapiens in Africa about 100,000150,000
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 15,000years ago respectively). Regional ('racial')
variation only developed during and afterthe 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 newforms of human in inhabited regions outside
Africa, but argues that these non-Africanlineages became extinct
without evolving into modern humans. Some, such as theNeanderthals,
were displaced and then replaced by the spread of modern humans
intotheir regions.
... and an African Eve? In 1987, research on the genetic
material called mitochondrial DNA (mtDNA) inliving humans led to
the reconstruction of a hypothetical female ancestor for
allpresent-day humanity. This 'Eve' was believed to have lived in
Africa about 200,000years ago. Recent re-examination of the 'Eve'
research has cast doubt on thishypothesis, but further support for
an 'Out of Africa' model has come from geneticstudies of nuclear
DNA, which also point to a relatively recent African origin
forpresent-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 onewould
expect from the multiregional model.
Related Articles: Africa anthropology Darwin, Charles Robert DNA
evolution fossil
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genetics human species, origins of natural selection Neanderthal
palaeontology
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