IS OUT OF AFRICA GOING OUT THE DOOR?
Reanalysis of gene studies and new fossil evidence cast doubts of a popular theory of
Anthropologists have long debated the origins of modern humanity, and by the mid-1980s two
main competing theories emerged. One, Multiregional evolution, posits that humans arose in
Africa some two million years ago, evolved as a single species spread across the Old World
and were linked through interbreeding and cultural exchange. The Out of Africa hypothesis,
in contrast, proposes a much more recent African origin for modern humans--a new species,
distinct from Neanderthals and other archaic humans, whom they then replaced. Emphatic
support for Out of Africa came in 1987, when molecular biologists declared that all living
peoples could trace a piece of their genetic legacy back to a woman dubbed
"Eve," who lived in Africa 200,000 years ago. Although that original Eve study
was later shown to contain fatal flaws, Out of Africa has continued to enjoy much
molecular affirmation, as researchers have increasingly turned to DNA to decipher the
history of our species.
But a closer look at these genetic studies has led some researchers to question whether
the molecular data really do bolster the Out of Africa model. And striking new fossil data
from Portugal and Australia appear to fit much more neatly with the theory of
The DNA from mitochondria, the cell's energy-producing organelles, has been key Out of
Africa evidence. Mitochondria are maternally inherited, so genetic variation arises
largely from mutation alone. And because mutations have generally been thought to occur
randomly and to accumulate at a constant rate, the date for the common mitochondrial DNA
(mtDNA) ancestor can theoretically be calculated. This "molecular clock"
indicates that the mtDNA ancestor lived a a mere 200,000 years ago, and the root of the
gene tree traces to Africa. These results, along with the observation that variation is
highest in Africa (indicating that modern humans had been in Africa the longest), seemed
to offer unambiguous support to a recent African origin for all modern humans.
OUT OF AFRICA THEORY posits that modern humans arose in Africa and replaced other human
species across the globe.
But the significance of each finding has been questioned. The date is suspect because the
molecular clock depends on problematic assumptions, such as the calibration date and
mutation rate. And if natural selection has shaped mtDNA, as some studies suggest, then
the rate of mutation accumulation may have differed at different times. The African root
for the mtDNA gene tree is compatible with Out of Africa, but it does not exclude
Multiregionalism, which predicts that the common ancestor lived somewhere in the Old
World, probably Africa. And neither does the high mtDNA variation in African populations
as compared with non-Africans uniquely support Out of Africa, according to anthropologist
John H. Relethford of the State University of New York College at Oneonta. "You could
get the same result if Africa just had more people living there, which makes sense
ecologically," he asserts.
Another problem plaguing the genetic analyses, says geneticist Alan R. Templeton of
Washington University, lies in a tendency for researchers to draw conclusions based on the
particular genetic system under study. "Very few people try to look across all the
systems to see the pattern," he observes. Some nuclear genes indicate that archaic
Asian populations contributed to the modern human gene pool, and Templeton's own analyses
of multiple genetic systems reveal the genetic exchange between populations predicted by
Still, Relethford and Templeton's arguments haven't convinced everyone. Henry C.
Harpending, a population geneticist at the University of Utah, finds Multiregionalism
difficult to swallow because several studies put the prehistoric effective population
size--that is, the number of breeding adults--at around 10,000. "There's no way you
can get a species going from Peking to Cape Town that's only got 10,000 members," he
remarks. (Other researchers counter that this number, based on genetic diversity, may be
much smaller than the census size of the population--perhaps by several orders of
magnitude.) And many geneticists, such as Kenneth K. Kidd of Yale University, insist that
"the overwhelming majority of the data is incompatible with any ancient
But those who believe that Out of Africa's genetic fortress is crumbling find confirmation
in fresh fossil data that pose new difficulties for the theory's bony underpinnings. Last
December researchers unearthed in western Portugal's Lapedo Valley a fossil that preserves
in exquisite detail the skeleton of a four-year-old child buried some 24,000 years ago.
According to Erik Trinkaus, a Washington University paleoanthropologist who examined the
specimen, the team fully expected the remains to represent a modern human, based on its
date and the style of the burial. But subsequent analysis, published in the June 22
Proceedings of the National Academy of Sciences USA, revealed a surprising combination of
features, such as a modern-looking chin and Neanderthal limb proportions. After reviewing
scientific literature on primate hybrids, Trinkaus concluded that this child resulted from
interbreeding between Neanderthals and modern humans.
Not everyone is persuaded. Christopher B. Stringer of London's Natural History Museum,
lead proponent of the Out of Africa model, wonders whether the fossil might simply
represent a cold-adapted modern human, because Portugal then was colder than it is today.
In any case, Stringer maintains that his model does not exclude occasional interbreeding.
Yet Trinkaus notes that because the fossil is dated to thousands of years after these
groups came into contact, "we're looking at populations admixing." Furthermore,
adult fossils from central and eastern Europe show the effects of mixing, too, states
paleoanthropologist David W. Frayer of the University of Kansas. And if the groups were
interbreeding across Europe, asserts University of Michigan multiregionalist Milford H.
Wolpoff, "that would mean you could make a strong case that [contemporary] Europeans
are the result of the mixture of these different groups." Another name for that, he
says, is Multiregional evolution.
Multiregionalism also best explains the surprising new date for a previously known fossil
from western New South Wales, according to paleoanthropologist Alan Thorne of the
Australian National University. In the June Journal of Human Evolution Thorne and his
colleagues report that the fossil, known as Lake Mungo 3, now looks to be some 60,000
years old --nearly twice as old as previously thought--and unlike the other early
Australian remains (all of which date to less than 20,000 years ago), this one bears
delicate, modern features. To Stringer, this gracile form indicates the arrival of modern
humans from Africa, albeit an early one. Over time, he reasons, selection could have led
to the robust morphology seen 40,000 years later.
But Thorne argues that such dramatic change is unlikely over such a short period and that
fossils from the only environmentally comparable region--southern Africa--show that people
have remained gracile over the past 100,000 years. Moreover, Thorne maintains, "there
is nothing in the evidence from Australia which says Africa"--not even the Mungo
fossil's modern features, which he believes look much more like those of contemporaneous
Chinese fossils. And Thorne observes that living indigenous Australians share a special
suite of skeletal and dental features with humans who inhabited Indonesia at least 100,000
Therefore, he offers, a simpler explanation is that the two populations arrived in
Australia at different times--one from China and the other from Indonesia--and mixed, much
like what has been proposed for Neanderthals and moderns in Europe. Exactly the same
pattern exists in recent history, Thorne adds, pointing to the interbreeding that took
place when Europeans arriving in North America and Australia encountered indigenous
peoples. "That's what humans do." The mystery of human origins is far from
solved, but because DNA may not be as diagnostic as it once seemed, Thorne says,
"we're back to the bones." University of Oxford geneticist Rosalind M. Harding
agrees. "It's really good that there are things coming from the fossil side that are
making people worry about other possibilities," she muses. "It's their time at
the moment, and the DNA studies can just take the back seat."