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After miners unearthed a skull and bones in a Neander Valley
cave in Germany in 1856—three years before the publication of On the
Origin of Species—the remains were initially described as either
those of a "brutish" race or of someone disfigured by disease.
As Darwinian evolution caught on, so did the realization that
these fossils were evidence of an earlier human species. Scientists
have been debating Neanderthal's place in human evolution ever
since.
An ongoing question concerns the possibility that Neanderthals
and early humans mated, since they likely crossed paths during
thousands of years of European cohabitation. In a new study, Mathias
Currat and Laurent Excoffier present a simulation model based on
what we know about the population density and distribution of
Neanderthals and Cro-Magnons.
Interbreeding?
Their results complement recent genetic and morphological
evidence indicating that early human and Neanderthal interbreeding
was unlikely.
The notion that modern Europeans directly descended from
Neanderthals has mostly yielded to two competing models: One
postulates that modern humans arose in Africa about 130,000 years
ago and completely replaced coexisting archaic forms with no
interbreeding, while the other proposes a gradual transition with
interbreeding.
Though mounting genetic evidence (based on mitochondrial DNA
extracted from fossils) suggests Neanderthals and early humans did
not breed, the evidence has been inconclusive.
It's possible, for example, that any Neanderthal gene "leakage"
could have been lost through genetic drift if the mating populations
were small. And because so few fossils are available to analyze,
previous studies could rule out only Neanderthal contributions over
25%.
Currat and Excoffier's model refines various parameters—such as
geographic boundaries, local population variations, range expansion,
and competition for resources—based on archeological and demographic
data for both populations. Evidence suggests modern humans replaced
Neanderthals over 12,500 years, for example, which constrains the
speed at which modern humans could expand.
The authors started with a scenario based on a set of
"plausible" parameter values—their basic scenario—and then varied
the local interbreeding rate and, for example, the population size
and location of Cro-Magnons, to produce eight alternate scenarios
describing how Cro-Magnon colonization of Europe might have
proceeded.
Gene Pool
They estimated the likely proportion of Neanderthal gene
contributions to the modern gene pool using "coalescent
simulations," which generate the genealogies and diversity of genes
in local populations based on simulations of their population
densities and migration histories.
The simulations show that if
Neanderthals bred with Cro-Magnons without constraints over
thousands of years, Neanderthal contributions to the modern gene
pool "would be immense." Surprisingly, the simulations also show
that even a very small mixing should lead to high levels of
Neanderthal DNA in modern humans.
What could account for this counterintuitive result? Given a low
population density with small local breeding populations, any
introduction of Neanderthal genes would decrease the frequency of
Cro-Magnon genes of that population; if these Neanderthal
integrations take place as the Cro-Magnon population is expanding,
newly acquired Neanderthal genes would also be amplified
Since no Neanderthal mitochondrial DNA has been found in
modern-day Europeans, the authors modeled the maximum number of
interbreeding events that would support this observation. The
estimated maximum number of events, it turns out, falls between 34
and 120—extremely low values, Currat and Excoffier conclude, "given
the fact that the two populations must have coexisted for more than
12,000 years."
While the authors acknowledge their simulations suggest rather
than reflect reality, their model does incorporate real historical
data such as Cro-Magnon expansion over time and local population
growth. At a value of only 0.1%, their new estimate of the rate of
interbreeding is about 400 times lower than previous estimates and
provides strong support that Neanderthals and Cro-Magnon didn't
interbreed.
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