By Will Dunham
WASHINGTON, Feb 13 (Reuters) - Scientists examining the
genomes of West Africans have detected signs that a mysterious
extinct human species interbred with our own species tens of
thousands of years ago in Africa, the latest evidence of
humankind's complicated genetic ancestry.
The study indicated that present-day West Africans trace a
substantial proportion, some 2% to 19%, of their genetic
ancestry to an extinct human species - what the researchers
called a "ghost population."
"We estimate interbreeding occurred approximately 43,000
years ago, with large intervals of uncertainty," said University
of California, Los Angeles (UCLA) human genetics and computer
science professor Sriram Sankararaman, who led the study
published this week in the journal Science Advances.
Homo sapiens first appeared a bit more than 300,000 years
ago in Africa and later spread worldwide, encountering other
human species in Eurasia that have since gone extinct including
the Neanderthals and the lesser-known Denisovans.
Previous genetic research showed that our species interbred
with both the Neanderthals and Denisovans, with modern human
populations outside of Africa still carrying DNA from both. But
while there is an ample fossil record of the Neanderthals and a
few fossils of Denisovans, the newly identified "ghost
population" is more enigmatic.
Asked what details are known about this population,
Sankararaman said, "Not much at this stage."
"We don't know where this population might have lived,
whether it corresponds to known fossils, and what its ultimate
fate was," Sankararaman added.
Sankararaman said this extinct species seems to have
diverged roughly 650,000 years ago from the evolutionary line
that led to Homo sapiens, before the evolutionary split between
the lineages that led to our species and to the Neanderthals.
The researchers examined genomic data from hundreds of West
Africans including the Yoruba people of Nigeria and Benin and
the Mende people of Sierra Leone, and then compared that with
Neanderthal and Denisovan genomes. They found DNA segments in
the West Africans that could best be explained by ancestral
interbreeding with an unknown member of the human family tree
that led to what is called genetic "introgression."
It is unclear if West Africans derived any genetic benefits
from this long-ago gene flow.
"We are beginning to learn more about the impact of DNA from
archaic hominins on human biology," Sankararaman said, using a
term referring to extinct human species. "We now know that
both Neanderthal and Denisovan DNA was deleterious in general
but there were some genes where this DNA had an adaptive impact.
For example, altitude adaptation in Tibetans was likely
facilitated by a Denisovan introgressed gene."
In watershed discovery, skull of ancient human ancestor
unearthed mysterious extinct humans, conquered high altitudes
cave findings shed light on enigmatic extinct human
species study implicates humans in demise of prehistoric cave
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