About 50,000 years ago, ancient humans in what is
now West Africa apparently procreated with another group of ancient humans that
scientists didn't know existed.
There aren't any bones or ancient DNA to prove it,
but researchers say the evidence is in the genes of modern West Africans. They
analyzed genetic material from hundreds of people from Nigeria and Sierra Leone
and found signals of what they call "ghost" DNA from an unknown
ancestor.
Our own species — Homo sapiens — lived alongside
other groups that split off from the same genetic family tree at different
times. And there's plenty of evidence from other parts of the world that early
humans had sex with other hominins, like Neanderthals.
That's why Neanderthal genes are present in humans
today, in people of European and Asian descent. Homo sapiens also mated with
another group, the Denisovans, and those genes are found in people from
Oceania.
The findings on ghost DNA, published in the
journal Science Advances, further complicate the picture of how Homo sapiens —
or modern humans — evolved away from other human relatives. "It's almost
certainly the case that the story is incredibly complex and complicated and we
have kind of these initial hints about the complexity," says Sriram
Sankararaman, a computational biologist at UCLA.
The scientists analyzed the genomes of 405 West
Africans. Sankararaman says they used a statistical model to flag parts of the
DNA. The technique "goes along a person's genome and pulls out chunks of
DNA which we think are likely to have come from a population that is not modern
human."
The unusual DNA found in West Africa isn't
associated with either Neanderthals or Denisovans. Sankararaman and his study
co-author, Arun Durvasula, think it comes from a yet-to-be-discovered group.
"We don't have a clear identity for this
archaic group," Sankararaman says. "That's why we use the term
'ghost.' It doesn't seem to be particularly closely related to the groups from
which we have genome sequences from."
The scientists think the interbreeding happened
about 50,000 years ago, roughly the same time that Neanderthals were breeding
with modern humans elsewhere in the world. It's not clear whether there was a
single interbreeding "event," though, or whether it happened over an
extended period of time.
The unknown group "appears to have split off
from the ancestors of modern humans a little before when Neanderthals split off
from our ancestors," he says.
Sharon Browning, a biostatistics professor at the
University of Washington who has studied the mixing of Denisovans and humans,
says "the scenario that they are discovering here is one that seems
realistic."
Browning notes that the ghost DNA appears
frequently in the genetic material. "That tells us that these archaic
populations might have had some DNA that did some useful stuff that's proved to
be useful to the modern population," she says.
But at the moment, Sankararaman says, it's not possible
to know what, if any, role these genetic materials have for modern humans who
carry them. "Are they just randomly floating in our genomes? Do they have
any kind of adaptive benefits? Do they have deleterious consequences?" he
added. "Those are all questions which would be fantastic to start thinking
about."
He says there is likely evidence of other ghost
populations in modern humans in other parts of the world. "I think as we
get the genome sequences from different parts of the world at different points
in time, there is always the possibility that we might discover these
as-yet-unidentified ghost populations," Sankararaman says.
It's also possible that the ghost DNA found in
this study comes from multiple groups, Browning added. "Within Africa, we
don't know how many archaic groups might have been involved, and the study
doesn't tell us that," she says. "It tells us that there was
integration, but it could have been from more than one archaic population, in
theory."
Compared with the Neanderthals, where there is
abundant DNA fossil evidence, physical samples are much harder to come by in
Africa. Browning says the climate on the continent has made it challenging.
"The conditions have to be right for the
fossils to not totally disintegrate" in order to recover DNA, Browning
says. Bones have been found in Africa from archaic populations, but no DNA has
been recovered. Still, she adds, "the technology is continuing to improve,
and people are still out there looking for more fossils."
So what happened to this mysterious group of
ancient humans? Scientists aren't totally sure.
They might have died off, or they might have
eventually been completely subsumed into modern humans.
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