Hints of an unidentified, extinct human species have been
found in the DNA of modern Melanesians – those
living in a region of the South Pacific, northeast of Australia.
According to new genetic modelling,
the species is unlikely to be Neanderthal or Denisovan – two ancient species
that are represented in the fossil record – but could represent a third,
unknown human relative that has so far eluded archaeologists.
“We’re missing a population, or we’re
misunderstanding something about the relationships,” Ryan Bohlender, a
statistical geneticist from the University of Texas, told Tina Hesman Saey at Science
News.
Bohlender and his team have been
investigating the percentages of extinct hominid DNA
that modern humans still carry today, and say they’ve found discrepancies in
previous analyses that suggest our mingling with Neanderthals and Denisovans
isn’t the whole story.
It’s thought that between 100,000 and
60,000 years ago, our early ancestors migrated out of Africa, and first made
contact with other hominid species living on the Eurasian landmass.
This contact left a mark on our
species that can still be found today, with Europeans and Asians carrying
distinct genetic variants of Neanderthal DNA in their own genomes.
And that’s not all they’ve given us.
Earlier
this year, researchers investigated certain genetic variants that people of
European descent inherited from Neanderthals, and found that they’re associated
with several health problems, including a slightly increased risk of
depression,, heart
attack and a number of skin disorders.
And a separate study published
earlier this month found evidence that modern genital warts –
otherwise known as the human papillomavirus (HPV) – were sexually transmitted
to Homo sapiens after our ancestors slept with Neanderthals and Denisovans once
they left Africa.
While our relationship with
Neanderthals has been widely researched, how we interacted with the Denisovans
– the distant cousins of Neanderthals – is less clear.
The problem is that Neanderthals are
well represented in the fossil record, with many remains having been uncovered
across Europe and Asia, but all we have of the Denisovans is a lone finger bone
and a couple of teeth that were found in a Siberian cave in
2008.
Using a new computer model to figure
out the amount of Neanderthal and Denisovan DNA carried by modern humans,
Bohlender and his colleague found that Europeans and Chinese people carry a
similar amount of Neanderthal DNA: about 2.8 percent.
That result is pretty similar to
previous studies have estimated that Europeans and Asians carry, on average,
between 1.5 and 4 percent Neanderthal DNA.
But when they got to Denisovan DNA,
things were a bit more complicated, particularly when it came to modern
populations living in Melanesia – a region of the South Pacific that includes
Vanuatu, the Solomon Islands, Fiji, Papua New Guinea, New Caledonia, West
Papua, and the Maluku Islands.
“Europeans have no hint of Denisovan
ancestry, and people in China have a tiny amount – 0.1 percent, according to
Bohlender’s calculations. But 2.74 percent of the DNA in people in Papua New
Guinea comes from Neanderthals.
And Bohlender estimates the amount of
Denisovan DNA in Melanesians is about 1.11 percent, not the 3 to 6 percent
estimated by other researchers.
While investigating the Denisovan
discrepancy, Bohlender and colleagues came to the conclusion that a third group
of hominids may have bred with the ancestors of Melanesians.”
This find is supported by a separate
study by researchers from the Natural History Museum of Denmark, who
analysed DNA from 83 Aboriginal Australians and 25 locals from the Papua New
Guinea highlands.
As
we reported last month, this was the most comprehensive genetic study of
Indigenous Australians to date, and it indicated that they are the oldest
continuous civilization on Earth, dating back more than 50,000 years ago.
But the results revealed something
else – DNA that was very similar to that of the Denisovans, but distinct enough
for the researchers to suggest that it could have come from a third,
unidentified hominid.
“Who this group is we don’t know,” lead researcher Eske Willerslev told Hesman Saey.
Until we have more concrete evidence
of this hypothesized third human species (some fossils would be nice), we can’t
prove this, and we should point out that Bohlender’s estimates have
yet to be formally peer-reviewed, so they might shift with further scrutiny.
And it could be that our
identification of Denisovan DNA is more ambiguous than we think, given that our
only source is a finger bone and a couple of teeth.
But the evidence is mounting that our
interactions with ancient humans were far more complex than we’d assumed, which
shouldn’t be much of a surprise, when you think about it.
Just because we don’t see them in the
fossil record doesn’t mean they didn’t exist – preserving the remains of
something for tens of thousands of years isn’t easy, and then someone has to be
in the right place at the right time to dig them up.
Hopefully, the more we investigate
the genetic make-up of our most ancient societies, the more hints we’ll get of
the rich and complicated history our species shared with those that didn’t make
it to modern times.
The results of Bohlender’s analysis
were presented last week at the 2016 American Society of Human Genetics meeting
in Canada.
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