[A gene that allows present-day
people to cope with life at high altitude was inherited from an extinct species
of human, Nature journal has reported.]
By Paul Rincon.jpg)
The variant of the EPAS-1 gene, which
affects blood oxygen, is common in Tibetans - many of whom live at altitudes of
4,000m all year round.
But the DNA
sequence matches one found in the extinct Denisovan people.
Many of us carry DNA
from extinct humans who interbred with our ancestors as the latter expanded out
of Africa .
Both the Neanderthals - who
emerged around 400,000 years ago and lived in Europe and
western Asia until 35,000 years ago - and the enigmatic
Denisovans contributed DNA to present-day
people.
The Denisovans are known only
from DNA extracted from the finger bone of a
girl unearthed at a cave in central Siberia . This 40,000-50,000-year-old
bone fragment, as well as a rather large tooth from another individual, are all
that is known of this species.
The tiny "pinky" bone
yielded an entire genome sequence, allowing scientists to compare it to the
genetic sequences of modern people in order to better understand the legacy of
ancient interbreeding.
Now, researchers have linked an
unusual variant of the EPAS1 gene, which is involved in regulating the body's
production of haemoglobin - the molecule that carries oxygen in the blood - to
the Denisovans. When the body is exposed to the low oxygen levels encountered
at high elevations, EPAS1 tells other genes in the body to become active, stimulating
a response that includes the production of extra red blood cells.
The unusual variant common among
Tibetans probably spread through natural selection after their ancestors moved
onto the high-altitude plateau in Asia several thousand
years ago.
"We have very clear evidence
that this version of the gene came from Denisovans," said principal author
co-author Rasmus Nielsen, from the University
of California Berkeley .
A tiny finger bone provided a high-quality DNA sequence for a
new species - the Denisovans
He told BBC
News: "If you and I go up to high altitude, we'll immediately have various
negative physiological effects. We'll be out of breath, we might suffer from
altitude sickness.
"After a little while, we'll
try to compensate for this by producing more red blood cells. But because we're
not adapted to the high altitude environment, our response would be maladaptive
- we would produce too many red blood cells.
"The blood becomes too thick
and raises our blood pressure, placing us at risk of stroke and pre-eclampsia (in
pregnant women)."
But Tibetans are protected
against these risks by producing fewer red blood cells at high altitude. This
keeps their blood from thickening.
The Tibetan variant of EPAS1 was
discovered by Prof Nielsen's team in 2010. But the researchers couldn't explain
why it was so different from the DNA
sequences found in all other humans today, so they looked to more ancient
genome sequences for an answer.
"We compared it to
Neanderthals, but we couldn't find a match. Then we compared it to Denisovans
and to our surprise there was an almost exact match," he explained.
He says the interbreeding event
with Denisovans probably happened very long ago.
"After the Denisovan DNA
came into modern humans, it lingered in different Asian populations at low
frequencies for a long time," Prof Nielsen said.
"Then, when the ancestors of
Tibetans moved to high altitudes, it favoured this genetic variant which then
spread to the point where most Tibetans carry it today."
He says it remains unclear
whether the Denisovans were also adapted to life at high altitudes. Denisova
Cave Altai Mountains
which rise above 3,000m.
Prof Nielsen said it was a "clear
and direct" example of humans adapting to new environments through genes
acquired via interbreeding with other human species.
Previous research has shown that
ancient humans introduced genes that may help us cope with viruses outside Africa .
And a study of Eurasian
populations showed that Neanderthal DNA is
over-represented in parts of the genome involved in making skin, hair and nails
- hinting, perhaps, at something advantageous that allowed Homo sapiens to
adapt to conditions in Eurasia .
@ BBC
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| A tiny finger bone provided a high-quality DNA sequence for a new species - the Denisovans |
@ BBC
