(BBC News, 30th January 2014) - Gene types
that influence disease in people today were picked up through interbreeding
with Neanderthals, a major study in Nature journal suggests.
Journal article: Resurrecting Surviving Neandertal Lineages from Modern Human Genomes
Journal article: Resurrecting Surviving Neandertal Lineages from Modern Human Genomes
They passed on variants involved in type 2 diabetes, Crohn's disease and - curiously - smoking addiction.
Genome studies reveal that our species
(Homo sapiens) mated with Neanderthals after leaving Africa.
But it was previously unclear what this
Neanderthal DNA did and whether there were any implications for human health.
Between 2% and 4% of the genetic blueprint
of present-day non-Africans came from Neanderthals.
By screening the genomes of 1,004 modern
humans, Sriram Sankararaman and his colleagues identified regions bearing the
Neanderthal versions of different genes.
That a gene variant associated with a
difficulty in stopping smoking should be found to have a Neanderthal origin is
a surprise.
It goes without saying that there is no
suggestion our evolutionary cousins were puffing away in their caves.
Instead, the researchers argue, this
mutation may have more than one function, so the modern effect of this marker
on smoking behaviour may be one impact it has among several.
Researchers found that Neanderthal DNA is
not distributed uniformly across the human genome, instead being commonly found
in regions that affect skin and hair.
This suggests some gene variants provided a
rapid way for modern humans to adapt to the new cooler environments they
encountered as they moved into Eurasia. When the populations met, Neanderthals
had already been adapting to these conditions for several hundred thousand
years.
The stocky hunters once covered a range
stretching from Britain to Siberia, but went extinct around 30,000 years ago as
Homo sapiens was expanding from an African homeland.
Neanderthal ancestry was found in regions
of the genome linked to the regulation of skin pigmentation.
"We found evidence that Neanderthal
skin genes made Europeans and East Asians more evolutionarily fit," said
Benjamin Vernot, from the University of Washington, co-author of a separate
study in Science journal.
Genes for keratin filaments, a fibrous
protein that lends toughness to skin, hair and nails, were also enriched with
Neanderthal DNA. This may have helped provide the newcomers with thicker insulation
against cold conditions, the scientists suggest.
"It's tempting to think that
Neanderthals were already adapted to the non-African environment and provided
this genetic benefit to (modern) humans," said Prof David Reich, from
Harvard Medical School, co-author of the paper in Nature.
But other gene variants influenced human
illnesses, such as type 2 diabetes, long-term depression, lupus, billiary
cirrhosis - an autoimmune disease of the liver - and Crohn's disease. In the
case of Crohn's, Neanderthals passed on different markers that increase and
decrease the risk of disease.
Asked whether our ancient relatives
actually suffered from these diseases too, or whether the mutations in question
only affected the risk of illness when transplanted to a modern human genetic
background, Mr Sankararaman said: "We don't have the fine knowledge of the
genetics of Neanderthals to answer this," but added that further study of
their genomes might shed light on this question.
Joshua Akey, from the University of Washington,
an author of the Science publication, added: "Admixture happened
relatively recently in evolutionary terms, so you wouldn't expect all the
Neanderthal DNA to have been washed away by this point.
"I think what we're seeing to a large
extent is the dying remains of this extinct genome as it is slowly purged from
the human population."
However, some regions of our genomes were
discovered to be devoid of Neanderthal DNA, suggesting that certain genes had
such harmful effects in the offspring of modern human-Neanderthal pairings that
they have indeed been flushed out actively and rapidly through natural
selection.
"We find that there are large regions
of the genome where most modern humans carry little or no Neanderthal
ancestry," Mr Sankararaman told BBC News.
"This reduction in Neanderthal
ancestry was probably due to selection against genes that were bad -
deleterious - for us."
The Neanderthal-deficient regions encompass
genes that are specifically expressed in the testes, and on the X (female sex)
chromosome.
This suggests that some Neanderthal-modern
human hybrids had reduced fertility and in some cases were sterile.
"It tells us that when Neanderthals
and modern humans met and mixed, they were at the very edge of being
biologically compatible," said Prof Reich.
Another genome region that lacked
Neanderthal genes includes a gene called FOXP2, which is thought to play an
important role in human speech.
Joshua Akey said his team's results were
compatible with there having been multiple pulses of interbreeding between
modern humans and Neanderthals.