17 September 2014
Blue-eyed, swarthy hunters mingled with
brown-eyed, pale skinned farmers as the latter swept into Europe from
the Near East.
But another, mysterious population with
Siberian affinities also contributed to the genetic landscape of the
continent.
The findings are based on analysis of
genomes from nine ancient Europeans.
Agriculture originated in the Near East
- in modern Syria, Iraq and Israel - before expanding into Europe
around 7,500 years ago.
Multiple
lines of evidence suggested this new way of life was spread not just
via the exchange of ideas, but by a wave of migrants, who interbred
with the indigenous European hunter-gatherers they encountered on the
way.
But assumptions about European origins
were based largely on the genetic patterns of living people. The
science of analysing genomic DNA from ancient bones has put some of
the prevailing theories to the test, throwing up a few surprises.
Genomic DNA contains the biochemical
instructions for building a human, and resides within the nuclei of
our cells.
In the new paper, Prof David Reich from
the Harvard Medical School and colleagues studied the genomes of
seven hunter-gatherers from Scandinavia, one hunter whose remains
were found in a cave in Luxembourg and an early farmer from
Stuttgart, Germany.
The hunters arrived in Europe thousands
of years before the advent of agriculture, hunkered down in southern
refuges during the Ice Age and then expanded during a period called
the Mesolithic, after the ice sheets had retreated from central and
northern Europe.
They were closely related to each other
and their genetic profile is not a good match for any modern group of
people, suggesting they were caught up in the farming wave of
advance. However, their genes live on in modern Europeans, to a
greater extent in the north-east than in the south.
The
early farmer genome showed a completely different pattern, however.
Her genetic profile was a good match for modern people in Sardinia,
and was rather different from the indigenous hunters.
The Sardinians could represent a
population of early farmers that became isolated on the Mediterranean
island, and were little affected by later migrations that shaped the
rest of Europe.
But, puzzlingly, while the early
farmers share genetic similarities with Near Eastern people at a
global level, they are significantly different in other ways. Prof
Reich suggests that more recent migrations in the farmers' "homeland"
may have diluted their genetic signal in that region today.
Prof Reich explained: "The only
way we'll be able to prove this is by getting ancient DNA samples
along the potential trail from the Near East to Europe... and seeing
if they genetically match these predictions or if they're different.
"Maybe they're different - that
would be extremely interesting."
These regions will be the most
challenging to get ancient samples from, however, because DNA breaks
down more readily in warmer climates.
Pigmentation genes carried by the
hunters and farmers showed that, while the dark hair, brown eyes and
pale skin of the early farmer would look familiar to us, the
hunter-gatherers would stand out if we saw them on a street today.
"It really does look like the
indigenous West European hunter gatherers had this striking
combination of dark skin and blue eyes that doesn't exist any more,"
Prof Reich told BBC News.
Dr Carles Lalueza-Fox, from the
Institute of Evolutionary Biology (CSIC - UPF) in Barcelona, Spain,
who was not involved with the research, told BBC News: "If you
look at all the reconstructions of Mesolithic people on the internet,
they are always depicted as fair skinned. And the farmers are
sometimes depicted as dark-skinned newcomers to Europe. This shows
the opposite."
Last year, Dr Lalueza-Fox published
genetic details of a 7,000-year-old hunter from Spain who was
similarly dark and blue-eyed, suggesting these looks were not a
one-off.
So where did fair pigmentation in
present-day Europeans, including their rich diversity of hair colour,
come from? The farmer seems to be on her way there, carrying a gene
variant for light skin that's still around today.
"There's an evolutionary argument
about this - that light skin in Europe is biologically advantageous
for people who farm, because you need to make vitamin D," said
David Reich.
"Hunters and gatherers get vitamin
D through their food - because animals have a lot of it. But once
you're farming, you don't get a lot of it, and once you switch to
agriculture, there's strong natural selection to lighten your skin so
that when it's hit by sunlight you can synthesise vitamin D."
 |
This reconstruction shows the dark skin
and blue eyes
of a 7,000-year-old hunter from northern Spain
|
When the researchers looked at DNA from
2,345 present day people, they found that a third
population was
needed to capture the genetic complexity of modern Europeans.
This additional "tribe" is
the most enigmatic and, surprisingly, is related to Native Americans.
Hints of this group surfaced in an
analysis of European genomes two years ago. Dubbed Ancient North
Eurasians, this group remained a "ghost population" until
2013, when scientists published the genome of a
24,000-year-old boy buried near Lake Baikal in Siberia.
This individual had genetic
similarities with both Europeans and indigenous Americans, but lacked
the East Asian ancestry present in Siberia and the Americas today.
The ghost had been sighted.
The 8,000-year-old Scandinavian hunters
already show some signs of mixture with this population, but the
ancient hunter from Luxembourg and the farmer from Germany do not,
implying that this third ancestor was added to the continental mix
after farming was already established in Europe.
Dr Lalueza-Fox commented: "The
interesting point is the idea that we can dissect these components in
any modern European and explain diversity in modern Europeans as
different proportions of these three populations."
The study also revealed that the early
farmers and their European descendents can trace a large part of
their ancestry to a previously unknown, even older lineage called
Basal Eurasians. This group represents the earliest known population
divergence among the humans who left Africa 60,000 years ago.
(source: BBC)
