8 | New Scientist | 2 April 2022
NATURAL selection was at work
on Bronze Age Britons, ancient
DNA reveals. Within the past
4500 years, evolution has acted on
genes involved in the production
of vitamin D – which people living
in Britain are sometimes short of
due to a lack of sunlight for much
of the year.
The genetic changes have had
knock-on effects on other traits,
from the ability of people to
digest milk to their skin colour.
One of the ways evolutionary
change can happen is through
natural selection: genetic variants
that are beneficial become more
common in the population
because individuals that carry
them are more likely to reproduce.
In recent years, geneticists have
collected DNA from the remains of
thousands of people who lived in
Britain over the millennia, so it is
possible to see natural selection
by looking for genetic variants
becoming more or less common.
“In some cases, the change is
so dramatic that you can rule out
this happening by chance, and
that’s when we would posit that
selection is driving this,” says
Jonathan Terhorst at the
University of Michigan.
Terhorst has developed a
new method of analysing ancient
DNA for signs of natural selection.
Unlike previous techniques, it
doesn’t assume that selection is
equally intense throughout the
study period, as that is unrealistic.
“The novelty here is that we can
really localise selection to within
a few thousand years, and say ‘this
is what’s being selected’,” says Iain
Mathieson at the University of
Pennsylvania, who has worked
with Terhorst to apply the
technique to ancient Britons.The new approach impresses
Claire-Elise Fischer, who is at the
University of^ York in the UK. “It’s
really amazing,” she says. “We’re
all going to use the method.”
Terhorst and Mathieson have
now used the new technique to
examine DNA from 529 ancient
Britons from the past 4500 years,
enhanced with genetic data from
98 present-day individuals.They found seven regions of the
genome with strong evidence of
selection (bioRxiv, doi.org/hnhz).
To their surprise, there was a
pattern. “All these genes that are
under selection can plausibly be
linked to natural selection for
increased vitamin D and calcium,”
says Mathieson.
That finding fits into an existing
body of knowledge about the role
of vitamin D in recent human
evolution, which has driven
genetic and cultural adaptations
in some populations. At the heart
of this is the health function ofvitamin D. It helps us to absorb
dietary calcium, strengthening
bones. Children who are deficient
in vitamin D can develop soft
bones, a condition called rickets.
Our bodies make vitamin D
when our skin is exposed to
ultraviolet radiation in sunlight.
When humans first evolved in
Africa, there was no shortage of
sunlight. However, when people
migrated away from the tropics,
they found themselves in places
where the sunlight reaching them
is less intense and the days can
be shorter. Britain’s cloudy skies
didn’t help either.
Nina Jablonski at Pennsylvania
State University, who has
spent decades unpicking the
significance of vitamin D on
human populations, says Britain
has “a punishingly low and highly
seasonal UV regime”.
Unable to produce enough
vitamin D, Bronze Age Britons
adapted. One shift, which
Mathieson and Terhorst saw in the
DNA they examined, was towards
lighter skin. Skin pigmentation
protects against UV, which is
good in the tropics as it guards
against skin cancer, but can limitvitamin D production in Britain.
It may seem odd that this
shift only happened in the past
few thousand years, when people
have lived in Britain for much
longer than that. However, the
earlier inhabitants were hunter-
gatherers who could get vitamin D
by eating oily fish.
Cheddar Man, who lived in
Britain about 10,000 years ago,
had very dark skin, and some Irish
people from about 5000 years ago
had moderately dark skin.
Natural selection for paler
skin only really kicked in when
people started getting most of
their food from crop farming,
leaving them prone to vitamin D
deficiency, says Mathieson.
Jablonski agrees this would
create evolutionary pressure.
“They’re going to be at the sharp
end of natural selection,” she says.
People can also compensate for
a lack of vitamin D by consuming
lots of calcium in their diet. Milk
is an excellent source, but for
most of our species’ existence we
could only digest it as babies. After
that, our bodies stopped making
the lactase enzyme that digests
the lactose sugar, so drinking milk
caused indigestion.
However, in Bronze Age Britain
there was strong selection for
producing lactase even into
adulthood, allowing people to
drink milk throughout their lives.
Today, most adults of European
origin can drink milk, whereas
many other populations remain
lactose-intolerant beyond infancy.
The study illustrates the
importance of vitamin D, says
Jablonski, because the lack of it
forced evolution to find “multiple
strategies” to help Bronze Age
Britons survive. “Virtually
everything in the body requires
vitamin D,” she says, so it isn’t
surprising that a shortage
provoked rapid evolution. ❚“All these genes under
selection can be linked
to a need for increased
vitamin D and calcium”ArchaeologyLENNART
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/NATIONALMUSEE
TAncient Britons evolved to survive
DNA from bones shows how these people adapted to keep healthy in their
sun-starved land once they had turned to farming, finds Michael MarshallA reconstruction
of a typical Bronze
Age settlementNews