14 September 2019 | New Scientist | 9THE number of mildly harmful
mutations among people of
European ancestry has been
gradually increasing ever since
modern humans first moved
into the continent. Although
many of these mutations are
linked to diseases, their individual
effects are minor, and it is unlikely
that they are causing the people
who have them significant harm.
Our species evolved in Africa
and made significant strides
into Europe only 45,000 years ago.
The groups that made this move
were relatively small.
To find out if the small size
of Europe’s founding populations
may have affected the DNA of
their descendants, Stéphane
Aris-Brosou at the University
of Ottawa in Canada examined
the genomes of 2062 people
of European ancestry, including
1179 ancient genomes dating back
to up to 45,000 years ago.
For each genome, he looked
at 1.2 million sites where a single
“letter” on the DNA varies from
person to person. Many of these
genetic variants have previously
been found to be associated
with diseases like asthma and
diabetes, although their effects
are often small: having a single
harmful variant would only
slightly increase a person’s
chances of developing a disorder.
Aris-Brosou found that the
number of mildly harmful
variants in the European
population has steadily increased
over time (Molecular Biology and
Evolution, doi.org/c99g). This is
probably because mutations were
harder to shake off in Europe’s
small initial populations, and
more able to spread.
The findings are in line
with existing studies, says
Laura Botigué at the Centre for
Research in Agricultural Genomics
in Barcelona, Spain. “What is
significant and new is the fact they
include ancient specimens,” she
says. Previous studies used only
modern people, for example
comparing living people of mainlyEuropean or African descent.
It may seem odd that harmful
mutations have become more
common, because natural
selection is supposed to winnow
out disease-causing genes.
However, in small populations,
the power of natural selection
is reduced, especially when it
comes to slightly negative genetic
changes. Previous studies have
found that mildly harmful
mutations seem to be more
common in populations that
have gone through bottlenecks. In
2016, Botigué helped to show that
populations outside Africa carry
more mildly harmful mutations
than populations in Africa.
However, natural selection can
still keep very bad mutations in
check in small groups. “I think
everyone would agree now
that highly deleterious mutations
do not increase with human
expansions,” says Botigué. This
is because people carrying such
mutations are unlikely to live
long enough to have children, no
matter what size the population is.
Aris-Brosou points out that his
study has a big limitation: he
could look only at genetic variants
known to be harmful in modernpopulations. “There could have
been some mutations that were
terribly deleterious, but that have
been driven to extinction, so the
mutations disappeared,” he says.
There is no way to identify such
harmful mutations in the ancient
DNA, if they aren’t present today.
Aris-Brosou and Botigué agree
that the lingering mutations in
European populations are nothing
to worry about. “I think it’s
obvious that it’s fine, because we
are fine,” says Botigué. “Humans
are still around,” says Aris-Brosou.
There is also little we could do
about the mutations. Although
the European population as a
whole carries more mildly
harmful mutations than African
populations, if you compared
individuals from the two
continents you wouldn’t see
a difference, says Botigué.
This is because we all carry so
few of the mutations. As a result,
it isn’t possible to identify
individuals or groups in Europe
who have greater or smaller
shares of the mutations. ❚Human evolution
Harmful gene variants
have become more
common in EuropeMichael Marshall
MAXIPHOTO/GETTYHidden mutations revealed
Europeans have been accumulating harmful DNA mutations for thousands of years
Smoking may change your children’s gene activity
It isn’t just DNA that can change
(see story above). The epigenetic
markers that affect the activity
of our genes can alter too. Now
it seems that lifestyle choices
such as smoking can lead to
epigenetic changes that may
affect your future children.
Kenneth Aston at the
University of Utah and his
colleagues have found that the
sperm of men who smoke have
different epigenetic patterns
from the sperm of men who don’t.
To see if such epigenetic
changes may affect children,
the team exposed male mice to
cigarette smoke and examined
offspring they subsequently had.The pups whose fathers had
been exposed to smoke had
altered epigenetic patterns,
and different gene activity
in the prefrontal cortex, a
region of the brain involved in
personality and social behaviour
(bioRxiv, doi.org/c99h).
This suggests that epigenetic
changes in sperm can go on to
affect future children.
Men need to be aware of this,
says Aston. “I don’t think many
men think about the impacts
that their behaviour prior to
conception has on their offspring.
That’s something I didn’t think
about when I was having kids.”
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