Is human evolution speeding up or slowing down Laurence Hurst
The Tibetan high plateau lies
about 4500 meters above sea level,
with only 60% of the oxygen found below.
While visitors and recent settlers
struggle with altitude sickness,
native Tibetans sprint up mountains.
This ability comes not from training
or practice,
but from changes to a few genes
that allow their bodies
to make the most of limited oxygen.
These differences are apparent from birth—
Tibetan babies have, on average,
higher birth weights,
higher oxygen saturation,
and are much likelier to survive than
other babies born in this environment.
These genetic changes are estimated
to have evolved
over the last 3,000 years or so,
and are ongoing.
That may sound like a long time,
but would be the fastest an adaptation
has ever evolved in a human population.
It’s clear that human evolution
isn’t over—
so what are other recent changes?
And will our technological and scientific
innovations impact our evolution?
In the past few thousand years,
many populations have evolved genetic
adaptations to their local environments.
People in Siberia and the high arctic are
uniquely adapted to survive extreme cold.
They’re slower to develop frostbite,
and can continue to use their hands
in subzero temperatures
much longer than most people.
They’ve undergone selection
for a higher metabolic rate
that increases heat production.
Further south, the Bajau people
of southeast Asia can dive 70 meters
and stay underwater
for almost fifteen minutes.
Over thousands of years living
as nomadic hunters at sea,
they have genetically-hardwired unusually
large spleens that act as oxygen stores,
enabling them to stay underwater
for longer—
an adaptation similar
to that of deep diving seals.
Though it may seem pedestrian
by comparison,
the ability to drink milk
is another such adaptation.
All mammals can drink
their mother’s milk as babies.
After weaning they switch off the gene
that allows them to digest milk.
But communities in sub-Saharan Africa,
the middle east and northwest Europe
that used cows for milk have seen
a rapid increase in DNA variants
that prevent the gene from switching off
over the last 7 to 8000 years.
At least in Europe, milk drinking may
have given people a source of calcium
to aid in vitamin D production,
as they moved north and sunlight,
the usual source of vitamin D,
decreased.
Though not always in obvious ways,
all of these changes improve people’s
chance of surviving to reproductive age—
that’s what drives natural selection,
the force behind all these
evolutionary changes.
Modern medicine removes
many of these selective pressures
by keeping us alive when our genes,
sometimes combined
with infectious diseases,
would have killed us.
Antibiotics, vaccines, clean water
and good sanitation
all make differences between our genes
less important.
Similarly, our ability to cure
childhood cancers,
surgically extract inflamed appendixes,
and deliver babies
whose mothers have life-threatening
pregnancy-specific conditions,
all tend to stop selection by allowing
more people to survive
to a reproductive age.
But even if every person on Earth
has access to modern medicine,
it won’t spell the end of human evolution.
That’s because there are other aspects
of evolution besides natural selection.
Modern medicine makes genetic variation
that would have been subject
to natural selection
subject to what’s called
genetic drift instead.
With genetic drift, genetic differences
vary randomly within a population.
On a genetic level, modern medicine
might actually increase variety,
because harmful mutations don’t kill
people and thus aren’t eliminated.
This variation doesn’t necessarily
translate to observable, or phenotypic,
differences among people, however.
Researchers have also been investigating
whether genetic adaptations
to a specific environment
could appear very quickly
through epigenetic modification:
changes not to genes themselves,
but to whether and when certain genes
are expressed.
These changes can happen
during a lifetime,
and may even be passed to offspring—
but so far researchers are conflicted
over whether epigenetic modifications
can really persist over many generations
and lead to lasting changes
in populations.
There may also be other contributors
to human evolution.
Modern medicine and technology
are very new,
even compared to the quickest,
most recent changes by natural selection—
so only time can tell how our present
will shape our future.