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
birthweights, 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 7-8000 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.