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When we think about evolution, we typically think about big changes that happened long
ago: single-celled organisms becoming multicellular, aquatic animals taking their first steps onto
land, that sort of thing.
But we often forget that we humans are still evolving.
Right now, today.
If you’re thinking you don’t feel any different, that’s because evolution isn’t
defined as changes that affect individuals.
Instead, evolution is the process of change in a population across generations.
Now, our anatomy and behavior haven’t changed much in at least the past 65,000 years.
If you met someone from back then, you would probably find them recognizably… human.
But we can still spot some things that have changed in ourselves.
And they’ve helped us live in harsher environments, avoid disease, and even just… get taller.
We’ve talked before about how drinking milk is pretty new.
But that’s not all.
Here are four more ways humans have changed recently — or are still changing today.
To most of us, evolution looks like changing physical traits — like a population gaining
the ability to drink milk.
But strictly speaking, evolution is defined as changes in the frequency of certain gene
variants in a population over time — the genes that control those traits.
We’re actually going to be talking a lot about gene variants in this episode, so for
context...
Except in specific cases, humans all have the same number of chromosomes and the same
basic set of genes, but the exact sequence of our DNA varies from person to person.
That’s what we mean when we refer to variants.
Different variants of a gene are referred to as alleles of that gene.
Those alleles create the variation that make us different from one another.
And if one individual survives to reproduce, those alleles can be passed on down to the
next generation, causing their frequency to increase over time.
Then, boom!
Everyone is drinking milk.
Since evolution is basically a change in gene frequencies, one way to tell if a population
has adapted to harsh conditions is to look for an increase in the frequency of alleles
that help you deal with those conditions.
In a 2014 paper, researchers showed that Aboriginal Australians have adapted to living in some
of the world’s hottest climates over the course of the last 65,000 years or so.
The deserts of Australia can be really hot — like 45 degrees plus hot!
But humans have lived there successfully for a long time.
In response to increasing temperatures, your body will typically release more of a hormone
called thyroxine.
And thyroxine has a ton of jobs in your body, like increasing your metabolic rate, helping
regulate your digestive system, and helping to keep your heart functioning.
But too much of it can actually be dangerous if you’re in a super hot environment.
However, this study showed that 40% of Aboriginal Australians have a pair of changes in th in
that gene that codes for a protein that normally holds onto thyroxine and releases it when
it’s needed.
These variants are associated with lower total thyroxine levels, and lower levels of the
binding protein itself.
In laboratory studies, this variant cut the temperature-associated release of thyroxine
almost in half.
This might be helping these populations keep their cool in warmer climates.
If something nasty is present in the environment, a population might adapt over time to resist
it.
So one way we can look for evidence for human evolution is by checking for genetic changes
that accompany some environmental variable.
At this point, we should stop and explain why that sentence is phrased like that: the
changes accompany the variable.
See, not all gene variants actually do anything.
A gene variant can be something as small as a single nucleotide.
If that single-nucleotide change is in the right place, it changes the instructions in
that gene slightly, and makes the protein it codes for do something a little different.
But if it’s somewhere else, either within a gene or nearby, it may not do anything.
We can spot those variants when we sequence DNA.
But a sequence alone doesn’t tell you if it’s one of those functional variants, or
just something that happens to be there.
Okay, here’s why that’s important.
In the Andes mountains, some indigenous populations are routinely exposed to naturally-occurring
arsenic in their drinking water.
And arsenic is very bad for you, causing issues from skin lesions to cancer.
But people who live in this area seem to process arsenic a bit differently.
If someone’s exposed to arsenic, their body will try to deal with it, producing a chemical
called monomethylarsonic acid in their urine.
But people from these Andean populations have less of it than you’d expect.
This suggests that they might have some kind of adaptation that protects them from arsenic
poisoning.
So in 2015, researchers from Sweden looked at these populations in a type of study called
a Genome Wide Association Study, or GWAS for short.
This is a widely-used study design that separates groups of people based on some characteristic
— in this case resistance to arsenic.
And it combs through their DNA, looking at millions of specific places to see if certain
gene variants show up more often in one group compared to another.
If so, it could be evidence that those variants are tied to the trait in some way.
They don’t say anything about whether the variants cause the trait, though.
It just tells you they’re there.
Basically, there’s definitely something going on — but while the study tells you
that a gene variant occurs more often in that group, it doesn’t tell you for sure what
the variant does.
The researchers found that these Andean people were much more likely to have specific variants
associated with a gene involved in arsenic processing, called AS3MT.
It’s likely that specific alleles of the AS3MT gene help people tolerate arsenic in
their drinking water.
The study doesn’t tell us how, but it points us there.
And since evolution is a simple change in the genetic makeup of a population, this population
seems to have evolved better arsenic tolerance.
It’s likely that a few of the early settlers in this region had this allele, and it allowed
them to be healthier and have more children in a region where the water’s mildly poisonous.
So that gene would have spread through the population.
What’s more, this would have happened within the last 11,000 years — not that long ago
in the scheme of things.
If an allele is being strongly selected for in a population, it might bring its whole
surrounding region of DNA with it.
Because we don’t always inherit genes independently of each other — they’re part of larger
chromosomes, and while those chromosomes can sometimes swap pieces, any given segment of
DNA tends to remain intact.
That can help researchers identify evidence of positive selection, when one of those regions
gets really popular really fast.
In this case, certain regions of DNA might look different from their ancestral counterparts.
If a region of DNA has recently been selected for and swept quickly through the population,
it will have had less time than its ancestral counterpart to pick up rare mutations — ones
not present in that region of DNA in many other people.
This makes sense, because any given region of DNA will accumulate mutations over time.
So if a given version of that region is newer, it will have fewer distinct mutations.
So in a paper published in 2016, researchers looked across the genome in a huge number
of European participants for alleles with fewer of these rare mutations in the DNA surrounding
them.
They wanted to find… really any evidence of positive selection in humans.
And what they found was evidence that we’re getting taller.
The strongest signal for positive selection they found was related to height.
Multiple alleles known to influence our height for the taller showed up in their screen.
Which suggests those genes are being selected for — and that this population has been
getting taller!
It’s unlikely that genes are the only thing that’s been affecting our height in recent
generations — stuff like better nutrition is definitely involved as well.
But this study clearly points to a genetic component.
This method detected changes that had happened in the last 2000 to 3000 years, or 100 human
generations.
Which is way more recent than what we’ve talked about so far — but hold onto your
hat.
Scientists have one more trick up their sleeves to really look at how people are evolving
now.
If a given allele occurs at low levels among people with long lifespans, that can be a
clue that it’s being selected against.
Because the folks who had it... didn’t make it that long.
This kind of analysis can be used to spot very recent adaptations in us humans — like
within living generations.
The idea is that if a gene variant doesn’t have much of a positive or negative effect
on us, it should occur at the same frequency in every age group.
If certain alleles are found at much lower levels in older generations than younger ones,
then it’s likely those alleles are somehow harmful — because if you had them, you may
not get to grow old.
In a study published in 2017, scientists looked at over 57,000 people and found changes in
the frequency of a specific allele of a gene called APOE.
We still don’t know exactly how Alzheimer’s works, but previous research has implicated
this particular allele of the APOE gene in putting people at risk of developing late-onset
Alzheimer’s.
But the researchers found that the frequency of this allele was dramatically reduced in
people over 70 years old.
This suggested that people who have this allele typically do not live as long, and that it’s
being selected out of the population.
They then looked at another group of almost 120,000 people, this time looking at the age
of their parents, or their age of death if they were deceased.
Since they couldn’t look at the parents’ genomes directly, they used the child’s
genome as a stand-in for which genetic variants the parents had.
And they found that that same APOE allele was less prevalent in people whose mothers
lived to old age — though the effect didn’t quite meet their statistical threshold.
So we can’t say for sure, but this study seems to provide support for the idea that
Alzheimer’s in general is being selected against.
Traditionally, geneticists have assumed that there’s not a lot of selective pressure
against genes that cause harm late in life.
Like, you’ve already given those “bad” genes to your kids, so they’re still out
there in the gene pool.
But this study seems to show the opposite: those genes DO get selected out.
One potential explanation relates to the so-called grandmother hypothesis.
This is the hypothesis that any genes that help you live longer will also help you take
care of your grandkids, specifically by helping provide extra food and resources — ensuring
they survive and reproduce, and pass on your genes to future generations.
But if a grandparent dies from Alzheimer’s and can’t help care for their grandkids,
those very genes that predisposed them to Alzheimer’s in the first place could get
selected out in the same way.
Other studies have drawn inferences between other Alzheimer’s-related variants and the
grandmother hypothesis, but it’s still all pretty speculative.
It’s important to note that the 2017 study did not turn up very many of these bad grandmother
genes, though.
The authors suggest that could be because the others have already been selected out.
Evolution isn’t just a thing of the past.
There’s not really a reason for it to stop happening to us humans just because we’ve
invented the wheel, wars, and New York City.
And even though it’s weird to think about that kind of change happening to us, it’s
probably nothing to fret about.
On the scale of a single lifetime, the changes are pretty small.
But if you’re not as tall your ancestors, you may have gotten the short end of the genetic
stick.
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