[ ♪INTRO ] 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. Thanks for watching this episode of SciShow,
and thanks to our awesome patrons for helping to make it happen. We really do depend on you guys to help make
tough, complicated topics accessible and bring them to the whole Internet. You rule. If you’d like to get involved, check out
patreon.com/scishow. [♪OUTRO]
oh cool