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  • [ ♪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 recognizablyhuman.

  • 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 justget 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 recentlyor are still changing today.

  • To most of us, evolution looks like changing physical traitslike 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 timethe 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 hotlike 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 onbut 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 yearsnot 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 otherthey'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 mutationsones

  • 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 findreally 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 forand that this population has been

  • getting taller!

  • It's unlikely that genes are the only thing that's been affecting our height in recent

  • generationsstuff 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 farbut 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 humanslike

  • 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 harmfulbecause 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 agethough 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 thosebadgenes 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 resourcesensuring