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[intro]

You have domestication to think for some of your favorite foods, furry friends, even a mode of transportation. Apples? Domesticated. Cats? Domesticated. Zebras? ... Well, we tried. It's such a huge part of your daily life that you might not know just how much of your world is domesticated. That's probably because of the broad effect that domestication can have.

Domestication can mean that we're selecting for one specific trait and narrowing the species diversity, but it can also mean introducing new diversity. Take for example how we have domesticated our food. Domestication made wild cabbage into many of the distinct greens we know today, like broccoli, kale, and Brussels sprouts. Here is why those foods are actually all the same species.

[slide: "Kale, Cauliflower, and Brussels Sprouts Are All the Same Species"]

One of the most extraordinary things about dogs is how different they all are. Like, we took one wolfy species and made over 200 breeds: from adorable, wrinkly pugs to lanky, powerful greyhounds. We didn't just do this kind of whole-body tinkering with dogs. We've done it with plants, too.

Just about all the fruits and veggies you can buy at the supermarket have been shaped by human breeding. Most look totally different from their wild ancestors. But there is one plant species that has produced so many different varieties that it's known to biologists as "the dog of the plant world." You probably know it as kale and broccoli and cabbage and Brussels sprouts. That's right, those are all the same species of plants.

Foodie favorites like kale and cauliflower are just a couple of cultivars, or human-modified and grown varieties, of Brassica oleracea. And there are dozens more: from the logarithmic spiral of Romanesco broccoli to the distinct pointed shape of caraflex cabbage.

And you might think that tons of variety is just what happens when humans selectively breed something or generations, but that's not entirely true. After all, we've been growing and breeding lettuce for about the same amount of time, and yet, all lettuce varieties look...you know, pretty lettuce-y.

It turns out that B. oleracea is kind of a special plant. It was so transformable because it underwent some massive genomic events during its evolution. The story of why we have such a variety of this kind of plant starts millions of years ago. Back then, an ancient Brassica ancestor did something quite remarkable - it tripled its genome. That massive genome was whittled back down to a more reasonable size by the time wild cabbage emerged as its own species around 4 million years ago. Still, it meant that wild cabbage ended up with a lot more genetic variation than your average garden plant.

You see, broccoli and kale and Brussels sprouts don't just look different; they're very genetically distinct, too. And we're not just talking about little tweaks to genes. In a 2016 paper, researchers sequenced the genomes of nine different cultivars to construct the plant's pangenome, the total genetic variation that exists in the species. And they found that nearly 20% of the genes in that pangenome are only present in some varieties. So not only do cultivars have a lot of mutational difference, they also have WHOLE genes that are not present in other members of their OWN species, even though they all came from the same wild cabbage.

That plant, as far as we can tell, originated in the coastal areas of southern and western Europe. We don't know exactly when our species first grew and domesticated it, but genetic evidence suggests it may have been around 2000 B.C.E. The earliest written records come from ancient Greece, and they suggest the first cultivars were leafy veggies like kale and collard greens. And the Greeks weren't the only ancient people who tinkered with wild cabbage; scientists are pretty sure that the plant was domesticated many times in several locations. Some of these domesticated varieties found their way back -

- into the wild, became feral, and then were redomesticated, adding to the genetic diversity of the species. 

And all that genetic diversity eventually allowed people to magnify different structural parts of the plant. The variety we now call cabbage, for example, seems to have arisen sometime before the 10th century when people bred a kale-like plant to have larger buds on the tips of its stems. Brussels sprouts are also enlarged buds, the buds that grow all around the length of the stem. And scientists aren't quite sure when that cultivar emerged, but it was definitely being grown in Belgium by the end of the 18th century. And then there's kohlrabi, which literally means "cabbage turnip" in German, presumably referring to its bulb-like enlargement at the base of the stem. It's not clear when it first came about either, but historical literature suggests it was grown throughout Europe by the 1500s. 

Then there's broccoli and cauliflower; both get their unique florets - the yummy parts we eat - from mutations to the flowering genes. In broccoli, those mutations lead to a lot of flower buds packed tightly together; cauliflower has a lot of tightly packed flowering structures, too, but most of them never actually flower. Instead, the white, pre-bud flower tissue replicates itself as it grows, leading to the familiar curd-like head. Since both have modified flowers, it's thought that one came from the other, but it's not totally clear which came first. '

As of 2018, genetic research seemed to be leaning toward Team Broccoli. In fact, scientists are still trying to piece together how we got all of these amazingly different versions of Brassica oleracea and in what order. Trouble is, the same genomic shuffling events which gave this species so much genetic diversity also make it challenging to figure out a precise timeline for these cultivars and their relationships to each other.

And researchers are eager to figure out as much of this as they can, not just because it's fascinating, but because it will also help them better understand how the different varieties tolerate different environments, resist different diseases, and produce different nutrients.

By better understanding these nutritious, delicious, and fascinating dogs of the plant world, scientists just might figure out how to make our favorite crops more hearty, sustainable, and nutritious. 

[slide]

So we could turn one green into many different veggies using the tool of domestication. But that same tool can keep apples weird without turning them into new fruits. That's how we ended up with varieties like red delicious and golden delicious. Here's Olivia to explain why domesticated apples went ina completely different direction from other foods.

[slide: "The Truth About Johnny Appleseed and Apple Genetics"]

Olivia: If you went to school in North America, you were likely introduced to tales of Johnny Appleseed, a well-intentioned - if slightly odd - gentleman who traveled the continent planting apple seeds everywhere he went. Which, if you know anything about apple genetics, might come across as a colossal waste of time. After all, every time you grow an apple from seed, you're actually rolling the dice; you don't know what's going to grow. So it's not like Johnny was spreading tasty apples across the US of A - just crabby, gross ones. But it turns out growing all those not-so-yummy apples was kind of a good thing because it ensured that apple growers have tools to continue to cultivate delicious varieties today.

Jonathan Chapman traveled hundreds of thousands of miles across what is now the American Midwest in the 19th century, toting the fruit seeds that would earn him his nickname, Johnny Appleseed. He made a living selling the trees that sprouted from those seeds, but here's the weird thing: He had no way of knowing what apples would come from those seeds. And a modern apple-grower couldn't tell you much better.

Suppose you go to the grocery store and buy yourself some nice Fuji's or Pink Ladies. You say to yourself, "Gosh, this is the best apple I've ever eaten." So you plant the seeds in your backyard in hopes that once the tree matures, you can experience that delicious apple all over again. But you wait about a decade until the tree finally produces fruit, and - surprise! Your apples are small or sour or kind of just ugly looking. Or maybe all of the above. Well, if you'd talked to an -

-apple-grower first, you would've expected that. Unlike planting seeds from your favorite store-bought tomatoes, the fruit of any apple tree you grow from seed will never look like the fruit it came from. They don't grow "true-to-type," as gardeners say. And that's because genes in those seeds are always from two genetically distinct trees. 

See, we can't just inbreed the trees to preserve the traits we like, like we do in dogs. Apples and other species like pears and sweet cherries won't let us do that. These species have a system called "self-incompatibiliy," where they're capable of recognizing genetically similar individuals and then not breeding with them. Generally for flowering plants, the process of seed production starts when a pollen grain falls onto an organ within the flower called a pistil. That pollen grain then grows a long tube down to the flower's ovaries and delivers its genetic material.

Plants are a bit oddball, so there's more to it than that, but that's the gist. Many flowering plants produce both male and female reproductive organs on the same flower. And if that's the case, they can often fertilize themselves. Like the reason Mendel's pea plants were so great for studying genetics was because they're self-fertilizing. If he'd be studying apples, he never would've gotten as far as he did. 

But even though apples do have the necessary parts in place for self-fertilization, they also have really robust ways of telling their own pollen from that of a genetically distinct tree. The female reproductive organ produces an enzyme called an S-RNase. That enzyme's job is to chop up RNA, which would be bad for a future seed since cells need RNA to make proteins and - by extension - live. Still, these enzymes are transported into the growing pollen tube. 

Luckily, it has a defense; it can degrade the S-RNase before the enzyme can do any degrading of its own. But it will only do that if its genes and the RNAs are a mismatch. If it recognizes the RNase as being from genetic stock similar to its own, the RNase gets to do its work unencumbered, and fertilization is stopped. This means apple blossoms won't pollinate themselves or other blossoms on their tree, even if the pollen happens to land in the right place. It even -

It even reduces the odds that parent or sibling trees can breed with them.

Most of the time, pollen from a totally different strain has to be carried by bees or the wind for flowers to produce fruit. That's good for the plant, because inbreeding can lead to a loss of resistance from pests and disease, as well as just being less healthy overall. But it's bad for us, because it means we can't pick a tree we like and force it to produce offspring with very similar genes. Instead, growers have to find another type of apple tree that blooms at the same time, produces compatible pollen, and carries desirable genes in order to breed new trees.

What that all means is that we've essentially been rolling the dice for literally thousands of years, hoping that two trees will mate and produce a nice apple. And it's not even like a six-sided die; it's more like a handful of d20s. That's because apples have remained almost as genetically diverse as their wild ancestors, starting from when they were first cultivated around 4,000 years ago.

Normally, domestication really hurts the genetic diversity of a population. As humans select for desirable traits, gene variants get left behind, creating what's referred to as a "domestication bottleneck." And more modern methods of cultivation can narrow the gene pool even further, creating a second "improvement bottleneck." Estimates vary, but improvement bottlenecks can remove as much as 25% of the wild genetic diversity.

But that's the not the case with apples. A 2014 paper surveyed the genetic diversity of modern cultivated apples and found it's basically equal to the very oldest varieties. That means that any genes that contribute to sweetness, or color, or pest resistance, or ability to grow in cold climates are mixed in with all sorts of other genes throughout the apple's gene pool. And that means that once apple-growers find an apple they like, they just can't risk letting it breed with other apple trees.

So if they hit the genetic jackpot, they usually propagate that tree by cloning: not modern molecular cloning, but a growing technique called grafting, where you take the fruit-bearing part of one tree and fuse it with the root of another, creating a new hybrid -

- tree that produces genetically identical fruit. It's a process so ancient, we've had it about as long as we've had cultivated apples. And it means that we can keep growing what's effectively the same tree for generations, like Golden Delicious apples go back to 1890.

There is still some room for genetic changes, even when you're cloning trees in this fashion, though. Like sometimes a new branch will turn up with a chance mutation that makes the apples on it a little different - a deeper shade of red, perhaps. Growers might select for that more appealing color, propagating the mutant branches over the older variety, even if the deeper color comes at the expense of flavor. You might see where I'm going with this.

Yes, the reason Red Delicious apples taste like misery incarnate is probably for the selection for color, at least according to some food scientists. By all accounts, they used to taste pretty good. Of course, good tasting apples have only really been the goal of apple-growers for the last century or two. Your Honeycrisps and your Galas are what the trade calls "dessert apples." They're sweeter than cider apples, and we tend to want them to be more consistent.

Apples that go into hard cider don't have to be sweet, or perfectly firm, or... well, good, really. They basically just have to have enough sugar to ferment, which in the end, is why our buddy Johnny probably wasn't wasting his time. Sure, he didn't know what would grow from his seeds exactly, but at the time, most apples ended up as hard cider, so pretty much any apple worked. And the genetic studies suggested he, or people like him, may actually have helped apples maintain their genetic diversity up to the present day.

The apples you see in the grocery store originate from the Tian Shan mountains in Central Asia. They traveled to Europe along the Silk Route, where they further interbred with European crab apples to produce the modern domesticated apple, Malus domestica. In fact, they've interbred so much that domesticated apples have more genetic material in common with the European apples than the Asian ones. And only modern genetic studies have been able to establish for certain where they came from.

Then, those European domesticated strains were introduced to North America, and somehow, they stayed super diverse. Some have suggested that that's because Malus Malus domestica interbred with North American species to adapt to a new climate. But others think it had more to do with our deal pal, Johnny, and others like him. Because even though different apple varieties were often kept apart in their European orchards, with people running around, planting them all over North America, some were bound to go wild. That let them get together, and they were different enough from one another to overcome self-incompatibility. So they made new varieties of trees called "chance seedlings."

That turned out to be a pretty good thing for us, because we've gotten more than a few delicious apples through these new offspring. Literally. Red Delicious and Golden Delicious apples were both chance seedlings. And the Macintosh - an apple so popular it's got a certain type of computer named after it - was also a chance seedling that was discovered all the way back in 1811.

So there's a lot to be said for planting apple seeds when you don't know what will sprout from them. Genetic diversity isn't just valuable for its own sake. Apple-breeders rely on that huge gene pool to create new varieties. Though these days, we're lucky enough to have genetic sequencing to cut down on the guesswork. And apple-growers aren't just looking for things that improve flavor.

Hiding amongst those genes are also the keys to resisting pests and diseases, growing in different climates, or making apples that are hardier and easier to transport. Or so breeders hope. In fact, there's some evidence that a gene for disease resistance made the jump from wild to domestic apples as recently as the 1970s. And the need for resistance isn't just theoretical. Both pests and a changing climate have been making life harder for North American apples in recent years. That's why efforts are ongoing to preserve apple diversity.

See, apples as a whole are diverse. But as of 2008, 90% of apples produced in the US consisted of just 15 varieties. And if we want to keep creating new, tasty apple varieties -

- that can survive whatever gets thrown at them, we'll need to do better than that. 

Fortunately, researchers are on it. Much like Johnny once did they're planting all sorts of seeds, and by doing so, they're ensuring that apples stay wonderfully diverse. The rest of us will just have to wait for the fruits of their labor.

[slide]

Hank: All this domestication might seem pretty high-tech, but it's nothin' compared to today's cloud computing technology. Cloud computing allows you to stream videos like this one, create websites and run analytics on them, store important information, and generally exist in the modern online world. And Linode cloud computing from Akamai keeps it all running with servers across the physical world. They're already a cloud-computing powerhouse, but they're still working to improve your access to cloud technology by adding at least a dozen more servers by the end of 2023.

To hit the ground running this year with those brand new servers, you can click on the link in the description down below, or go to linode.com/scishow for a $100 60-day credit on a new Linode account. Thanks to Linode for supporting this SciShow compilation, and now back to the episode.

And we're not just talking about domesticated food, because the domestication of our food may have led to the domestication of cats. Here's how.

[slide: "Where Do Domestic Cats Come From?"]

Cats. We know that they like to chase lasers and lick their own butts, but there's a lot that we don't know about cute, little Whiskers: like where her cuddly domestic ancestors came from, and when she evolved from wild animals.

We used to think that the earliest historical evidence for domestic cats was from ancient Egypt, like art and mummified remains from around 4,000 ago. But now, some clues are pointing to domestic kitties older than that from separate places across the globe. The oldest, probably-domestic cat skeleton we've found was in 2001 on the island of Cyprus in the Mediterranean Sea. Scientists guessed that this cat 9,500 years ago, which makes sense historically. That's after people started farming in the Fertile Crescent - that not-totally desert region in parts of Western Asia and Northern Africa.

Farming means you have to store extra crops somewhere, -

- and piles of tasty grain attract rodents, and for hungry cats, that's an all-you-can-eat buffet. So one hypothesis is that feral cats might have started snagging some meals and getting cozy with humans. Humans were happy to have them, too, because they took care of the pests and were fluffy and cute. By this time, we think humans had domesticated other animals - like dogs, cattle, and sheep - so adding another furry friend wouldn't seem all that unusual. And we think this cat from Cyprus was a pet for a couple of reasons:

First of all, Cyprus is an island with no native cats, so someone must have brought them over on a boat, and if they weren't a little tame, that would have been a scratchy, panicky animal mess. Like, you might know how hard it can be to get an uncontrollable kitty just to the vet and back. Plus, the cat was buried with a person presumably its owner, and surrounded with carved seashells. Wild animals wouldn't get this special treatment, and if the cat was a meal, its bones would have been separate and probably scattered.

All of this evidence lines up with a study published in the journal Science in 2007, which looked at the genetic origins of domestic cats. Those researchers found that our feline friends are most closely related to the wildcat Felis silvestris, specifically the near-Eastern subspecies. Your eyes, also, if you look at this cat, will back this evidence up, because they look a lot like domestic cats. So lots of signs point to domestic cats splitting off from their wildcat cousins in the Fertile Crescent.

But hold on. Some other scientists discovered probably-domestic cat bones in 2001 in an ancient millet-farming village (Quanhucun) in central China. A close computer analysis of jawbone shapes showed that these cats weren't related to the wildcat at all; instead, they were a kind of leopard cat, which is ain an entirely different genus. From small animal tunnels throughout the excavation site and ceramic containers that looked like they stored grain, the researchers were pretty sure that this village had a rodent problem. And by looking at the carbon isotopes in cats' bones, it was clear that they ate lots of small animals that ate lots of human-grown millet.

This was the first convincing evidence to support the "domestic cats eat pests that eat grain" hypothesis. But this domestication happened in different kinds of cats around -

- 5,300 years ago on the other side of this huge landmass. So, what's the real story? The Middle Eastern or the Chinese domestication of cats? Well, there's no reason to think that domestication couldn't have happened twice in two separate places with two separate cat species when people started farming grain. But remember - genetically, all of our modern cats seem to be descended from the wildcat, not the leopard cat. Maybe the domestic wildcats were just snugglier and had a leg up to win our favor.

See, domestication leaves its fingerprints in an animal's genome, so even though any cat person will joke that their cats are too independent to really be considered domesticated, we can look at these genetic fingerprints. A 2014 collaboration between a bunch of American universities took a close look at the domestic cat genome, using 22 different breeds from different places. The study found recent changes in genes that control the development of the cat's nervous system. These genes could play a role in how domestic cats, for example, behave less defensively in new situations and can change their behavior in response to rewards. In other words, compared to a wildcat, Fluffy is genetically more likely to walk up to you with a friendly headbutt and beg for treats. This could explain why our cats are extra snuggly; the ones that got along best with humans could take advantage of our rodent pests and table scraps, and survived to pass on their genes. So in a way, cats did domesticate themselves, and it seems like they did it more than one time. Which kinda of means that the rise of cat videos was practically inevitable.

[slide]

The most cat thing a cat could do was probably domesticate themselves. But in the end, cats and dogs might not be so different, because dogs may have been domesticated for the same reason - to eat our leftovers. And we played a bigger role in their domestication. Here's Michael's list of the three weird things we did to dogs, like accidentally giving them floppy ears.

[slide: "3 Weird Things That Domestication Did to Dogs"]

Michael: A world without dogs sounds like no fun at all. But many of the hundreds of dog breeds we know today are only a few centuries old. And according to current research, if you go back in time at most 34,000 years, -

- dogs as we know them didn't even exist. Even though we know that modern dogs and modern wolves share a wolf ancestor. No one's exactly sure how dogs were first domesticated or even when. But we do know that at some point dogs evolved from mostly ignoring humans to wanting to be best friends with us. That process of domestication came with consequences. Some of them you'd expect, like dogs becoming tamer over time, and others are just...strange.

For instance, there's a trait that dogs share with other domesticated animals that even Darwin thought was weird; lots of dogs have floppy ears. Evolutionarily, this doesn't make much sense. It's the result of deformed ear cartilage, and it can actually make it harder for a dog to hear, so why would we breed dogs to have deformed ears? Well, we didn't. At first. At least, not on purpose. Instead, floppy ears seem to have a lot to do with other traits that domesticated animals have, like patches of white fur and adorable little faces that retain their juvenile features into adulthood. 

According to a new hypothesis, it turns out that in the process of domesticating dogs, we might have actually been affecting some of their stem cells. In a dog embryo, there's a group of stem cells called the neural crest, and these cells are responsible for forming a specific set of physical features: like the dog's coat, the structure of its face, and its adrenal glands. And according to this new research, a lot of the features that we associate with tameness may actually come from changes that have been made to this neural crest.

The earliest dogs may have been less aggressive because they had smaller adrenal glands, so when early humans bred for tameness, the dogs probably ended up with changes to other traits that are controlled by the neural crest, like floppy ears in the faces with more juvenile features, such as smaller jaws. So basically, by domesticating dogs, we may have ended up selecting for mutations in their stem cells that made them less like wolves and more like the animal that's probably sleeping in your living room right now.

But domesticating dogs has had other useful side effects, too. For example, dogs are a whole lot better than their wild cousins at digesting starch. A study published in 2013 analyzed the genomes of 12 wolves and 60 dogs of different breeds. The researchers were looking for genetic differences that showed up in all of the dogs but none of the wolves. They found changes in 36 regions of all of the dogs' DNA. Some of the results were somewhat predictable, like changes to genes that were involved in brain development, which account for how friendly and tame dogs could be.

 But they also found something they didn't expect: the dogs had three genetic variations that helped them digest starch. And this fits with a theory that dogs first started to be domesticated when humans settled down to agrarian life.

At some point, hungry wolves might have started venturing into human settlements and eating their leftover starchy good, something that a lot of modern dogs seem to be into as well. The wolves that were best able to digest the starch were better fed, so they survived to reproduce. 

And speaking of things that dogs are really into, you know how dogs really like chewing on bones but it seems to take them forever to actually finish eating one? Well, that has a lot to do with their ancestry, too. Wolves eat meat, and they're really into the stuff. It doesn't matter how delicious you think your pie is; a wolf is going to pick the steak any day. So they've got really sharp teeth that are perfect for tearing flesh apart and powerful jaws that polish off a bone pretty quickly, especially if they're looking to get to the marrow inside.

But even though dogs inherited wolves' desire to gnaw on bones, they have smaller jaws and a less powerful bite, which means that it takes them a whole lot longer to finish a bone, if they can even make a dent in it at all. But if you've ever seen a dog chewing on a bone, it doesn't seem like they mind how long it takes to finish.

[slide]

Hank: While we made dogs more time, our interaction with zebras went in the other direction. I mean, we've managed to domesticate horses for writing, but we pretty much totally failed with zebras. Here's how to TRIED to domesticate them for transportation.

[slide: "Why Do We Ride Horses But Not Zebras?"]

Michael: I don't know if you've noticed, but zebras look an awful lot like horses; they're even part of the same equine family, and you can crossbreed them into an animal called a zorse. But no matter how similar they look, don't be fooled. Thanks to their biology and evolution, you just can't ride a zebra. It's a beautiful, stripy, majestic trap.

According to archaeological evidence, humans have been taming horses for at least 5,500 years, but our relationship with zebras goes back much further. Humans and zebras have spent millions of years together because both species evolved alongside each other in Africa, and that's actually where the problems with zebra-riding start. Because even though we've spent millennia together, it's not like humans and zebras were best buddies.

For quite a while, early humans saw zebras as food and not much else. There's even evidence that we hunted them, so our striped neighbors -

- kind of grew up knowing that people were bad news. According to some researchers, that means they could be predisposed to fear us. Horses, meanwhile, didn't encounter people until much later, and we likely didn't hunt them long enough or them to pick up the same fear.

But even though we totally started our relationship off on the wrong foot, there's actually an even bigger problem with zebras. Their evolutionary history has made them just plain nasty. They spend their whole lives surrounded by large predators like lions, cheetahs and hyenas, so they have a really well-developed fight and flight response. This means that zebras are flightier than horses and a lot more aggressive. Corner a zebra and it will bite, and kick, and in general, try to end you, because those are the kind of skills it needs to survive in the wild. 

And a zebra's kick is serious business. An adult zebra can kick hard enough to break a lion's jaw, and zebras injure more American zookeepers than any other zoo animal. This aggressiveness is such a big deal that some researchers have even looked for a genetic component to it, although they haven't found anything conclusive yet. Either way, trying to convince a zebra to let you ride it is just asking to get hurt, and I don't know about you, but I would rather walk somewhere than deal with that. 

Finally, temperament aside, zebras aren't built for riding. Even though they're from the same family, they're smaller than domestic horses, and their backs aren't as strong, so they're not able to comfortably carry as much weight. They also have thick necks, so it's not easy to direct them with reins. And because they're so ill-tempered, they're a lot more prone to getting fed up when they're tired, and that's likely to end with your swift introduction to the hard ground.

This isn't just a hypothesis either. During the Victorian era when Europeans were attempting to colonize parts of Africa, taming zebras was a popular idea. Their horses weren't that useful because in Sub-Saharan Africa, they were susceptible to a fatal disease commonly called "animal sleeping sickness," which is carried by tsetse flies. Zebras, meanwhile, almost never catch this disease because they're only rarely bitten by tsetse flies, possibly because the flies are put off by all those stripes, or maybe because tey have natural fly repellent in their skin. Whatever the reason, Europeans took note of this and famous attempted to domesticate the horse's striped cousin, and while there were a few individual -

- successes, this was - for the most part - an abysmal failure. 

Today, most people have given up on riding zebras, partly because we've come to our senses, and mostly because we have Jeeps now. Still, it goes to show that no matter what we mighty humans do, Nature can still sometimes get the upper hand. And that's probably okay.

[slide]

Hank: So. We don't successfully domesticate everything we touch, but we sure do give it the old college try.

Now aside from food, friends, and transportation, there's one big group that we have not talked about yet - us. Here is the totally-not-creepy way we accidentally domesticated ourselves.

[slide: "The Future of Human Evolution"]

Michael: Evolution has given us some useful abilities. Over the past millions of years, we've evolved to stand on two legs, use tools, and develop language. And as recently as a few thousand years ago, humans were still evolving. Part of what drives evolution is natural selection, where those with traits that make them better adapted to their environment tend to survive long enough to pass on those traits to their offspring. Modern medicine can sometimes interfere with that process, since it keeps people alive who maybe otherwise wouldn't be. But that doesn't mean humans are done evolving.

One recent example of human evolution is the fact that 35% of adult humans can digest milk; 7,000 years ago, not nearly as many people could do that. Instead, mainly babies drank and digested milk so they could get energy from the lactose in their mother's breast milk. Once they grew up though, the gene that allowed them to digest milk was switched off. But when we domesticated cattle, the mutation that allowed certain people to digest milk as adults became something that helped them survive, since they had an extra source of nutrition. A few thousand years later, being able to digest lactose is pretty normal.

So humans were still evolving in the not-so-distant past. And a lot of scientists think that we still are, and we might even be able to predict what future humans will be like. For one thing, it's possible that in the future, women will be shorter and slightly heavier than they are now. Researchers figure this out from using the data from the Framingham Heart Study, which tracks the medical histories of more than 14,000 people who live in Framingham, Massachusetts.

The team wanted to know what traits gave women higher reproductive success. They found that short, slightly heavier women tended to have more children, and those traits were passed on to their children.

If this trend continues, in 10 generations, the average woman might be about 2 centimeters shorter and a kilogram heavier.

Another possible difference between us and future humans is that their brains might be a bit smaller, which is weird, because for most of human history, brain sizes tended to get bigger over time. But over the last 20,000 years, the brain size of Homo sapiens has decreased. During that time, the average volume of the male brain has shrunk from about 1,500 cubic centimeters to about 1,350 - about a tennisball-size difference. A similar decrease in brain size happened for women, too. But why the change?

One possibility is that our brains are smaller thanks to the emergence of more complex societies. As population density increased and there was more division of labor, humans didn't have to be as smart to stay alive, so their brains didn't have to be as big. Other scientists think our brains have been shrinking because we've become more tame. See, domesticated animals have smaller brains than their wild counterparts. Probably because if a wild animal isn't smart enough, it won't survive too long. So, like domesticated animals, maybe our brains have gotten smaller because we aren't constantly worried about things like being attacked by predators.

But a shrinking brain might NOT mean the human species is getting dumber. Instead, our brains might just be getting more efficient. Having a big brain uses up a lot of energy, so if it's a little smaller and more efficient, we don't need as much energy to stay alive. That said, a recent study of the skulls of Americans showed that they've actually been getting bigger since the mid-1800s. But that's probably because of better nutrition and not an evolutionary change. So if we factor out the effect of better nutrition, it's possible that the shrinking trend will continue into the future.

Even if natural selection isn't what drives the future evolution of our species, we might artificially evolve ourselves through genetic engineering. As we learn more about what all of our genes do and how to modify them, we'll be able to eliminate certain genetic disorders and maybe even diseases related to getting older. And eventually, we might be able to design people like we design avatars in video games: altering things like height, intelligence, athleticism, or any other trait.

So whether the changes are natural or artificial, humans of the -

- future might be very different.

[slide]

Hank: So, we have domesticated plants like cauliflower and animals like dogs on purpose, we tried to domesticate zebras but couldn't quite pull it off, and some animals - like cats and people - were accidentally domesticated. We haven't domesticated everything in the world, but we sure do seem to do it more than most people consider, and that's not even the full list. To learn about the first mammal to be domesticated for scientific research, you can watch our video about the lab rat.

Thanks for watching this SciShow compilation.

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