字幕表 動画を再生する 英語字幕をプリント There's a certain set of humans in the world that hears the "Why do I have to brush my teeth?" question pretty much every evening. Right around, say 8 o'clock. Almost always posed by the offspring of that person. Usually under the age of 10. And in those instances, the answer almost always is "Because your mother and I told you to!" But one of our followers on Tumblr asked a more thoughtful version of this same question: "Why do I have to brush my teeth from an evolutionary perspective?" The answer is still because your mother and I told you to! Okay, actually, it is a pretty cool question. Cause our primate ancestors didn't brush their teeth, and surely they didn't have their teeth falling out all over the place, and they weren't dying of gingivitis. Right? Right. Well some of them did, but not all of them. I mean all animals that have teeth likely have to deal with tooth decay to some extent. Veterinarians will tell you that they see cats and dogs all the time that have cavities or caries, as scientists call them. And biologists have reported finding wild animals as diverse as lions, and bears, and orcas with huge painful abscesses. But part of why we have to brush our teeth has to do with longevity. Most wild primates just don't live long enough for things like tooth and gum disease to catch up with them. We do. For example, the life expectancy of our nearest evolutionary relative the chimpanzee is just 40 years in the wild. In captivity, though, it's around 60. And tooth decay can become a problem with captive apes and monkeys, which is why keepers have to regularly brush their teeth. Which I imagine is not super fun. Our life expectancy, meanwhile, is around 80 in most industrialized countries, which means we're living twice as long as our closest genetic relatives. So we gotta keep our parts working for a lot longer than they were probably supposed to work. And this means brushing your teeth a couple times a day. Consider it a small price to pay for having benefited so much from evolution. But beyond that, by far the most important factor that affects our dental health, as opposed to that of our genetic predecessors, is our diet. Most primates live quite nicely on diets of fruits, and seeds, and nuts, and leaves. And in the case of chimps, the occasional insect or piece of meat. But since we started cultivating grain some 12,000 years ago, our diets have become totally dominated by starches and the simple sugars that they contain. The bacteria that form cavities really love these sugars, and for the same reason that we do: they are a very readily accessible form of energy. So the more sugar you eat, the more susceptible you are to tooth decay. And this is true not just for leftover Halloween candy and Coca-Colas, but any kind of cereal grain you can think of like wheat, or oats, or corn. The evolutionary connection between tooth decay and agriculture is so distinct. In fact that anthropologists can actually determine whether a culture started farming in part by studying when they started to get cavities. Again, it's just part of the cost of being such an awesomely successful species. Now seriously, brush your teeth! Go to bed! It's late; you got school in the morning! Thanks for watching this SciShow Quick Question. If you have a question that you would like us to answer, you can ask us on our tumblr, or our facebook, or our twitter, or in the comments of this video. And if you want to keep getting smarter with us, you can go to youtube.com/scishow and subscribe.