字幕表 動画を再生する 英語字幕をプリント Guys, have you ever looked at your hands, and you're like, why do I have these? Hey, peeps, this is D News. Thanks for watching. Sometimes when you're sitting around in your friend's basement and you're all zoned out, and you look down at your heavy limbs and you start pondering, why do we have four of these, man? Bats got four, and monkeys and lizards and frogs and birds and drag-- you get the idea. I'm not saying that's what happened. But a team of scientists from the University of Vienna wanted to know exactly why limbs always come in pairs, and published their model in the journal Evolution and Development. By the way, it has to deal with our tummy. I like to point out that this is the hypothesis. The researchers have created a model. And now that it's published, other researchers can begin to study it. They will pick it apart for accuracy and tweak it over time. This is science in action. Of all the known species on the planet, both living and extinct, those with both a backbone and a jaw all have paired appendages . Using information from molecular embryology, paleontology, and classical morphology, the researchers examined all these jawed vertebrates known as gnathostomes in as basic a state as we can, by looking at their gene expression and embryonic development. Their model explains the reason we have paired appendages is because of the three types of basic tissues in a developing embryo. The ectoderm becomes the skin and nervous system, the mesoderm, which forms muscles, bones, and organs, and the endoderm, which forms the digestive tract. Usually the mesoderm splits into two layers to line the body cavity and the gut. And if those layers are sufficiently separated, they'll form into limbs right below the gut. Why? Because the developing gut keeps the layers nearby too close together to form those appendages. As the body forms, the end of the digestive tract fits between the legs, and everything else just kind of closes up down there. Above the gut, the mesoderm and ectoderm are messing around. And they form another set of limbs with the separated tissue. Two sets of opposing appendage pairs, all because that digestive tissue sits in the middle. Thanks, belly. If this works out, we'll have a great theory on why all the gnathostomes have two pairs of limbs. We'll have to wait and see if the science pans out. Would you rather have more arms? Maybe another set, like down here or something? Where? How? Grab your keyboards and head down to the comments to share your [INAUDIBLE] dreams. Thanks a lot for watching D News, everybody.