字幕列表 影片播放 列印英文字幕 Congratulations! This is our last episode of our section on Evolution and Genetics, which puts us at the halfway mark of CrashCourse Biology. So far we've learned about DNA, genetics, natural selection, how cells multiply, populations, speciation, replication, respiration, and photosynthesitation. I'm so proud of you. But I couldn't let this section end without discussing the iscussion that everybody can't help but discuss these days: Evolution. It's a thing. It's not a debate. Evolution is what makes life possible. It allows organisms to adapt to the environment as it changes. It's responsible for the enormous diversity and complexity of life on Earth, which not only provides organisms with sources of food and some healthy competition. It also gives us some truly awesome stuff to marvel at. And even though evolution makes living things different from one another, it also shows us how we're all the same. All of life, every single thing that's alive on the Earth today, can claim the same shared heritage, having descended from the very first microorganism when life originated on this planet 3.8 billion years ago. There are people who will say that this is all random- It's not. And that this clumsy process could not be possible for the majestic beauty of our world. To them, I say, well at least we agree that our world is beautiful but, well you're probably not going to enjoy the rest of this video. To me, there are two sorts of people in the world, those who are excited about the power and beauty and simplicity of the process of evolution, and those who don't understand it. And somehow, I live in a country where only 40% of the population believes that evolution is a thing. The only possible reason for that that I can accept is that they just don't understand it. It's time to get real, people. First, let's understand what we mean when we talk about the theory of evolution. Evolution is just the idea that gene distribution changes over time, which is an indisputable fact which we observe all the time in the natural world. But the THEORY of evolution is a large set of ideas that integrates and explains a huge mass of observations from different disciplines including embryology, paleontology, botany, biochemistry, anatomy and geophysics. In every day language, the word "theory" means "hunch" or even "hypothesis." But in science, a theory is an idea that explains several phenomena at once. Thus, The theory of evolution is a bunch of ideas that explain many things that we, as humans, have observed for thousands of years. It's the theory that meticulously and precisely explains the facts, and the facts are indisputable. So let's spend some time going through the facts, and how evolution explains them all so well. First, fossils: The fossil record shows that organisms that lived long ago were different from those that we see today. Sounds obvious, but two hundred years ago it seemed a little bit crazy. When scientists first started studying dinosaur fossils in the 1820s, they thought that all dinosaurs were basically giant iguanas. That's why the first fossil dinosaur was named Iguanodon. It wasn't until the fossils of two-legged dinosaurs started showing up in the 1850s that scientists had to grapple with the idea that organisms of the past were somewhat similar to ones today like, dinosaurs were reptiles, but many of them took on a diversity that's barely recognizable to us. And of all those ancient not-really-iguanas were all extinct, either dying out completely or evolving into organisms that survive today, like birds. Fossils make it clear that only evolution can explain the origin of these new kinds of organisms. For instance, fossils taught us that whales used to walk. Whales are cetaceans, a group of mammals that includes porpoises and dolphins, and biologists long suspected that whales descended from land mammals. Partly because some modern whales still have the vestigial remnants of a pelvis and hind-limb bones. But it wasn't until recently, the 1990s and 2000s, that the pieces really came together. First, paleontologists discovered fossils of DOR-oo-dons, cetaceans that had different skulls from modern whales but still had the same vestigial leg bones. Then they found even older fossil remains of another cetacean that actually had hind legs and a pelvis. The pelvis wasn't fused to the backbone like ours is, so it did swim like a whale, but more importantly, it still had ankle bones And they were ankle bones that are unique to the order that includes bison, pigs, hippos and deer. So by following these clues left behind in fossilized bones, paleontologists were able to track the origin of whales back to the same origin as bison and pigs. This leads us to another series of facts that evolution explains: Not how animals were different, but how they are incredibly similar. Last week we talked about Carl Linnaeus and how he classified organisms by their structural similarities. Well he didn't know anything about evolution or genetics, but when he began grouping things in this way, he hit upon one of evolution's most prominent clues: homologous structures. The fact that so many organisms share so many finely detailed structures shows us that we're related. Let's go back to the whale. Like my dog, Lemon, and me, the whale has two limbs at the front of its body, its front flippers. And so does this bat, its wings. Inside our limbs we all have the very same structure: one longish bone on top, connected to two thin bones at the joint, followed by a cluster of small bones called the carpals, and then our fingers, or digits. We each use our forelimbs for totally different purposes: the bat flies, the whale swims, Lemon walks and I... you know, jazz hands! Building limbs like this isn't the most efficient way to swim or fly or walk. Our limbs have the same structure because we descended from the same animal, something like this more-gan-uh-cah-don here, which, yeah, has the same forelimb structure. In the first stage of our existence, every vertebrate looks almost exactly the same. Why? Because we're all descended from the same initial vertebrates. So our structures are the same as other mammals and other vertebrates, sure, but it also turns out that our molecules are the same as, like, everything. In fact, if we were ever to find life on Mars or something, the sure fire way of knowing whether it's really extra-terrestrial is to check and see if it has RNA in it. All living things on our planet use DNA and/or RNA to encode the information that makes them what they are. The fact that we all use the same molecule itself suggests that we are all related, even if very distantly. But what's more, by sequencing the DNA of any given creature, we can see precisely how alike we are. The more closely related species are, the more of the same DNA sequences they have. So the human genome is 98.6% identical to that of the chimpanzee, our closest evolutionary relative, and fellow primate. But it's also 85% the same as a mouse. And I wonder how you're going to feel about this, about half of our genes are the same as in fruit flies, which are animals, at least. So, just as your DNA proves that you descended from your parents, your DNA also shows that you descended from other organisms and ultimately, from that one prokaryotic microorganism 3.8 billion years ago that is the grandparent of us all. Now when it comes to species that are very similar, like say, marsupials, their distribution around the world or their biogeography, is also explained extraordinarily well by the theory of evolution. Animals that are the most similar, and are the most closely related, tend to be found in the same regions, because evolutionary change is driven in part by geographical change. As we talked about in our speciation episode, when organisms become isolated by physical barriers, like oceans or mountains, they take their own evolutionary courses. But in the time scales we're talking about, the geographical barriers are much older, and are often even the result of continental drift. So, marsupials.