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  • 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.