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  • Stated Clearly Presents:

  • What is the evidence for evolution?

  • The theory of Biological Evolution makes two very bold claims about living creatures:

  • First: All living things on earth are related. They evolved from a common ancestor.

  • Second: The evolution of living things is powered by natural processes. Things which

  • can be studied and understood.

  • But is there really any evidence that these two claims are true?

  • Yes. There are so many observable facts from so many different fields of study that the

  • only way we can even begin to talk about them is to group them into categories or lines

  • of evidence.

  • To keep things simple, here well focus on Evolution’s first claim that: All living

  • things on Earth are related.

  • We cannot tackle the entire tree of life at once (after all there’s an estimated 8.7

  • Million species alive today), so instead well focus most of our attention on one fairly

  • small but fascinating branch of the evolutionary tree: Cetaceans. This branch includes whales,

  • dolphins and porpoises.

  • Biologist claim that all these creatures are closely related, and that the entire group

  • evolved from an ancient 4 legged land mammal.

  • Instead of taking their word for it, let’s look at the facts. Well start with a few

  • from field of comparative anatomy: the study of differences and similarities between living

  • things.

  • Whales live in water and from a distance, they sort of look like giant fish. A close

  • inspection of their anatomy however, tells us a very different story.

  • Whales, just like land mammals but unlike fish:

  • have placentas and give live birth

  • They feed milk to their young

  • They are warm blooded (which is extremely rare for a fish)

  • and whales do not have gills, instead, just like us, they breath air with 2, fully developed

  • lungs.

  • Whales don’t seem to have noses like mammals do. Instead they breathe through blowholes

  • coming out the tops of their heads. Some whales have two blowholes that almost look like nostrils,

  • but dolphins and porpoises only have one. Surprisingly, if you look at their skulls,

  • you find that the blowhole splits into 2 nasal passages inside the head. Could it be that

  • the blowhole is actually a highly modified mammal nose? It looks that way but well

  • need more evidence to be sure.

  • Many whales have hair, just like land mammals. In this photograph, you can actually see the

  • whiskers of this baby gray whale as he rests his chin on mama's back.

  • Strangely, whales have arm, wrist, hand, and finger bones inside their front flippers.

  • Here’s a photo of these bones, the same bones that bats, hippos and people have in

  • their front appendages: One bone, two bones, wrist bones and finger bones.

  • Modern whales do not have back legs but they do have a pair of strange tiny bones where

  • the hips and hind legs should be. Here’s a picture of these bones from a bowhead whale.

  • They almost look like shriveled hip, thigh, and shin bones. This one even has what looks

  • like a deformed ball and socket joint between the hip and thigh bone, just like the ball

  • and socket joint in your own hip. Is this resemblance a mere coincidence or are these

  • real leg bones? Perhaps leftovers from the whales evolutionary history?

  • Before we draw any bold conclusions, let’s see if a completely separate line of evidence

  • will confirm our suspicions.

  • Embryology is the study of how creatures develop before being born or hatching from an egg.

  • Here we see a dolphin and a human embryo, side by side, at similar stages of development.

  • Notice that they both have what look like arm buds, and leg buds. In humans, the leg

  • buds grow to become legs. In whales, they grow for a while, but then stop, effectively

  • fading away as the rest of the whale continues to grow.

  • These are all photographs of a common dolphin at different stages of development. Notice

  • that early on, we see two nostril grooves on the front of the face, just like you’d

  • expect in a puppy or a human.

  • As the dolphin continues to grow, the nostril groves migrate to the top of the head and

  • fuse together becoming the dolphin’s blowhole.

  • So far we have multiple facts from two independent lines of evidence, comparative anatomy, and

  • embryology, both telling us the exact same story: The ancestors of whales were once 4

  • legged land creatures! Will the fossil record act as a third witness confirming this idea?

  • These are two species of extinct basilosaurid whales!

  • These creatures are known from multiple well preserved skeletons. They appear to have lived

  • side by side roughly 34 to 40 million years ago.

  • In this photo we are looking down at the top of a basilosaurid skull. This is not a model

  • or a cast, these are the actual bones which were pulled from the ground. Notice that the

  • nasal opening is not on the top of the head like those of modern whales, and not at the

  • end of the snout like those of most land mammals. Instead their nostrils sit right in the middle,

  • this is an intermediate species, exactly what the theory of evolution tells us we should

  • find!

  • At the back-end of a basilosaurid’s body, there are small, yet fully developed hips,

  • legs, ankle, feet and we suspect they had at least 3 toes though weve only found

  • the bones for one.

  • These legs are far too small for walking on land, but may have been useful for mating

  • or scratching away parasites and itchy skin.

  • Evolutionary theory tells us that the further we go back in time, the harder it will be

  • to distinguish whales from regular land mammals.

  • Meet Maiacetus. Scientists have found multiple well preserved skeletons of this species,

  • one of which appears to be a pregnant mother.

  • The hip bones of Maiacetus do seem sturdy enough to walk on land, but this animal is

  • considered to be a whale for many reasons: Their skeletons have all been found among

  • fossils of sea-creatures

  • Their short legs combined with long flat fingers and toes, suggest they were strong swimmers

  • with webbed hands and feet.

  • Here we see the bottom side of a maiacetus jaw and skull as it looked at the dig site.

  • Her teeth match those of the basilosaurid whales we saw earlier.

  • And unique structures of her middle ear bones, the bulbs behind her jaw, match those of basilosaurid

  • whales and modern whales.

  • Maiacetus appears to be, a walking whale!

  • The fossils of many ancient whale-like mammals have been found, and people continue to find

  • more. Together, these fossils blur the line between 4 legged land mammals and fully aquatic

  • whales, solidifying the idea that whales indeed, evolved from land creatures.

  • Now lets look at a 4th line of evidence: DNA?

  • DNA molecules contain chemical codes which act like recipes for living things.

  • Without ever looking at bones, embryos, or anatomy, researchers can compare the DNA code

  • of different living creatures to find out who is most closely related to who.

  • Whale DNA has been compared to all kinds of other animals: fish, sea lions, you name it,

  • and so far, the closest genetic match, is to the pudgy, water-loving hippopotamus.

  • This does not mean that whales evolved from hippos, but if this genetic finding is correct,

  • whales and hippos both evolved from a common ancestor which lived roughly 54 million years

  • ago.

  • At first the link between whales and hippos surprised researchers. Whales are mainly carnivores

  • - they eat things like fish and small crustaceans, while hippos are mostly vegetarian.

  • A closer look however, reveals that hippos and whales, actually share many strange features,

  • some of which may have come from their common ancestor.

  • Ancient walking whales have specially shaped ankle bones, found only in hippos and the

  • close relatives of hippos, hippos, just like whales, often give birth and even nurse their

  • young underwater, they both have multi chambered stomachs (which is common for herbivores but

  • unheard of in fish-eating mammals), they are both missing a coat of fur, and here’s a

  • fun fact - whales and hippos are some of the only mammals on earth that have internal testicles.

  • So there you have it, dozens of facts from 4 independent lines of evidence, all tell

  • us the exact same story, whales evolved from 4 legged land mammals, but the history of

  • whales isn’t the only evolutionary history that weve been able to work out.

  • We know from fossils, DNA, embryology and many other lines of evidence that bird wings

  • are actually modified arms and claws! Birds evolved from dinosaur-like ancestors.

  • We can also clearly see that bat wings evolved from 5 fingered hands, similar to those of

  • monkeys and shrews.

  • Weve found that humans share a fairly recent common ancestor with chimpanzees, that mammals

  • evolved from reptile-like creatures, those reptile-like creatures evolved from amphibian-like

  • creatures, those amphibian-like creatures evolved from fish-like creatures, and fish

  • if you go back far enough, share a common ancestor with segmented worms.

  • So to sum things up, thousands of observable facts from completely independent fields of

  • study, are coming together to tell us the exact same story.

  • All living things on earth are related.

  • I’m Jon Perry and that’s a basic overview of the evidence for evolution, Stated Clearly.

  • Thanks for watching our show, if you enjoyed it, be sure to share it with your friends

  • on facebook and twitter.

  • If you want to learn more about whale fossils, and who doesn't, we have links in the description

  • of this video which take you to articles on our website.

  • Special thanks to Tyler Proctor, Zaid Ghasib, Anthony Danzl, Leon Z Newman, Rosemary Mosco,

  • and Jordan Collver. All of these folks volunteered their time and talents to make this animation

  • happen.

  • Special thanks to Dr. Philip D. Gingerich and Dr. Hans Thewissen. They both volunteered

  • hours of their time advising us, and preparing images of fossils and embryos.

  • I'm proud to announce that for the first time in Stated Clearly's history, this animation

  • was completely paid for with contributions from our viewers. If you are able to help

  • support our please visit us at statedclearly.com and click "contribute"

  • So long for now, stay curious.

Stated Clearly Presents:

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進化の証拠とは? (What is the Evidence for Evolution?)

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    Zenn に公開 2021 年 01 月 14 日
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