Animal development: We're just tubes발음듣기
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Animal development: We're just tubes
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Do you know that? Today we're going to talk about animal development and the miracle of life.발음듣기
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The process that animals go through to turn, like a sperm cell and an egg cell into a multi-cellular organism is incredible. No, it's not just incredible.발음듣기
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It's unbelievably, transcendentally magnificent man, magnificent and dudes, the thing is, we're all just like, tubes.발음듣기
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(music) So animals, they come in all sorts of shapes and sizes and like smartness-es and things.발음듣기
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And in our infinite wisdom, humans have come up with a system for classifying animals based on how similar they are to each other.발음듣기
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Today we're going to be talking about some differences between animals at the phylum level, here.발음듣기
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That's because a bunch of really big decisions are made within a few moments of the sperm fertilizing the egg and how this early embryonic ground work is laid makes a big difference when it comes to what kind of amazing, multi-cellular being you're gonna end up being.발음듣기
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Signs of an animals complexity include; how symmetrical it is, how many organs it has, and how specialized it's cells are.발음듣기
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A sea sponge, for instance, is a total fricken' mess symmetry-wise and it doesn't really have any organs to speak of.발음듣기
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In fact, if you were to like blender-ize a live sea sponge and then leave the sponge smoothie to settle overnight, you'd wake up the next morning to find the surviving cells had found each other and were reforming themselves into a sea sponge again.발음듣기
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My point is, that most animals are much more complicated than sponges and an animals complexity has everything to do with what happens in the first couple hours of it's development.발음듣기
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The more complex an animal is, the more it resembles a tube with some different stuff layered around it and that is when you're like, "Uh, Hank. Uh, what?"발음듣기
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A really important clue indicating that you're dealing with a complex life form is how many layers of tissue it makes in it's very early stages of development. Sea sponges make just one.발음듣기
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Remember sperm cells and egg cells are both gametes, haploid cells that only carry one set of chromosomes.발음듣기
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Once the sperm fertilizes the egg the two haploid cells fuse their information together and form a zygote.발음듣기
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One beautiful diploid cell with two sets of chromosomes that contain all of the instructions needed to create a new living thing.발음듣기
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Fast forwarding to like an hour-and-a-half after fertilization, the zygote has started divided and cleaving through mitosis resulting in two, four, eight, 16 cells, until it creates a solid ball of 32 cells.발음듣기
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Hank: And the morula actually looks a lot like a raspberry or a mulberry, which is what it's named after in Latin. Mmm, juicy. Morula pie.발음듣기
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(laughing) As more cells are created, these solid wad of cells begins to secrete the fluid that forms a space in the center, resulting in a hollow sphere of cells called a blastula.발음듣기
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And yeah, you go ahead and get your giggles out now because I'm going to be saying anus a lot in this video, for example, right now.발음듣기
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Sponges don't have a mouth or an anus and there are also other animals like your sea anemones, your jellyfish, your corals, that have just one hole that serves as both mouth and anus. (laughing) Aren't you glad we're a little bit more complicated than that.발음듣기
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That is their mouth hole/poo hole and that is a little bit more sophisticated than having no symmetry at all, like a sponge, but just barely.발음듣기
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But more complex animals with the notable exception of the echinoderms, like starfish and sand dollars, exhibit bilateral symmetry.발음듣기
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Something else we have in common is that we have an anus that is, get this, in a different place than our mouth.발음듣기
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This separation is pretty key because it means that we, as animals, are basically built around a tube, a digestive tract, with a mouth at one end and an anus at the other.발음듣기
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So, when we left our little blastula it was still just hanging out a little round hollow ball of cells.발음듣기
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This place on the blastula that starts to invaginate or fold in on itself is called the blastopore.발음듣기
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Now, for animals whose mouth and anus are the same thing, this is where the development stops, which is why they only have one hole for all their business, but in everything else the invagination continues until the indentation makes its way all the way through and opens on the other side creating what is essentially a hollow bead made of cells. Now we have a gastrula.발음듣기
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Now two different things can happen at this point, depending on what kind of animal this is going to be.발음듣기
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It can either be an animal whose mouth is the orifice that's formed by the blastopore, called a protostome or one whose anus is the structure that's created by the blastopore and that's called a deuterostome.발음듣기
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Chordates, that is to say, all vertebrates and a couple of our relatives like starfish are deuterostomes.발음듣기
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Meaning that we were once just a butt hole attached to a little wad of cells and that includes you and me, congratulations!발음듣기
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And hopefully you're getting the idea here the formation of the digestive tract is the first thing that happens in the development of an animal and it happens to every living thing, whether it's going to be a tardigrade or a polar bear or a [tepane]. The miracle of life!발음듣기
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An outer layer called the ectoderm and an inner layer called the endoderm and these are called your germ layers.발음듣기
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For those organisms that stop developing at this point with that classy mouth/anus combo, they only get two germ layers.발음듣기
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It's totally okay, but for us more complex animals whose mouths are separate from our anuses, yes!발음듣기
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Here the ectoderm is going to end up being the animals skin and nerves and spinal cord and most of it's brain, while the endoderm ends up forming the digestive tract, the esophagus and stomach and colon and stuff.발음듣기
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And in addition, some of the cells start breaking off between the endoderm and the ectoderm and form another layer called the mesoderm.발음듣기
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These cells will eventually end up as the muscles and the circulatory system and the reproductive systems and in the case of vertebrates, most of the bone.발음듣기
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Awesome! From here, this little guy is gonna go on to fulfill his destiny as a ladybug or a walrus or whatever.발음듣기
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And now this seems to me like a great time to take a look at a completely dis-proven theory that biologists hold in the highest contempt, but which is actually a kind of useful way to think about the way that an animal embryo develops into a fully formed animal.발음듣기
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(music) Back in the mid-1800's a German zoologist named Ernst Haeckel tried to prove what we now refer to as recapitulation theory.발음듣기
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Basically, and this is not basic at all, recapitulation theory states that, "Ontogeny recapitulates phylogeny."발음듣기
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Uhh! In other words, ontogeny, or the growth and development of an embryo recapitulates or sums up phylogeny, which is the evolutionary history of a species.발음듣기
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So this means, for instance, that a human embryo over the course of it's development will go through all of the hundreds of millions of years worth of evolutionary steps that it took for a single celled organism to evolve into a fully tricked out person.발음듣기
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Haeckel was a contemporary of Darwin and on the origin of species, made a giant impression on him, especially a section of it that notes how cool it is that all vertebrate embryos look pretty similar to one another, regardless of whether they're a mammal or bird or reptile.발음듣기
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Darwin, however, cautioned that this probably wasn't a very good way of reconstructing the history of evolution.발음듣기
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He just thought it meant that the embryological similarities were evidence of common ancestry between species.발음듣기
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Well, Haeckel was kind of a spaz and he definitely heard the first part of Darwin's idea, but not the rest, so Haeckel jumped onto this idea and very quickly wrote a couple of books about how the development of an embryo mirrors the evolutionary development of adults of a species, which is exactly what Darwin said was not happening.발음듣기
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Anyway, Haeckel did spend a lot of time looking at embryos and observed that the slits in the neck of a human embryo resembled the gill slits of fish, which he took to mean that we must have, at one point, had a fish-like ancestor.발음듣기
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He drew tons of figures of different animal embryos in different stages of development to prove his theory and his illustrations of embryos started to make their way into textbooks all over the world.발음듣기
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Haeckel is exactly the sort of person who really ticks other scientists off because real science loving scientists like to sit and think about stuff and find out all the problems with an idea before they start publishing books about it.발음듣기
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And here, Haeckel was firing off volume after volume and before long all the "data" he had "collected" convinced a bunch of other people, including Darwin actually, that he was on to something, but in the end it turned out that Haeckel was kind of fiddling with his drawings of embryos to make the data fit his recapitualtion theory instead of, you know, making the theory to fit the data,발음듣기
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but by that time everybody already knew about the theory and if there's anything harder than teaching people something, it's un-teaching them something.발음듣기
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So here we are, almost 150 years later, and we're still talking about the recapitulation theory, but that might have less to do with the stubbornness of a bad idea, than it does with the fact that it actually makes a kind of sense when you don't take it literally.발음듣기
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At some point in our embryonic development, humans actually do have gill slits like a fish and tails like a dog or a pig or a jaguar and webbed fingers and toes like a frog.발음듣기
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So, while it's not true that every zygote reenacts all of animal evolution, the way that an animal develops does remind us that we are, in fact, related to other chordates.발음듣기
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