If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Comparative anatomy: What makes us animals

Hank introduces us to comparative anatomy, which studies the similarities and differences in animal anatomy to support the theory of evolution and the shared ancestry of living things. Created by EcoGeek.

Want to join the conversation?

  • leafers ultimate style avatar for user Prithvi Shetty
    Are sea cucumbers considered as a animal ?
    (7 votes)
    Default Khan Academy avatar avatar for user
  • hopper jumping style avatar for user Rory Avera
    How does blood replenish a part of the body(like arms, or legs)?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • winston baby style avatar for user Ivana - Science trainee
      Who told you that? Are we speaking of humans?

      No, blood per se cannot replenish missing limb. Even reprogramming stem cells is quite hard to do to grow new limb (since many tissues and neural signaling is evolved and neural tissue almost impossible to regenerate).

      Blood alone can only deliver oxygen tot he tissues (I mean that is not only but super important role). It does not play any regenerative role.
      (2 votes)
  • aqualine ultimate style avatar for user Krithik Venugopal
    At and many other times it is "unless you're a sponge" which leads me to wonder, why are they even considered animals with such differences?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • male robot donald style avatar for user illicit
      Sponges are... sponges for a lack of a better word. They are "animals" in that they are multicellular creatures that "eat" and have "sex". Sponges are pretty much that kid in class that only shows up to class hoping for participation points for being there. They are primarily made up of undifferentiated/specialized cells, and their main goals in "life" are to maximize surface area and efficiency, and also to procreate. Imagine a somewhat rigid and responsive mess of hermaphroditic stem cells that knew mainly how to become 3 things... digestive epithelial type cells, sperm type cells, and egg type cells... this jumbled mess would do what any such mess would do... grow towards food and make sex cells to release/receive. Sponges show that they are multicelled creatures that can be adaptive, eat, "breathe", excrete, and procreate... which is enough to earn the token "unless you're a sponge" in any related bio conversation.
      (2 votes)
  • starky sapling style avatar for user Eric
    Clovers and other weeds slowly spread on the ground does that count as moving?
    (4 votes)
    Default Khan Academy avatar avatar for user
  • leaf orange style avatar for user Panzer Selbstfahrlafette IVc
    Why is the Sea-Sponge the only exception? Are there no similar animals?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user lg64
    I thought that in convergent evolution, the organisms developed similar traits, but they lived in different environments. Please correct me if I'm wrong.
    (1 vote)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user 😊
    Where the collagen fibres are present?
    (2 votes)
    Default Khan Academy avatar avatar for user
  • eggleston blue style avatar for user kevenw4446
    i love gordon the plant
    (2 votes)
    Default Khan Academy avatar avatar for user
  • piceratops tree style avatar for user rollandsroshan91
    Since all living things originated from one ancestral prokaryote, does that mean that there was first divergent evolution followed by convergent evolution ? If so, isn't convergent evolution some attempt by nature to return all species to a more sophisticated version of the aforesaid ancestral prokaryote?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user Just Keith
      I think you are misunderstanding convergent evolution or perhaps just evolution in general.

      Nature is NOT attempting to do ANYTHING via evolution. Nature has no mind, has no plans, has no intentions or desires (apart from what goes on in the brains of organisms with brains). Evolution has neither a plan nor a goal.

      Convergent evolution is not a goal nor anything nature is "trying" to do. Instead, it is just a fact that certain body layouts are inherently better for certain environmental niches than others. For example, for burrowing deep underground and not often coming to the surface, pointed heads with a cylindrical body shape is inherently better than other shapes because any body part that sticks off to the side can cause the animal to get stuck. The pointed head helps reduce the effort of digging. Thus a very large number of burrowing animals, no matter how distantly related, tend to evolve toward a cylindrical body shape with a pointed head.

      But, no, nature is not attempting to return eukaryotes to being prokaryotes -- nor, for that matter, anything else. Nature is not attempting anything. It is just a matter of what works adequately in a given environmental niche: those with body layouts and other traits that are more adequately suited to their environmental niche tend to be more reproductively successful than those that are less well suited to their environmental niche.
      (5 votes)
  • leaf orange style avatar for user Click here
    At , Hank says that an organism probably developed both a muscular and digestive system 1.6 billion years ago. Necessarily, it would take evolution to develop such structures. Since they are a system, one organ couldn't exist without the other. How could all the necessary organs be developed at the same time, given that evolution is a gradual process?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Charles LaCour
      You are using the argument of irreducibility but you are assume that each part has always existed the way it is not. When the parts originally started to collaborate they were able to operate independently but over time the duplicated functions disappeared because it was inefficient to keep both.
      (2 votes)

Video transcript

- Hi, I want you to meet my friend Shoshana, she's a zebra finch and she's very good at it. She's here to help me talk about comparative anatomy which is the study of similarities and differences between the anatomies of animals. We study comparative anatomy because it helps us learn more about our evolution and our shared ancestries. Organisms have their evolutionary history written all over them if you know what to look for. For instance, which of these two living organisms would you say I'm more closely related to? Shoshana the finch, or Gordon the plant? This isn't a quiz, but sure, sure, it is a quiz, it is the easiest quiz that you will ever take in your life. (upbeat music) Well Gordon is green and can make is own food with just sunlight, water, carbon dioxide, while Shoshana can't make her own food. She has to move around to find stuff to eat, to escape predators, to find mates, to poop on bark benches, just like me, except not the pooping on park benches, I mean the moving around. So yeah, shocker, I'm more closely related to a bird than to a plant. You get a gold star. So that one's obvious, but as the relationships between organisms get closer, the questions get a lot more interesting. So what is an animal? I mean I know you know what an animal is, but when you're looking at Shoshana and me here, what clues you in to the fact that we are members of the Kingdom Animalia? Two things. For starters, we're both moving. Locomotion is a really good sign that an organism is an animal, unless you're a sponge. Now I know what you're thinking though, protists, bacteria, and archaea they all move around using flagella and cilia, but they also only have one cell. It's the multicellular locomotion that's so peculiar and specific to animals. So animals move because of the second trait that we have in common, we're heterotrophs. We get our energy from eating other lifeforms. Locomotion also helps us avoid predation and seek out mates for reproduction. Now plants, they can mate by dispersing their seed to the wind or having an insect come by and fertilize them. But if land animals did that, things would get, like really messy and gross. Some aquatic animals actually do just release their sex cells into their surroundings and cross their fingers and presumably close their mouths and hope that somebody gets pregnant. So since animals have to eat and move around they evolved anatomical forms that help them do those things. But obviously those forms are the same in all animals. For instance, in order to move Shoshana and I both have to be able to apply force to the ground or the air to propel ourselves. Here is me pushing off the ground with my feet. And now here's Shoshana applying force to the air with her wings which keeps her afloat and moving. And if I had a shark in the studio with me, which thankfully I do not, so I'll just pretend to be a shark, my fins would apply force to the water to propel me forward. I have to be careful with this stuff, because even though similar body structures like fins or wings or feet can mean animals have a close common ancestor, it can also mean that they animals just evolved similar forms because that's the best structure for the job. When this happens it's called convergent evolution. For example, a tuna, a penguin and a seal are all animals that spend all or a lot of their time in the water. One's a fish, ones's a bird, and one's a mammal, but all three of them have a suite of similar features, the most notable being a really sleek fusiform body that can move through the water like nobody's business and fins for propelling those bodies. But of course those three animals have very different evolutionary origins. Each of these three marine animals have independently converged on similar body shapes because they live in the same environment and need to do the same sorts of things. So instances of convergent evolution can make linking physical structure of an animal to its evolutionary history a little bit tricky, which is why for a long time nobody really put much stock into comparative anatomy as proof of evolution. That is, until Thomas Henry Huxley came along. (lively ragtime piano music) Thomas Henry Huxley was the father of comparative anatomy and the father of modern paleontology and he invented the word agnostic to describe his spiritual views and he was the first person to conclude that birds evolved from small carnivorous dinosaurs. (sighs) I'm glad I'm sitting down for this. Plus we have much respect for his facial hair. Huxley was born in England in 1825 and though he started out as a doctor after serving as a ship's surgeon on a voyage to Australia in his 20s, he took to studying marine invertebrates. During his voyage he sent all of his papers back to England and when he got home he found that he had become a kind of famous marine invertebrate expert and he was admitted into the Royal Society. Huxley made friends with some other hot shot natural scientists including Charles Darwin and a few years later, when Darwin outlined his theory of evolution and on the Origin of Species, Huxley is reported to have said, "how extremely stupid to not have thought of that." In fact he became such a huge Darwin supporter that everybody started calling him Darwin's bulldog because threatened to cut the fool who badmouthed evolution. This is a good one. Huxley said when he was talking about On the Origin of Species, "old ladies of both sexes consider it "a decidedly dangerous book." You just got Hux slapped. With this new tool of the theory of evolution, and in part to help promote the theory of evolution, Huxley connected paleontology and biology together by looking for similarities in anatomy in the fossil record, where he found all kinds of interesting stuff, like some really obvious similarities between prehistoric horse fossils and modern day horses as well as between dinosaurs and birds, though nobody really bought his insights into the resemblance between birds and dinosaurs for another 100 years. And just in case you were still on the fence as to whether intelligence is heritable, Thomas Henry Huxley is the grandfather of Brave New World writer Aldous Huxley and of Sir Andrew Huxley who won the Nobel Prize for Physiology or Medicine in 1963. And because all animals come from the same evolutionary origin, in addition to sharing some anatomical structures like Huxley studied, we're also built from the same rudimentary blueprint. Our cells work pretty much the same not matter what sort of animal we are. So while animals have different strategies for moving around and acquiring food, once the food is gotten, all animals break it down, turn it into useful energy and distribute nutrients and eliminate waste in pretty similar ways, unless you're a sponge. Each of those functions is performed by collections of cells that group together in the body to form tissues. There are four primary types of tissues in the human body: the epithelial tissue, the connective tissue, muscle tissue, and nerve tissue. Epithelial tissue is formed by cells that bind very closely together. A layer of it covers every organ and lines the digestive tract to prevent crazy acids and poop and stuff from going where it's not supposed to go. Epithelial tissue can also produce a slippery fluid to let your organs slide over each other like the membrane that lines the inside of your ribs so that your inflating lungs don't build up friction as they expand. Most types of connective tissue are made up of fibrous strands of collagen protein and it adds support and structure to your body and holds your parts together. Some examples of connective tissue include the inner layers of your skin, your tendons, ligaments, cartilage, and bone. But oddly enough, connective tissue isn't defined by it's ability to connect, but instead by the presence of an extracellular matrix, meaning that part of the tissue extends outside of the cell. And so somewhat confusingly, blood and fat are also considered connective tissues. Muscle tissue is made up mostly two specialized proteins, actin and myosin, which can slide past one another and allow for movement. It also includes a bunch of other proteins including that longest word in the world one, titan. And finally there's nerve tissue, which generates and conducts electrical signals in the body. These electrical messages are managed by the nerve tissue in the brain and transmitted down the spinal cord to the rest of the body. Nerve tissue is made up of two types of cells, the neurons which do the electrical work and the glial cells which insulate and support the neurons. These tissues are then organized into organs which perform different functions in the body and these organs work together in organ systems. For instance, most animals have a digestive system made up of like a mouth and an esophagus and a stomach and intestines and an anus. And a lotta animals have a skeletal system made up of bones and tendons and ligaments and cartilage. We're gonna be talkin' about each of these systems in a lot more detail in a few weeks. These organ systems, like many different kinds of anatomical structures are shared by lots of different kinds of organisms, unless you're a sponge. Because about 1.6 billion years ago, an organism developed that had a digestive system and a muscular system and suddenly that organism was in it to win it. That organism was the common ancestor for all animals today and it's the reason why Shoshana and I are gonna hang out at the animal family reunion.