Tetrapods represent the evolutionary step of life moving on to land, and these four-limbed vertebrates include amphibians, reptiles, and mammals like us. Now, these limbs, as we said, are derived from those muscular fish fins to support the organisms' weight on land. And eventually, digits, such as fingers and toes, will evolve to efficiently transfer muscle force. It's a common misconception that all of a sudden one of these fish just crawled up on land. Early tetrapods were actually fully aquatic, so they had four limbs, but they used them just to move around in the water, less in a swimming-like fashion and more in a walking-like fashion. And eventually, these organisms had to evolve a heart and a circulatory system in order to deliver sufficient oxygen to their limb muscles while they lived on land. So we're going to see one of the important features of these organisms is the closed circulatory system.
Now here in this image, you can kind of see the evolution of what you could almost think of as an in-between species. In between those lobe fin fish and the eventual tetrapods. This organism basically, a very early tetrapod, one of those fully aquatic guys, will eventually evolve into something more akin to an early prototype of an amphibian. And eventually, these lineages give rise to an organism like this guy behind my head, not my bed—God, I would be terrified if this guy was behind my bed. Never sleep in there again. But this organism, which is kind of going to be like a proto-reptile, sort of like this guy was the in between on the way to the tetrapod. This guy here behind my head, not my bed, is going to be that in between, on the way to reptiles.
So modern ancestors of the early tetrapods, as we've already said, are amphibians. And that word amphibian actually comes from Greek, it kind of means "double life," you could think of it as. And that's because these organisms are going to live both in water and on land. Now these organisms are ectothermic, meaning they are going to get some of their heat, their body heat, from external sources. As we said, they live in both water and land, and we're going to see their living ancestors in the forms of salamanders, frogs, and these guys you probably never heard of before called apodens, which kind of look like worms. Believe it or not, they are amphibians. It's kind of like a snake situation where you had reptiles with limbs and then some of them lost their limbs, and that's what gives rise to snakes, sort of similar to what's going on here, these apodens sort of meaning like without feet, these guys lost their limbs.
So as we said, ectothermic means that the main source of body heat is external to the organism, and those organisms absorb it. This is in contrast to the term endothermic, which means that the main source of body heat is internal as we have it, and that's because our metabolism generates that heat. In fact, this is something that we've talked about previously in a video when we discussed cellular respiration because our body actually uses ATP to help generate body heat. Anyway, amphibians do have lungs, but they also can respire through their skin, which is why their skin has to be kept moist.
Amphibians generally have this metamorphosis where they go from a larval stage in the water to a terrestrial stage. Although some amphibians live almost entirely on land, and some live almost entirely in water. So there's a lot of variation, but the important trend is that most of them will lay their eggs in the water, have this larval stage, and then undergo a metamorphosis to the terrestrial stage. So, with frogs, probably the easiest example to point to, you know, you have those tadpoles where they look like aquatic organisms, and eventually, they lose their long tail and turn into the adult stage of a frog after undergoing metamorphosis. That's all I have for this video. See you guys next time.