by Jason Amores Sumpter
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tetrapods represent the evolutionary step of life moving on tow 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 will be evolved to help. As digits is in fingers, toes, that sort of thing will be evolved. Thio help 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 these four limbs. But they used them just to move around in the water, less in a swimming like fashion and Maurin a walking like fashion. And eventually, uh, these organisms had thio evolve heart and 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 sort of this. You could almost think of this as an in between species in between those lobe fin fish and the, you know, eventual tetrapods. This organism basically a very early tetrapod, one of those fully aquatic guys who will eventually evolved into something more akin to this guy, which is going to be, you know, sort of like your amphibian. You know, early early, uh, sort of prototype of an amphibian. And eventually we're going to see these or these, uh, lineages give rise to a new organism like this guy behind my bed in my head. Rather 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 reptiles, sort of, uh, you know, 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 Thio reptiles. So modern ancestors of the early tetrapods, as we've already said, our M for Vivian's and that word amphibian actually comes from Greek. It kind of means a 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 ecto thermic, meaning they are going thio get some of their heat, their body heat from external sources, Azzawi said. They live in both water and land, and we're going to see, uh, they're living ancestors in the forms of salamanders, frogs and these guys you probably never heard of before, called ape Oden's, which kind of looked 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 right. And that's what gives rise Thio snakes sort of similar to what's going on here. These ape Oden's ape Oden sort of meaning like without feet. These guys lost their limbs. So as we said, extra thermic 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 endo thermic, which means that the main source of body heat is internal, like we have it, and that's because our metabolism generates that heat. In fact, this is something that we've talked about, uh, previously in a video when we discussed cellular respiration because our body actually uses a teepee to help generate body heat. Anyways, uh, amphibians do have lungs, but they also can respond iron through their skin, which is why their skin has to be kept moist. Um, and amphibians uhh. Also generally have this metamorphosis where they go from a larval stage in the water to a terrestrial stage. Although some amphibians, uh, 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 sort of larval stage and then undergo a metamorphosis to the terrestrial stage. See, uh, with frogs. Probably the easiest example. 2.2. You know, you have those tadpoles. Where there they look like aquatic organisms. And eventually they lose their long tail and turn into the adult stage of frogs after undergoing metamorphosis. That's all I have for this video. I'll see you guys next time