Eli5: How was the first ancient animal to ever step (foot?) out of the water, able to survive breathing air instead of water?
In: Biology
Never seen a frog? Never caught a fish that can literally jump its way back to the river bank and then just swim off?
Evolution doesn’t happen immediately. It’s countless billions of tiny steps, with *many* (but not all) a slight improvement on before. A fish that can last two seconds when it’s accidentally plucked out of the water by a wave or beaching itself is *a tiny, tiny, tiny bit* more likely to survive and reproduce than one that can only last 1 second. Thus creating the potential for a fish that can survive 2.1 seconds out of the water without ill effect.
And so on. For millions upon millions of years, sea creatures ruled the ocean and they were beached, washed up, caught up in hurricanes, etc. Eventually, after enough time, this creates something which was probably amphibian – frogs, toads, that kind of thing. Then that evolves further millions of times until it’s actually discovered that it’s the only thing on land and, hey, look at all this food, and no predators yet! And now it has *slightly* more chance of surviving, reproducing, and creating offspring ever-so-slightly more adapted to life on land.
Even the humble woodlouse is actually evolved from shrimp (and they still taste like it, apparently!). A tiny little sea-insect that realises it can survive for a moment on land, so long as it doesn’t dry out completely, and a few million years later, you have a bug that lives in your house and still likes damp areas but can survive perfectly well on land during the summer too.
There was no first animal to do this… there were probably countless millions of individual animals that died in all kinds of new, interesting and unfortunate ways, even if they could have been the best tree-climbers in the world before they ever saw a tree. There was no one “first leg”, there was no one “first air-breather”, there was no one “first walker”, and so on. Countless BILLIONS of individual animals, from microscopic algae to huge sea creatures like whales, were just chugging along for millions of years doing what they do. And every now and then some of them happened to survive better in a little rock pool, or got washed ashore into a lake, or ended up swimming the wrong way and ended up upstream in a river, or a mangrove, or a swamp, or a puddle, or a bog, or any (and all) of those kinds of things. And they just tried to survive by any means in their capability. And the ones who did survive? They had kids. The ones who didn’t survive… they just died and were forgotten about… a billion times over.
The ones with kids, their kids survived in the only way they could. Even if the conditions weren’t ideal and were very different to their parent, grandparents, or 28,000 generations ago. Eventually the ones who “won” in that environment got better and better at being in that environment, and even moved onto other worse environments (either through accident, force of nature, or just ran out of food and were forced to improvise to stay alive).
Evolution isn’t a targeted instantaneous thing. It’s largely accidental, has countless billions of casualties, is very inefficient, and based solely on “winner stays on”. When people say “what came first, chicken or the egg?” the answer is really neither. They both co-evolved from other things that were neither chicken nor born in an egg. But over millions of years, eventually something that resembles an chicken bears something that looks like an egg which bears resemblance to something slightly more chicken-y by a fraction of a billionth of an animal.
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