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
That’s likely not how it happened.
We have some fish today that can live out of water for a little while. This is super helpful when your tide pool evaporates, you’re not dead when the next tide comes in, so evolution selects for this feature in this fish.
Long ago some fish that had a feature like this probably needed to move across land to an adjacent tide pool. This gave them more ways to survive, so the individuals that could survive on land the longest won. After a while, it became more efficient not to breathe in the water at all any more. That makes the individuals that dropped the “breathe in water” feature the first “land animals”.
Some fish (such as bettas) can actually breathe a bit. They come to the surface and gulp some air into an organ called a labyrinth organ. From there, they can absorb oxygen. Also, amphibians can absorb some oxygen through their skin.
Additionally some species of fish “walk” on the ocean floor and mudskippers walk on land as well.
The line between aquatic and terrestrial is blurrier than people expect.
As animals that lived close to the edge of the water, there would have been some incentive to get closer and closer to the edge. Whether that was predation or access to food, who knows. The ones that could get closest survived and passed on their genes by breeding. Next they could live semi submerged after a very long time. Then they could take short trips on land, then long trips, and eventually stay on land.
Keep in mind that each of these steps took thousands of generations to accomplish (unless looking at great leap type evolution.)
It wouldn’t have been a switch, or a moment. Like, one day it’s a fish the next day it’s a lizard, it would have been a transition.
For example, if you look up videos of an octopus walking across land, it becomes easy to imagine how generations of octopi with stiffer and stiffer cartilage would be more & more efficient, and that’s how bones evolve. Similarly, when a creature peaks its head out of water and a mutation makes it 1% better at using atmospheric oxygen, maybe that’s not enough to leave the water, but one of their offspring mutates to be 1% better, and one of their offspring is 1% better, and one of their offspring is 1% better, and, and, and…. enough mutations happen over enough generations, and you have a set of lungs.
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.
Nothing like this happens in one step. Some fish that live entirely in water have evolved the ability to “gulp air” which allows them to survive in poorly-oxygenated waters. Fish living in shallow areas that dry up periodically or on tidal flats started to be able to stay out of water for longer periods of time, to get from one body of water to another. Mutations that allowed some of them to stay out of the water for longer meant they’d survive better. Mutations that strengthened their fins for faster movement over land also helped them.
Some of those forms of fish exist today: lungfish, mud skippers and catfish that migrate between pools. Fossil examples of these transitional creatures like tiktaalik show clear features that are between fish and amphibians. So there was no “first ancient animal” on land, there were many that hovered on the margin between the water and land, and gradually were able to stay on land more.
Over time, some of them became able to stay out of water for very long times, eventually indefinitely. Amphibians like salamanders are still closely tied to water: their skin needs to stay wet, they lay their eggs in water (mostly), some have to mate in water.
Fish evolved lungs while still living in water. They started as swim bladders, which are air filled sacs that help fish control their buoyancy so they don’t have to spend as much energy swimming all the time they can just float statically in the water.
Then they evolved the ability to get oxygen from the air in their swim bladder, allowing them to get extra oxygen if they were swimming though low oxygen water. Some fish still do this.
Finally this ability to get oxygen from air allowed some fish to leave the water completely.
The specific family of fishes you should be looking at are *lungfishes*. This family, as the name suggests, has primitive lungs. These primitive lungs evolved to help fish in oxygen-poor environments, so that they could hold a reserve of sorts of oxygen that they could use when they need to exert themselves, either to escape a predator or catch prey.
That same organ also evolved into swim bladders.
Some of these fish, like bichirs, can survive *entirely* on land as long as the environment stays humid so their gills don’t dry and get damaged.
It didn’t happen overnight. There are plenty of creatures today that live in the interface between water and air and spend the time in either that suits their capabilities and needs.
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