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
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.
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