Eli5: How was the first ancient animal to ever step (foot?) out of the water, able to survive breathing air instead of water?

Question

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

Answer 1

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”.