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

You have a fish population. The water there tends to dry up sometimes, and many fish die. Over time, the fish that survive the dry spells are the ones who could stand those conditions. Over time they are able to be out of water for longer periods of time.