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 can see that working today with mudskippers. They keep water in their mouths to go over the gills and can absorb oxygen through their skin. Fish have an internal swim bladder that they could inflate with air, that eventually became lungs.