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

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.