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

The specific family of fishes you should be looking at are *lungfishes*. This family, as the name suggests, has primitive lungs. These primitive lungs evolved to help fish in oxygen-poor environments, so that they could hold a reserve of sorts of oxygen that they could use when they need to exert themselves, either to escape a predator or catch prey.

That same organ also evolved into swim bladders.

Some of these fish, like bichirs, can survive *entirely* on land as long as the environment stays humid so their gills don’t dry and get damaged.