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
They didn’t! They essentially “held their breath” on land for short periods before going back into the water. Essentially the reverse of what a lot of land animals do – including humans! Over time they evolved to hold their breath longer and longer, which eventually evolved into a way to breathe air.
Fish biologist here: because fish could breathe air long before they left the water. In fact lungs are ancestral to all bony fish. In most modern ray finned fish those lungs have been modified into a swim bladder, while some more primitive ray finned fish, as well as most lobe finned fish like lungfish and tetrapods, retain the original lungs.
Now, you probably ask, why do fish need lungs? Because there’s a lot more oxygen in the air than in water. This is especially true when water is warm, there is lots of decaying organic material, or global oxygen levels are low.
Early bony fish used their lungs to get access to this supply of oxygen, providing them an extra advantage over other animals and fish in these lower oxygen habitats who couldn’t breathe air.
The ability to breathe air is so handy that many modern groups of fish which have transformed their lungs into swimbladders have secondary adaptations for getting oxygen from air.
Anyway, when fish moved on land, they were already well able to breathe air.
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.
> How was the first ancient animal to ever step (foot?) out of the water, able to survive breathing air instead of water?
The first animal to do that was a single celled creature, and used osmosis to pull oxygen out of the air or the water.
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.
We still breathe water. The oxygen from air first dissolves into the water layer covering our lungs. It is from that water that the oxygen then moves into our bodies.
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