Why did animal life originate in the oceans rather than on the land?
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It all comes down to ozone. Back before oxygen was released into the atmosphere and converted from O2 (oxygen) into O3 (ozone) the sun was a deadly laser that would give off a lot of radiation. So life could only exist under the sea where it had some protection.
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Life in general originated in the oceans, it split into animal, plant and fungi and others long before any of them specialized to work outside of the water.
We are really just bits of chemistry floating in water, when we walked on land we took our original environment with us inside us.
There is no reason why life might not as well have originated on land at or in the air at first, but it didn’t.
It is thought that the water made it easier for life to evolve. But that is just a snake biting its own tale: *It is easier to evolve in water for the sort of live that evolved in water.*
First of all there was the primordial soup which was amino acids combining to for proteins and eventually life which required oceans as the medium https://youtu.be/Kq3Os00pPJM then eventually you got cyanobacteria which could produce oxygen and photosynthesise https://youtu.be/ile60Q3zsMU from that you have the oxygen that “animals” can use and they can evolve from cyanobacteria to eat cyanobacteria.
So many reasons.
1. Water does not change temperature very quickly. Before life colonized the surface there would have been much stronger temperature shifts (somewhat like what we see in deserts today), making it much less hospitable to life.
2. Before oxygen (free oxygen is only produced by life) there would have been no ozone, meaning a lot more radiation.
3. Life as we know it is all about water. Inside the cells is water, and living in water means that you can take in and eject stuff easily. None of that water preservation or complex nutrient uptake systems.
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Temperatures in water don’t change very quickly whereas temperatures in the air can change much faster. Stability was precious for creatures to thrive in environments with limited oxygen. Only a few species could weather the hellish environment on the surface.
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