Eli5 How did mammals end up in the ocean?

Question

I’m watching dolphins pop out of the water to breathe, and it just seems absurd that animals that live in water need to breathe air.

In: 9

Answer 1

Breathing air can give an animal oxygen at a higher rate than extracting it from the water. More oxygen can mean more energy to power a bigger brain, suitable for sophisticated sensors like sonar.

Mammals went into the ocean because there are lots of tasty fish in the ocean. Until humans got the idea to use tools to collect the fish, fish lovers from bears to whales have gone into the water to get the tasty fish.