All our ancestors came from the sea and obviously went through great trouble to leave the sea and radically change to live on land.
Some of them, whales, sealions, penguins, turtles, went back into the sea which would seem to undo a lot of hard research and development and the need to reverse it.
Now as they went back to sea... their hind limbs gradually shrank in evolution. Not visible outside at all, but amazingly if you look inside, deep inside a whale you’ll find vestiges of hind limbs, little tiny remanants of bones which shows that their ancestors had real hind limbs that they actually used for walking.
I think it must be that something about living on land equips animals to do things which they could not have done had they not had this sort of long apprenticeship on land.
For example, they got warm blood, a slight misnomer, they got the ability to keep their temperature constant, which is extremely valuable for biochemical reasons.
Having got that then it became an advantage to do that even back in the water, and so going back into the water was a reasonable thing to do.
Whales are descended from land animals, their ancestors used to gallop over the country side, now they live in the sea. In a sense you could say they gallop through the sea, because the spinal movement of a whale as it swims through the sea is up and down, which is similar to what mammals do when they are galloping on land, unlike lizards, salamanders and indeed fish, from which they are all descended, where the spinal movement is side to side. At some point in the history of mammals they must have switched from a side to side spinal movement to an up-down spinal movement, and the whales even when they went back to the water retained the up down spinal movement.
I think it's something about being freed up from the constraints of gravity. A land animal has an immense weight to carry about and big animals like elephants have big tree trunks of limbs, they need them to carry them about.
Whales freed themselves up from that, it's almost like going into space or becoming a balloon floating through the air. They are no longer constrained by gravity, they are supported by the water. It then becomes a possibility to become more or less indefinitely large...
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