[dyauspitr]

That question is analogous to asking why did some fish grow legs and become mammals.

The answer is mutations sometimes specific members of a group will gain a mutation that will overtime cause that group to split off away from the ancestor group. It’s all a matter of chance evolution doesn’t have a direction.

[WalterBright]

Multiple lines of mammals have independently evolved back into being marine life. Whales, seals and manatees do not have a common ancestor, for example.

[mouse_8b]

They do have a common ancestor, it's just not a marine mammal.

[WalterBright]

> not a marine mammal

which is the point. They were not evolved from the same species of land animal.