|Skeleton of a baleen whale with the hind limb and pelvic bone structure circled in red. This bone structure stays internal during the entire life of the species.|
The evolutionary history of cetaceans is thought to have occurred in the Indian subcontinent from even-toed ungulates 50 million years ago, over a period of at least 15 million years. Cetaceans are fully aquatic marine mammals belonging to the order Artiodactyla, and branched off from other artiodactyls around 50 mya (million years ago). Cetaceans are thought to have evolved during the Eocene or earlier, sharing a closest common ancestor with hippopotamuses. Being mammals, they surface to breathe air; they have 5 finger bones (even-toed) in their fins; they nurse their young; and, despite their fully aquatic life style, they retained many skeletal features from their terrestrial ancestors. Discoveries starting in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea.
Wikipedia - Evidence of common descent explains tons of evidences for evolution. Numerous links provide more tons of evidences.
There are still evolution deniers in the 21st century because looking things up makes them cry.