Mammals developed from land-based reptiles two hundred million years ago; most of the mammals we see today are still land based. A notable exception are the cetaceans, which we all know as dolphins and whales. How did they move from land to sea?
Whales With Legs
The ancestor of modern whales moved into the water about fifty-two million years ago. One whale fossil, about forty million years old, provides an interesting snapshot from this gradual change.
This early whale fossil had rudimentary legs growing from its sides. These were puny compared to the bulk of the whale, so they were probably vestigial, in the slow process of disappearing.
Moving from land to salt water requires more changes than simply trading in legs for fins, however. For example, the kidney of a modern whale allows it to drink salt water, but early whales must have been restricted to fresh water like other mammals.
Scientists have recently discovered that this important kidney change happened about forty eight million years ago to a whale called Indocetus. Kidneys don't leave behind fossils like bones do, so how can these scientists be sure?
It All Lies In The Tooth
The answer comes from the ancient whales' tooth enamel. Oxygen in an animal's tooth records information about the type of water they drank. The very earliest proto-whales had tooth enamel that indicated a fresh water diet. They probably lived in rivers, or at least returned to rivers to drink.
Indocetus, however, had teeth that indicated a salt water diet. This means it had specialized kidneys, and was probably the first mammal to swim the open ocean. That's a lot of information from some old teeth!