Whales, dolphins and porpoises evolved from a tiny deer-like mammal that adapted to living at sea millions of years ago, a new study says.

Whales are long suspected to have originated from four-footed mammals called artiodactyls that walked on land in South Asia and gradually adapted to live in the sea.

Evidence to back this comes from fossils of even-toed ungulates, artiodactyls that spread their weight on their third and fourth toes, which date from before the emergence of the whales, and of whale fossils that date from the first 10 million years of whale evolution.