The jawbone, complete with teeth, is 53.5m years old - 3.5m years older than previous record holder - and was found in the Simla Hills of northern India.

Chemical analysis revealed whales swam in both rivers and the sea

The rock layer which yielded the jawbone is littered with oyster shells and was deposited in a shallow ocean that once separated India and Asia. This is significant because the previous oldest-known whale fossil, unearthed in Pakistan, lay buried with the remains of only land mammals.

Scientists believed that whales evolved from land-living animals which were tempted to return to the ocean by the plentiful supply of fish in the now-disappeared Tethys ocean.

The researchers, from the University of Roorke, India and the University of Michigan, USA, analysed the newly discovered teeth and found the chemical composition was halfway between values expected for fresh and marine water.

Whales were land animals tempted back into the sea by plentiful fish

This, they believe, shows that the first whales swam in rivers, estuaries and oceans in search of fish, as well as spending time on land. Modern whales have become entirely adapted to ocean life, but have retained the need to breathe.

The fossil belongs to a previously unknown genus and species. It has been named Himalayacetus subathuensis in a report in the Proceedings of the National Academy of Sciences.

The ocean it once inhabited was destroyed when the Indian continent collided with Asia, creating the Himalayan mountains.