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.
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.
![[ image: Chemical analysis revealed whales swam in both rivers and the sea]](http://news.bbc.co.uk/olmedia/235000/images/_239966_whale_tooth180.jpg)
Chemical analysis revealed whales swam in both rivers and the sea
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.
![[ image: Whales were land animals tempted back into the sea by plentiful fish]](http://news.bbc.co.uk/olmedia/235000/images/_239966_beluga180.jpg)
Whales were land animals tempted back into the sea by plentiful fish