The find sheds new light on the evolution of one of the most successful groups of sea mammals, which became adapted to a semi-aquatic life in river estuaries and shallow seas before becoming fully marine.
Scientists have dated the fossil to about 53.5 million years old, making it 3.5 million years older than the previous oldest known member of the whale family.
The ancient whale, called Himalayacetus subathuensis, probably only spent some of its time in water, returning to dry land to rest and breed.
Its jawbone contains teeth that are clearly adapted to eating fish, according to Philip Gingerich, of the University of Michigan, and Sunil Bajpai, of the University of Roorkee, in northern India.
In a paper published in the Proceedings of the National Academy of Sciences, the scientists say the fossil is a significant find because of both its extreme age and because it was found in a layer of sediments clearly associated with marine animals rather than freshwater species.
H. subathuensis is considerably older than a more recent whale ancestor, Pakicetus, which has also been linked with the ancient Tethys Sea separating Asia and the Indian subcontinent before they collided to form the Himalayan mountains.
Pakicetus is believed to have been the ancestor of the first truly ancient whale, archaeocetus, a fish-eater that grew to about the size of a modern porpoise and lived more than 35 million years ago.
"When first described, pakicetus was interpreted as an amphibious initial stage of whale evolution that rested and reproduced on land and entered Tethys opportunistically to feed on fish," the scientists say.
The latest fossil jawbone was recovered from a sedimentary layer 100 metres deeper than previous pakicetus finds, Bajpal and Gingerich say. "This not only extends the fossil record of Cetacea [the whale family] back in time, but also reinforces the idea that whales originated on the margin of Tethys and corroborates interpretation of pakicetids as an initial amphibious stage of cetacean evolution entering Tethys to feed on fish."
The chemical composition of other early whale fossils showed evidence of life in freshwater rather than sea environments. Analysis of phosphate in the newly discovered fossil teeth revealed values half way between those associated with freshwater and sea-living species, the scientists report.
"Himalayacetus came from a shallow, oyster-bearing marine deposit, whereas Pakicetus and the other oldest pakicetids known previously came from continental red beds and were found in association with land mammals," they say.
Although modern whales have lost their hind legs, their earlier ancestors evidently had functional limbs that allowed them to roam around on land. Archaeocetus had two vestigial hind legs that protruded from its body but which seemed to serve little or no function. Further adaptations allowed modern whales to exploit the rich ocean environment to become, in the case of the blue whale, the largest animal on Earth.