An adult humpback whale

New Delhi, May 24: An Indo-American research team has discovered how millions of years ago the land-dwelling ancestors of whales around the Indian subcontinent lost their hindlimbs and became marine mammals.

Palaeontologist Sunil Bajpai at the Indian Institute of Technology in Roorkee is among the team that has unravelled the genetic basis of the evolution of whales from four-footed land animals into the graceful swimmers they are today.

In a report published on Monday in the US journal Proceedings of the National Academy of Sciences, the researchers said whales gradually lost their hindlimbs through small genetic changes in a number of genes over millions of years.

“From fossil records, we’ve known a long time that ancestors of whales originated on the Indian subcontinent,” said Bajpai. “This new study combines fossil records with genetics to reveal how whales evolved,” he told The Telegraph.

Over the past two decades, India’s Kutch region has yielded hundreds of fossils of whales ' skulls, jawbones, fore-limbs and backbones ' about 45 million years old. Even older fossils of whales have been discovered in Solan near the Himalayas.

The lead author of the study, Hans Thewissen, at the Northeastern Ohio Universities College of Medicine and his colleagues studied embryonic development of dolphins ' cousins of whales. As embryos, dolphins sprout tiny hindlimbs for a brief period but they vanish in the second month of gestation which lasts for 12 months. Limb development occurs through a series of gene-triggered events.

The researchers showed that a gene called “Sonic hedgehog”, which is important at several stages in limb formation, is absent from the dolphin hindlimb bud. “That is why dolphins lose their legs,” Thewissen said.

However, fossil records suggest whales lost their hindlimbs in a different manner. “Fossils from Kutch in India show that the hindlimbs of whales reduced in size over millions of years,” Bajpai said.

Whale fossils about 45 million years old from Kutch show that the hindlimbs had shrunk, but the basic bone structure remained unchanged, implying that “Sonic hedgehog” gene was still functional. It must have been lost later.

However, 35-million-year-old whale fossils found elsewhere in the world show that the hindlimbs had vanished. “Over time, as the ancestors of whales moved into the sea, the importance of the hindlimbs in locomotion diminished,” Bajpai said.

Scientists say the migration into the sea was likely to have been triggered by a search for food. Between 45 and 35 million years ago, India was colliding with Asia, and the Tethys Sea that separated the land masses had become shallow.

Shallow seas are abundant in marine organisms that might have provided an abundant food resource for whales, Bajpai said. “Whale ancestors might have ventured into the sea to eat the food in the shallow Tethys Sea. In the sea, the need for hindlimbs reduced.”