A langur baby rests in its mother’s lap at the Khandagiri cave hills in the eastern Indian city of Bhubaneswar. (Biswaran Jan Rout/Associated Press)

About 120 million years ago, the supercontinent of Gondwana broke into a jigsaw puzzle of continents and islands in the Southern Hemisphere. One of those was a giant island forming what we now call India.

About 55 million years later, an asteroid, perhaps, or a comet, hit Earth, and the planet’s dinosaurs died out. Mammals seized the opportunity and began diversifying on the major continents. Occasionally, an individual might have crossed over to the Indian island by chance. But even then, two of a kind had to make the journey and then adapt and avoid extinction. This would have been a rare event.

In fact, there is no fossil evidence of ancestors of any of the large mammals we now associate with India — elephants, tigers, lions, primates — and that creates a great scientific mystery: How did India get its mammals?

The leading theory has been that after tectonic forces caused India to crash into Asia between 55 and 35 million years ago, the ancestors of these mammals walked in from Africa, Southeast Asia and northern Asia.

Scientists are now studying India’s smallest mammals — rats — to prove it.

For a month this summer, a group of graduate students fanned out across a grassy valley in northern India, baiting traps with peanut butter and oats. The snow-covered Himalayas formed a ring around them. Each morning, the students examined the contents of the traps and found rats and voles inside.

They snipped off bits of the rodents’ ears and recorded the size of their skulls, the length of their bodies and hind feet, their weight and other physical traits before releasing them. The students then climbed into the mountains to trap species of rodents that have evolved to survive at higher altitudes. The students brought their specimens back to Uma Ramakrishnan, a scientist at the National Center for Biological Sciences in Bangalore, India.

Ramakrishnan realizes that rodents may seem an unusual choice to shed light on the origins of, say, the tiger or the leopard. But there are benefits to using rodents.

First, clipping the ear of a rat is far easier than handling a tiger. More important, rats do not travel great distances, and a hilly terrain that may be no great shakes for a snow leopard will probably be insurmountable to a rat. In this landscape, two groups of rats living on different mountaintops are unlikely to ever come into contact with each other and, over thousands of years, they may diverge into different species.

Ramakrishnan will mine DNA of the samples collected by the graduate students to understand how the species are related to each other and compare them with rodent species from such places as China, Thailand and Burma, which is also known as Myanmar. These comparisons, and an analysis of DNA mutations, will reveal whether the Indian species are descendants of their Asian neighbors. If they are, it would mean that rodents walked into India, out of Asia. And the genetic analysis will reveal when the rats first migrated. If rats walked into India following the collision with Asia, the theory goes, so probably did other mammals.

Not everyone buys this “out of Asia” theory, particularly when they consider mammalian evolution more broadly. In contrast to the theory that the Indian Island was a blank slate for animals, some scientists have suggested that the Indian island was a Noah’s ark. It carried the ancestors of plants, a certain type of placental mammals and insects from Gondwana to the Northern Hemisphere. After colliding with Asia, this precious cargo diversified into modern mammals and populated the world with horses, rhinos, tigers.

But no one has found fossil evidence of these ancient creatures on the subcontinent, said Kenneth Rose, a paleontologist at Johns Hopkins University who often excavates in India and has a self-described penchant for digging up teeth and jawbones. “We don’t have evidence of that yet,” he said.

The oldest fossil of a mammal found in India so far is a 66-million-year-old tooth that may have belonged to a condylarth, primitive ancestors of hoofed mammals, some of which looked like a cross between a lioness and a deer. Condylarth fossils have more commonly been found north of India.

By contrast, scientists have found thousands of fossils of early mammals in China and other parts of the Northern Hemisphere, and some of these date back 125 million years. This, says David Archibald, curator of terrestrial mammals at the San Diego State University Museum of Biodiversity, suggests that mammal ancestors most likely originated in Asia and moved into India.

Ramakrishnan takes up the story after India’s collision into Asia, an event so powerful it created the world’s tallest mountains, the Himalayas. There was a great influx of mammals into the Indian subcontinent, and these spread out, occupying climates best suited to them and at times diversifying into new species.

Marmots, which are related to groundhogs, thrived in the cool Himalayas in the north. Leaf monkeys, or langurs, from Southeast Asia migrated into eastern India between 7.5 million and 2.5 million years ago, said Praveen Karanth, an ecologist at the Indian Institute of Science in Bangalore. Seven species of langurs that descended from these early immigrants live in present-day India, he said.

Gerbils and antelopes and other mammals came from Africa to settle in the deserts of western India. The migration is encapsulated in the fossil record of lions, which originated in east Africa. The earliest fossils are from 2 million years ago. By 500,000 years ago, the felids had spread throughout Europe and by 10,000 years ago, they had occupied most of Asia, including the deserts of western India. Meanwhile, rodents from Southeast Asia went everywhere.

Ramakrishnan would like to do genetic studies and reconstruct the species distribution of India’s larger mammals, maybe even some carnivores, though the logistics would be complicated. The surprise would be if genetic analysis of any of these species revealed a history so deep that it is tied to ancient Gondwana.

That possibility is what drives her and paleontologists such as Rose to continue to probe for the origins of mammals in India.

“I’m fascinated by the modern groups and what they come out of and what made them successful,” Rose said. “And especially their relation to the earliest members: How did they get their start? What were those earliest members like?”

Vaidyanathan, a reporter based in Washington, writes about science in the developing world and energy issues in the United States.