Horses and rhinos likely originated in the Indian subcontinent, over 54 million years ago, according to a new study.
Working at the edge of a coal mine in India, researchers at the Johns Hopkins University and colleagues have filled in a major gap in science's understanding of the evolution of a group of animals that includes horses and rhinos.
The group likely originated on the subcontinent when it was still an island headed swiftly for collision with Asia, the researchers said.
Modern horses, rhinos and tapirs belong to a biological group, or order, called Perissodactyla. Also known as "odd-toed ungulates," animals in the order have, as their name implies, an uneven number of toes on their hind feet and a distinctive digestive system.
Though paleontologists had found remains of Perissodactyla from as far back as the beginnings of the Eocene epoch, about 56 million years ago, their earlier evolution remained a mystery, said Ken Rose, a professor of functional anatomy and evolution at Johns Hopkins.
In 2001, Rose and Indian colleagues began exploring Eocene sediments in Western India because it had been proposed that perissodactyls and some other mammal groups might have originated there.
In an open-pit coal mine northeast of Mumbai, they uncovered a rich vein of ancient bones. The mine yielded what Rose said was a treasure trove of teeth and bones.
More than 200 fossils turned out to belong to an animal dubbed Cambaytherium thewissi, about which little was known.
Researchers dated the fossils to about 54.5 million years old, making them slightly younger than the oldest known Perissodactyla remains, but, it provides a window into what a common ancestor of all Perissodactyla would have looked like.
"Many of Cambaytherium's features, like the teeth, the number of sacral vertebrae, and the bones of the hands and feet, are intermediate between Perissodactyla and more primitive animals," Rose said.
"This is the closest thing we've found to a common ancestor of the Perissodactyla order," said Rose.
In 1990, researchers at the Stony Brook University, published a paper suggesting several groups of mammals that appear at the beginning of the Eocene, including primates and odd- and even-toed ungulates, might have evolved in India while it was isolated.
Cambaytherium is the first concrete evidence to support that idea, Rose said.
"Around Cambaytherium's time, we think India was an island, but it also had primates and a rodent similar to those living in Europe at the time," he said.
"One possible explanation is that India passed close by the Arabian Peninsula or the Horn of Africa, and there was a land bridge that allowed the animals to migrate. But Cambaytherium is unique and suggests that India was indeed isolated for a while," said Rose.
The study was published in the journal Nature Communication.
