From Bambi to Moby Dick: how a small deer evolved into the whale

Indohyus - earliest known ancestor of the whale
Indohyus - earliest known ancestor of the whale. Photograph: Getty

Fossil hunters have discovered the remains of the earliest ancestor of the modern whale: a small deer-like animal that waded in lagoons and munched on vegetation.

The landmark finding represents a long-sought "missing link" in the 10m-year journey that saw ancient land mammals evolve into modern cetaceans, a group that includes whales, dolphins and porpoises.

Scientists have long known that whales are mammals whose ancient ancestors walked on land, but only in the past 15 years have they unearthed fossils that shed light on the creatures' dramatic evolutionary history. The latest discovery, named Indohyus, is the first whale ancestor known to have lived on land.

A team led by Hans Thewissen at the Northeastern Ohio Universities College of Medicine reconstructed a near-complete skeleton of the animal, close in size to a domestic cat, dating from 48m years ago.

Analysis of the bones revealed they had a thick and heavy outer coating, a characteristic common among modern wading mammals such as hippos. Chemical tests on the animals' teeth found they were similar to those of aquatic animals, suggesting that Indohyus spent much of its time in water.

The evolutionary path of the whale is one of the most extraordinary on record. In less than 10m years, the whale's ancestors completely transformed as they shifted from a four-legged life on land to a life in the ocean.

The first whales, Pakicetidae, emerged around 50m years ago and resembled land mammals rather than the giant marine creatures of today. These evolved into large, powerful coastal whales, or Ambulocetidae, that had big feet and strong tails. Later, whales lost their hind limbs and hair and developed powerful tail fins and flippers.

The latest fossils were discovered among rocks that were originally collected more than 30 years ago from an ancient site in Kashmir by an Indian geologist called A Ranga Rao. He had painstakingly recovered a few teeth and parts of a jawbone from some of the rocks, but when he died a few years ago, many had still not been broken open. Professor Thewissen began working on the remaining rocks after being given them by Ranga Rao's widow.

Thewissen realised Indohyus was the missing link in the whale's evolutionary path when his technician accidentally broke one of the skulls they had found while working on it. The fracture revealed an unusual bone structure around the animal's ear.

"When I saw it, I said 'Oh my God!' In most mammals the bone is a little bowl-shaped structure, but in whales the shape of the bone that makes up the ear is unique and in Indohyus it is the same. The inside of that bone is very thick, and on the outside, it's very thin, the difference is giant. No other mammal has that," said Thewissen.

On closer inspection, the team also found similarities between Indohyus's front teeth and those seen in whales. The research is published in the journal Nature.

The discovery overturns a previous assumption that the ancestors of whales were already carnivores before they left land for a life beneath the waves. Indohyus was a herbivore, suggesting the earliest ancestors of whales took to the water and only later adapted their diet to become carnivorous.

Whale evolution is thought to have begun with creatures like Indohyus becoming more adapted to a watery environment to avoid land-based predators. The animal's heavy bones would have made Indohyus a slow beast on land, but in the water, they would help it stay on the bottom, where it could forage and hide.

In Africa, the mousedeer or water chevrotain uses a similar strategy to avoid one of its major predators, the eagle. When it is threatened, the mousedeer dives into water and hides beneath the surface for up to four minutes.

Fred Spoor, an anthropologist at University College London, said the significance of the latest find was comparable to Archaeopteryx, the first fossils to show a clear transition between dinosaurs and birds. "For years cetaceans were used by creationists to support their views because for a long time the most primitive whales known had bodies that looked like modern whales, so there seemed to be this enormous gap in evolution. But since the early 1990s, there's been a rapid succession of fossils from India and Pakistan that beautifully fill that gap," he said.

"The tables are turned now because we have fossils that show that dramatic transition step by step. Cetaceans are almost the only group that has made such a rapid change from a land environment to an aquatic one.

"Unlike sealions and seals, which still spend some time on land, cetaceans are completely committed to the water now, and it had an enormous effect on their physiology. They had to change everything."

Thewissen's group will next study Indohyus further to learn more about its diet and habitat. One critical change that occurred when whales took to the water involved its sense of balance and orientation. In land mammals, this is governed by a vestibulary system in the inner ear, but whales had to adapt to moving in three dimensions, driving the evolution of a more complex system.

"This fossil completes the picture in terms of the whales' evolution, but what's next is to look at these other evolutionary adaptations," said Thewissen.