Whales are superbly adapted to life in the sea, and can never come onto the land. Yet they are not fish, but mammals. Among mammals, whales are the most extreme in their adaptation to an ocean environment; seals and sea lions also live in the sea, but need to come ashore to give birth; and although manatees and dugongs never come ashore, they are restricted to shallow waters, whereas whales roam the ocean from the shallows to the deepest trenches. Yet mammals originated on land, not in the sea, and this means that the ancestors of whales must have been land creatures. Hints of these terrestrial origins can be found in the whales' lack of gills, which means they have to return to the surface to breathe; in the bones of their fins, which look like huge, jointed hands; and in the vertical movement of their spines, which is more characteristic of a running mammal than of fish, whose spines move horizontally.

The question of how land animals evolved into creatures so superbly adapted for ocean living is a fascinating one, but the exact process of the change has been difficult to understand because of gaps in the fossil record. Now, however, new discoveries in India and Pakistan are shedding fresh light on the transition of the whale family from wolf-like shore-dweller to ocean-going behemoth. This article is intended as a summary of this new understanding of whale evolution.

Introduction: Modern Whales

Before we begin, we should look briefly at the nature of modern whales. As stated, whales are mammals, a fact betrayed by their warm-blooded circulatory system, by the small hairs that can be found on their bodies, by the fact that they give birth to live young, not to eggs, and by the fact that they inhale air into their lungs rather than filter oxygen through gills. Collectively, all whales and dolphins are known as the order Cetecea.

Modern whales are splendidly adapted to a permanent life in the sea.

• Their unique ears enable them to hear perfectly underwater.

• They have no legs, moving instead with flippers and the flukes of their huge tails.

• They do not have nostrils, having instead a single blowhole on top of their heads, which allows them to breathe while mostly submerged.

• They are able to give birth underwater, instead of coming onto the land like seals.

• The Odonteceti can use ultrasound to hunt, while all whales communicate with a complex language of 'songs' that carry for miles underwater.

Within the Order Cetecea, whales are divided into two sub-orders: the Odonteceti (toothed whales), which includes sperm whales, killer whales and dolphins, all of which eat fish and flesh; and the Mysticeti (baleen whales), which live by filtering plankton through a hairy sieve-like substance called baleen, these include blue whales, humpback whales and right whales. The difference in feeding habits might lead one to theorise an entirely separate ancestry for the two groups, but in fact studies have shown that all whales are descended from a common ancestor. The nature of this ancestor has been the subject of great debate and speculation, because of the amazing adaptations of these mammals to life in the water.

Why Leave the Land?

Although the transformation from land animal to sea creature may seem extraordinary, the origins of such an occurrence may be observed among some communities of sheep that live on the coast of Scotland. These wild, goat-like sheep have lived on the coast for hundreds of years, and like to eat seaweed and kelp. They like seaweed so much that they are often observed swimming out into shallow waters to find it. Perhaps if we returned in ten million years, the descendants of these sheep would be seal-like or even whale-like creatures. And if herbivorous creatures are prepared to brave the seas for food, it would be even more attractive for those that were able to eat fish.

However, while such theories are easy to hypothesise, proof has been lacking, and for a long time the fossil evidence for early whales was virtually non-existent.

The First Finds

The first fossil evidence for early whales arrived with the 1840 discovery in Egypt of Basilosaurus, an enormous, 40-million-year-old creature with a long, serpentine body, which was very whale-like in appearance, but also had tiny, useless hind legs indicative of a land-based origin. However, the discovery of Basilosaurus did not help greatly with the major questions of whale ancestry, since the creatures were so similar to modern whales that it remained difficult to imagine what their land-based ancestors were like.

More intriguing evidence for the land-based ancestors of whales arrived with the discovery of the Mesonychids, an extinct type of mammal that flourished between 60 and 30 million years ago. Mesonychids were ungulates (hoofed animals), but unlike all other known ungulates they were meat-eaters. Many of them, including the terrifying Andrewsarchus, looked like wolves with hooves. Their most important feature, for our purposes, was their unusual, triangular teeth. The only other animals that have teeth similar to these are whales. For this reason, scientists long believed that whales must have evolved from a form of Mesonychid. Better evidence, however, remained elusive, and it was still difficult to imagine a transitional form between a Mesonychid and the whale-like Basilosaurus.

A further suggestion was made by scientists studying DNA rather than fossils. They found that whale DNA was more similar to that of the Hippopotamids than any other living animal, and some subtle physical similarities between whales and hippos have also been noted. Could the origins of whales and hippos be linked?

All of these suggestions have been clarified, and considerably altered, by a number of recent discoveries in Pakistan and India, which not only tell us more about the earliest whales, but also question the Mesonychid theory. Let us now look at the 'proto-whales' in chronological order.

Pakicetids

The earliest proto-whale that has been discovered is now Pakicetus, one of a group of creatures labelled the Pakicetids which lived around 52 million years ago. Pakicetus was about the size of a wolf. It looked nothing like a whale, and more like a cross between a large dog and a rat, having a long, thick tail. Despite this, many features of its skeleton link Pakicetus more closely to whales than to any other family. In particular, its ears, though not as sophisticated as those of modern whales, show the beginnings of this adaptation to underwater hearing. It is not known exactly how the Pakicetids lived, but they may have roamed the seashore, or hunted in rivers.

The discovery of Pakicetus demonstrated the falsity of the Mesonychid theory; although the teeth indicate a shared ancestry, the skeletons of Pakicetus demonstrate that whales did not derive directly from Mesonychids. Instead, they are a form of Artiodactyl (another type of ungulate) which began to take to the water after the Artiodactyl family split from the Mesonychids. Thus, Pakicetids are early Artiodactyls that retain aspects of their Mesonychid ancestry which modern Artiodactyls have since lost.

The Pakicetid fossils also helped to clarify the relationship of whales and hippos. It now seems that they are not very close relatives, except in that both are Artiodactyls. The likeliest explanation for the physical and genetic similarities is that hippos split off from the main Artiodactyl branch shortly after the whales did, and thus, like whales, retain some characteristics of early Artiodactyls. Both hippos and whales are Artiodactyls that became adapted to life in the water, but they did so separately and evolved in quite different directions.

Ambulocetids

While Pakicetus was clearly a land animal with minor adaptations to life in the water, this is less true of the recently discovered Ambulocetus, a 50 million-year-old skeleton of which was recently unearthed in Pakistan. The three-metre long Ambulocetus was an alarming animal that looked like a mammalian crocodile, having a long snout filled with teeth. It was amphibious - its back legs are better adapted for swimming than walking on land - and it probably swam by undulating its back vertically, as otters, seals and whales do. Like Pakicetus, Ambulocetus had developed an early form of the specialised ears possessed by modern whales, but there was still no blowhole. It is not certain whether these animals lived in rivers, the sea, or both. It has been suggested that Ambulocetids hunted like crocodiles, lurking in the shallows to snatch unsuspecting prey.

Remingtonocetids

The Remingtonocetid family were smaller cousins of the Ambulocetids, and lived at a similar time. They had longer snouts than those of Ambulocetus, and were slightly better adapted for underwater life. They probably lived similarly to modern sea otters, hunting for fish in the shallows.

Protocetids

The Protocetids were better-adapted for the water and lived around 45 million years ago. The best-known is Rhodocetus. The major Protocetid adaptation was the appearance of flukes (horizontal bars) on their tails, which enabled faster swimming. However, the skeletons of Rhodocetus indicate that they retained substantial hind legs. They lived in shallow seas, and may have had a similar lifestyle to seals, or even dolphins; it is not clear whether they ever came onto the land.

Basilosaurus and Dorudon

These new discoveries make it possible to understand how the descendants of creatures like these could look like Basilosaurus¹. Basilosaurids lived around 38 million years ago and retained tiny, useless back legs. They were monsters, up to 18m long. Meanwhile, Dorudontids, which lived at the same time, were dolphin-sized, about 5m long. Although they look very much like modern whales, Basilosaurids and Dorudontids lacked the 'melon organ' that allows their descendants to sing and use ultrasound. They also had small brains, which suggests that they were solitary and didn't have the complex social structure of modern whales.

Summary

The transition from hoofed carnivore to sea leviathan is now understood in more detail than was possible even ten years ago. Pakicetids evolved into whales in a remarkably short time - about eight million years. However, these amazing transformations can now be observed in the fossil evidence of creatures that represent intermediate stages in the process. Gaps in our knowledge still remain - in particular the evolution of the blowhole remains mysterious - but hopefully further discoveries will provide answers to these questions.