How Did Whales Evolve? From Wolf-Sized Walkers to the Largest Animals Ever

The evidence that whales were once land mammals was there before the bones. Whales have lungs, warm blood, milk and hair as embryos; their flippers contain the five-fingered hand of a land vertebrate; their vestigial hip bones serve no function in swimming; and their embryos briefly grow hind limb buds that are then reabsorbed. Their nearest living relatives, found by DNA in the 1990s to the surprise of nearly everyone, are the hippopotamuses, and beyond them the even-toed hoofed mammals, cattle, pigs, deer and camels. The whale is a hoofed animal that went back to the sea, and the question was what the intermediate stages looked like.

The fossils from the Eocene of Pakistan, India and Egypt, roughly 50 to 35 million years ago, run in order:

• •Pakicetus, 50 million years ago: a wolf-sized animal with hooves and a long snout, found in river deposits, whose skull has the thickened ear bone unique to whales; it walked and waded
• •Ambulocetus, 49 million years ago: the walking whale, three metres long, crocodile-shaped, with large webbed hind feet, found in coastal deposits; it swam by undulating its back and hunted at the water's edge
• •Rodhocetus and Kutchicetus, 47 million years ago: hind limbs still present but the hip loosening from the spine, nostrils moving back along the snout, and the ear adapted for hearing underwater
• •Dorudon and Basilosaurus, 40 to 35 million years ago: fully marine, five and eighteen metres long, with tail flukes and tiny hind limbs no longer connected to the spine, found in Egypt's Valley of the Whales by the hundred
• •The first toothed and baleen whales, about 34 million years ago, from which every living species descends

The transition rebuilt the body from the ear outward. The nostrils migrated from the tip of the snout to the top of the head, becoming the blowhole; the ear sealed itself from the outside air and learned to hear through the jaw; the spine lengthened and gained the vertical flex of an animal that swims by moving its tail up and down, as mammals run, rather than side to side, as fish swim; the hind limbs shrank to nothing and the front limbs became flippers; and the teeth of the early whales, with their differentiated molars, became the uniform pegs of a toothed whale or were replaced altogether, in the baleen whales, by plates of keratin for straining. The genes for smell, for the hind limb and for the enamel of teeth are still present in whales as broken copies, pseudogenes, that record the losses.

The Eocene coasts of the Tethys Sea, the ocean that lay where the Middle East and the Himalayas now are, were warm, shallow and full of fish, and the reptiles that had ruled the seas in the Cretaceous were gone; a mammal that could hunt in the shallows had a niche with few competitors. Each step was an advantage on its own terms, the webbed foot of a wader, the flexible spine of a swimmer, the sealed ear of a diver, and selection did the rest over about fifteen million years, which is fast for so large a change. The whales then did what mammals do in an empty sea: they diversified, and the baleen whales, filter-feeding on the krill that a cooling ocean produced in vast quantities from about 30 million years ago, grew into the largest animals that have ever lived, a size that only water can support.

The whale sequence is the textbook case of a major transition documented in fossils, and it was assembled within living memory. When the biologist Stephen Jay Gould wrote in 1994 that the whale fossils were the sweetest series of transitional forms an evolutionist could hope for, he was reacting to finds that had arrived in a single decade and answered a question that had stood since Darwin; the Valley of the Whales in Egypt is a World Heritage site for the same reason. The animal that Darwin's critics thought absurd, a land mammal wading into the sea, is in the rocks, and its descendants carry its pelvis still.

When DNA first suggested that whales were close relatives of hippos, some palaeontologists were sceptical, because earlier fossil work had pointed to a different, extinct group of meat-eating hoofed mammals. The dispute was settled in 2001 by a small bone. Even-toed hoofed mammals, such as pigs, deer and hippos, have a distinctive ankle bone, the astragalus, with a pulley-shaped surface at both ends, which lets the foot flex in a particular way. Early whales found in Pakistan still had hind legs complete enough to preserve this bone, and it had the same double-pulley shape. The fossils and the genes now agreed.

The teeth of early whales carry a different kind of evidence. The oxygen in tooth enamel records the water an animal drank while the tooth was growing, and fresh water and sea water contain slightly different proportions of heavy and light oxygen atoms. By measuring these proportions, researchers found that the earliest whales such as Pakicetus drank fresh water, while later forms show a shift towards sea water. This lets scientists track the move into the ocean not only from the shape of bones but from the chemistry of the water the animals were actually living in.

The DNA result is often repeated as whales evolved from hippos, which is not what it shows. Whales and hippos share a common ancestor, and the two lines separated more than 50 million years ago, long before any animal that looked like a hippo existed; the earliest known members of the hippo family are more than twenty million years younger than Pakicetus. The relationship is like that of cousins rather than parent and child. Each line went its own way from an ancestor that was neither, and the hippo's love of water may have evolved separately from the whale's.

Close to that ancestor is a small animal called Indohyus, known from fossils about 48 million years old found in Kashmir and described in 2007. It was roughly the size of a raccoon, with slender legs and small hooves, and looked nothing like a whale, yet its skull has the same thickened ear bone that marks the whales, and its limb bones were dense and heavy, as in animals that wade. Researchers have compared it to the water chevrotain of Africa, a small deer-like animal that escapes predators by diving into streams. Whales may have begun with an animal that used water to hide before it used it to hunt.

Modern whales still have small pelvic bones buried in the muscle of their bodies, no longer connected to the backbone and with no legs attached. For a long time they were described as useless leftovers from the whales' land-living ancestors, a classic example of a vestigial structure.

A study published in 2014 found that the story is more interesting. Comparing many species, researchers showed that the size and shape of the pelvic bones were linked to the size of the males' testes, a sign of competition between males for mating. The bones anchor muscles that control the reproductive organs, so they have been reshaped by sexual selection. A remnant of walking legs has been put to a completely different use.

Whales evolved from wolf-sized hoofed land mammals, relatives of the hippopotamus, that took to the shallow coasts of the Tethys Sea about 50 million years ago, and the fossils from Pakistan and Egypt show the stages in order: the wading Pakicetus, the crocodile-like Ambulocetus, the semi-aquatic Rodhocetus and the fully marine Basilosaurus with its vestigial legs. The nostrils became a blowhole, the ear sealed, the spine learned to flex up and down and the hind limbs vanished, and the pelvis bones buried in a blue whale record the journey.