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.

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.