By adapting to water, early whales gained access to an environment closed to most other mammals, rich in food and shelter, and short on competitors and predators—perfect conditions for an evolutionary explosion. What followed was a starburst of idiosyncratic experiments in being a whale, most of which ended in extinction long before modern times. There was the hulking, 1,600-pound Ambulocetus, an ambush hunter with squat legs and huge snapping jaws, like a hairy saltwater croc; Dalanistes, with a long neck and head like a heron; and Makaracetus, with a short, curved, muscular proboscis that it may have used for eating mollusks.
Around 45 million years ago, as the advantages of a water environment drew whales farther out to sea, their necks compressed and stiffened to push more efficiently through the water, behind faces lengthening and sharpening like a ship's prow. Hind legs thickened into pistons; toes stretched and grew webbing, so they resembled enormous ducks' feet tipped with tiny hooves inherited from their ungulate ancestors. Swimming methods improved: Some whales developed thick, powerful tails, bulleting ahead with vigorous up-and-down undulations of their lower bodies. Selection pressure for this efficient style of locomotion favored longer and more flexible spinal columns. Nostrils slid back up the snout toward the crown of the head, becoming blowholes. Over time, as the animals dived deeper, their eyes began to migrate from the top toward the sides of the head, the better to see laterally in the water. And whale ears grew ever more sensitive to underwater sound, aided by pads of fat that ran in channels the length of their jaws, gathering vibrations like underwater antennae and funneling them to the middle ear.
Though finely tuned to water, these 45-million-year-old whales still had to hitch themselves ashore on webbed fingers and toes, in search of fresh water to drink, a mate, or a safe place to bear their young. But within a few million years whales had passed the point of no return: Basilosaurus, Dorudon, and their relatives never set foot on land, swimming confidently on the high seas and even crossing the Atlantic to reach the shores of what is now Peru and the southern United States. Their bodies adjusted to their exclusively aquatic lifestyle, forelimbs shortening and stiffening to serve as flippers for planing, tails broadening at the tip in horizontal flukes to create a hydrofoil. The pelvis decoupled from the spine, allowing the tail a broader range of vertical motion. Yet like talismans from a long-forgotten life ashore, their hind legs remained, complete with tiny knees, feet, ankles, and toes, useless now for walking but good perhaps for sex.