Philip Gingerich had unintentionally taken up this challenge in the mid-1970s. After earning his Ph.D. at Yale, he began excavating in Wyoming's Clarks Fork Basin, documenting the meteoric rise of mammals at the beginning of the Eocene, after the extinction of the dinosaurs ten million years earlier. In 1975, hoping to trace migrations of mammals from Asia to North America, he started fieldwork in middle Eocene formations in the Punjab and North-West Frontier (now called Khyber Pakhtunkhwa Province) Provinces of Pakistan. He was disappointed to discover that the 50-million-year-old sediments he had targeted were not dry land but marine beds on the eastern edge of the Tethys Ocean. When his team uncovered some pelvic bones in 1977, they jokingly attributed them to "walking whales"—a preposterous notion. At that time the best known fossil whales were thought to be similar to modern whales, with sophisticated mechanisms for underwater hearing, powerful tails with broad flukes, and no external hind limbs.
Then in 1979, a member of Gingerich's team in Pakistan found a skull about the size of a wolf's but with prominent—and very unwolflike—sails of bone at the top and sides of the skull to secure robust jaw and neck muscles. Stranger still, the braincase was little bigger than a walnut. Later the same month Gingerich came across some archaic whale specimens in museums in Lucknow and Kolkata (Calcutta), India. "That's when the tiny braincase started to make sense, because early whales have big skulls and relatively small brains," Gingerich remembers. "I began to think that this small-brained thing might be a very early whale."
When Gingerich freed the skull from its matrix of hard red stone back in his lab in Michigan, he found a grape-size nugget of dense bone at its base called the auditory bulla, with an S-shaped bony crest on it known as the sigmoid process—two anatomical features that are characteristic of whales and help them hear underwater. Yet the skull lacked several other adaptations that living whales use to hear directionally beneath the waves. He concluded that the animal had probably been semiaquatic, spending significant time in shallow water but returning to land to rest and reproduce.
Discovering this most primitive known whale, which Gingerich named Pakicetus, made him see whales in a new light. "I started thinking more and more about the huge environmental transition that whales had made," he remembers. "This was a creature starting out as a terrestrial animal and literally turning into an extraterrestrial. Since then, I've been consumed by the search for the many transitional forms in this huge leap from land back into the sea. I want to find them all."