A startling discovery made in the arid sands of Pakistan announced by University of Michigan paleontologists Philip Gingerich and Donald Russell in 1981 finally delivered the transitional form scientists had been hoping for. In freshwater sediments dating to about 53 million years ago, the researchers recovered the fossils of an animal they called Pakicetus inachus. Little more than the back of the animal’s skull had been recovered, but it possessed a feature that unmistakably connected it to cetaceans.
Cetaceans, like many other mammals, have ear bones enclosed in a dome of bone on the underside of their skulls called the auditory bulla. Where whales differ is that the margin of the dome closest to the midline of the skull, called the involucrum, is extremely thick, dense, and highly mineralized. This condition is called pachyosteosclerosis, and whales are the only mammals known to have such a heavily thickened involucrum. The skull of Pakicetus exhibited just this condition.
Even better, two jaw fragments showed that the teeth of Pakicetus were very similar to those of mesonychids. It appeared that Van Valen had been right, and Pakicetus was just the sort of marsh-dwelling creature he had envisioned. The fact that it was found in freshwater deposits and did not have specializations of the inner ear for underwater hearing showed that it was still very early in the aquatic transition, and Gingerich and Russell thought of Pakicetus as “an amphibious intermediate stage in the transition of whales from land to sea,” though they added the caveat that “Postcranial remains [bones other than the skull] will provide the best test of this hypothesis.” The scientists had every reason to be cautious, but the fact that a transitional whale had been found was so stupendous that full-body reconstructions of Pakicetus appeared in books, magazines and on television. It was presented as a stumpy-legged, seal-like creature, an animal caught between worlds.
Throughout the 1990s, the skeletons of more or less aquatically adapted ancient whales, or archaeocetes, were discovered at a dizzying pace. With this new context, however, the stubby, seal-like form for Pakicetus depicted in so many places began to make less and less sense. Then, in 2001, J.G.M. Thewissen and colleagues described the long-sought skeleton (as opposed to just the skull) of Pakicetus attocki. It was a wolf-like animal, not the slick, seal-like animal that had originally been envisioned. Together with other recently discovered genera like Himalayacetus, Ambulocetus, Remingtonocetus, Kutchicetus, Rodhocetus and Maiacetus, it fits snugly within a collection of archaeocetes that exquisitely document an evolutionary radiation of early whales. Though not a series of direct ancestors and descendants, each genus represents a particular stage of whale evolution. Together they illustrate how the entire transition took place.
The earliest known archaeocetes were creatures like the 53-million-year-old Pakicetus and the slightly older Himalayacetus. They looked as if they would have been more at home on land than in the water, and they probably got around lakes and rivers by doing the doggie paddle. A million years later lived Ambulocetus, an early whale with a crocodile-like skull and large webbed feet. The long-snouted and otter-like remingtonocetids appeared next, including small forms like the 46-million-year-old Kutchicetus. These early whales lived throughout near-shore environments, from saltwater marshes to the shallow sea.
Living at about the same time as the remingtonocetids was another group of even more aquatically adapted whales, the protocetids. These forms, like Rodhocetus, were nearly entirely aquatic, and some later protocetids, like Protocetus and Georgiacetus, were almost certainly living their entire lives in the sea. This shift allowed the fully aquatic whales to expand their ranges to the shores of other continents and diversify, and the sleeker basilosaurids like Dorudon, Basilosaurus and Zygorhiza populated the warm seas of the late Eocene. These forms eventually died out, but not before giving rise to the early representatives of the two groups of whales alive today, the toothed whales and the baleen whales. The early representatives of these groups appeared about 33 million years ago and ultimately gave rise to forms as diverse as the Yangtze River dolphin and the gigantic blue whale.
Studies coming out of the field of molecular biology conflicted with the conclusion of the paleontologists that whales had evolved from mesonychids, however. When the genes and amino acid sequences of living whales were compared with those of other mammals, the results often showed that whales were most closely related to artiodactyls—even-toed ungulates like antelope, pigs, and deer. Even more surprising was that comparisons of these proteins used to determine evolutionary relationships often placed whales within the Artiodactyla as the closest living relatives to hippos.
This conflict between the paleontological and molecular hypotheses seemed intractable. Mesonychids could not be studied by molecular biologists because they were extinct, and no skeletal features had been found to conclusively link the archaeocetes to ancient artiodactyls. Which were more reliable, teeth or genes? But the conflict was not without hope of resolution. Many of the skeletons of the earliest archaeocetes were extremely fragmentary, and they were often missing the bones of the ankle and foot. One particular ankle bone, the astragalus, had the potential to settle the debate. In artiodactyls this bone has an immediately recognizable “double pulley” shape, a characteristic mesonychids did not share. If the astragalus of an early archaeocete could be found it would provide an important test for both hypotheses.
In 2001, archaeocetes possessing this bone were finally described, and the results were unmistakable. Archaeocetes had a “double-pulley” astragalus, confirming that cetaceans had evolved from artiodactyls. Mesonychids were not the ancestors of whales, and hippos are now known to be the closest living relatives to whales.
Recently scientists determined which group of prehistoric artiodactyls gave rise to whales. In 2007, Thewissen and other collaborators announced that Indohyus, a small deer-like mammal belonging to a group of extinct artiodactyls called raoellids, was the closest known relative to whales. While preparing the underside of the skull of Indohyus, a student in Thewissen’s lab broke off the section covering the inner ear. It was thick and highly mineralized, just like the bone in whale ears. Study of the rest of the skeleton also revealed that Indohyus had bones marked by a similar kind of thickening, an adaptation shared by mammals that spend a lot of time in the water. When the fossil data was combined with genetic data by Jonathan Geisler and Jennifer Theodor in 2009, a new whale family tree came to light. Raoellids like Indohyus were the closest relatives to whales, with hippos being the next closest relatives to both groups combined. At last, whales could be firmly rooted in the mammal evolutionary tree.