Not long after the true identity of Basilosaurus was resolved, Charles Darwin’s theory of evolution by means of natural selection raised questions about how whales evolved. The fossil record was so sparse that no definite determination could be made, but in a thought experiment included in On the Origin of Species, Darwin speculated about how natural selection might create a whale-like creature over time:
In North America the black bear was seen by [the explorer Samuel] Hearne swimming for hours with widely open mouth, thus catching, like a whale, insects in the water. Even in so extreme a case as this, if the supply of insects were constant, and if better adapted competitors did not already exist in the country, I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale.
Darwin was widely ridiculed for this passage. Critics took it to mean he was proposing that bears were direct ancestors of whales. Darwin had done no such thing, but the jeering caused him to modify the passage in subsequent editions of the book. But while preparing the sixth edition, he decided to include a small note about Basilosaurus. Writing to his staunch advocate T.H. Huxley in 1871, Darwin asked whether the ancient whale might represent a transitional form. Huxley replied that there could be little doubt that Basilosaurus provided clues as to the ancestry of whales.
Huxley thought that Basilosaurus at least represented the type of animal that linked whales to their terrestrial ancestors. If this was true, then it seemed probable that whales had evolved from some sort of terrestrial carnivorous mammal. Another extinct whale called Squalodon, a fossil dolphin with a wicked smile full of triangular teeth, similarly hinted that whales had evolved from meat-eating ancestors. Like Basilosaurus, though, Squalodon was fully aquatic and provided few clues as to the specific stock from which whales arose. Together these fossil whales hung in a kind of scientific limbo, waiting for some future discovery to connect them with their land-dwelling ancestors.
In the meantime, scientists speculated about what the ancestors of whales might have been like. The anatomist William Henry Flower pointed out that seals and sea lions use their limbs to propel themselves through the water while whales lost their hind limbs and swam by oscillations of their tail. He could not imagine that early cetaceans used their limbs to swim and then switched to tail-only propulsion at some later point. The semi-aquatic otters and beavers, he claimed, were better alternative models for the earliest terrestrial ancestors of whales. If the early ancestors of whales had large, broad tails, that could explain why they evolved such a unique mode of swimming.
Contrary to Huxley’s carnivore hypothesis, Flower thought that ungulates, or hoofed mammals, shared some intriguing skeletal similarities with whales. The skull of Basilosaurus had more in common with ancient “pig-like Ungulates” than seals, thus giving the common name for the porpoise, “sea-hog,” a ring of truth. If ancient omnivorous ungulates could eventually be found, Flower reasoned, it would be likely that at least some would be good candidates for early whale ancestors. He envisioned a hypothetical cetacean ancestor easing itself into the shallows:
We may conclude by picturing to ourselves some primitive generalized, marsh-haunting animals with scanty covering of hair like the modern hippopotamus, but with broad, swimming tails and short limbs, omnivorous in their mode of feeding, probably combining water plants with mussels, worms, and freshwater crustaceans, gradually becoming more and more adapted to fill the void place ready for them on the aquatic side of the borderland on which they dwelt, and so by degree being modified into dolphin-like creatures inhabiting lakes and rivers, and ultimately finding their way into the ocean.
The fossil remains of such a creature remained elusive. By the turn of the 20th century the oldest fossil whales were still represented by Basilosaurus and similar forms like Dorudon and Protocetus, all of which were fully aquatic—there were no fossils to bridge the gap from land to sea. As E.D. Cope admitted in an 1890 review of whales: “The order Cetacea is one of those of whose origin we have no definite knowledge.” This state of affairs continued for decades.
While analyzing the relationships of ancient meat-eating mammals in 1966, however, the evolutionary biologist Leigh Van Valen was struck by the similarities between an extinct group of land-dwelling carnivores called mesonychids and the earliest known whales. Often called “wolves with hooves,” mesonychids were medium- to large-sized predators with long, toothy snouts and toes tipped with hooves rather than sharp claws. They were major predators in the Northern Hemisphere from shortly after the demise of the dinosaurs until about 30 million years ago, and the shape of their teeth resembled those of whales like Protocetus.
Van Valen hypothesized that some mesonychids may have been marsh dwellers, “mollusk eaters that caught an occasional fish, the broadened phalanges [finger and toe bones] aiding them on damp surfaces.” A population of mesonychids in a marshy habitat might have been enticed into the water by seafood. Once they had begun swimming for their supper, succeeding generations would become more and more aquatically adapted until something “as monstrous as a whale” evolved.