The Greeks dubbed them “river horses,” and for years, archeologists and biologists agreed that they were probably (evolutionary) cousins to modern-day pigs, but recently, DNA analysis has shown that hippos have a much stranger history than that. Descendants of a 60-million-year-old ancestor that’s long since gone extinct, hippos’ closest living relatives don’t live nearby and don’t look much like them (and presumably never call): they’re whales.
It makes sense that scientists grouped the hippos with the pigs; they share a four-toed ungulate foot, stubby legs, and a barrel-chested build. For years, the accepted theory, based on the fossil evidence, was that hippos were in fact a sub-group of pigs.
As it happens, however, DNA shows that hippos and pigs aren’t even all that closely related (hippos are closer relatives of giraffes than pigs).
At some point in the very distant past, first camels (likewise even-toed ungulates), then pigs split away from other hooved creatures, forming their own branches on the evolutionary tree. Then, around 59-60 million years ago, great-great-(100,000 more greats)-grandpa hippo-whale split off again, parting ways with the branch that would become modern-day ruminants (a large group comprised of most of the remaining hooved animals, including cattle, deer, giraffes, and antelopes). After a few million years together, hippos and whales eventually split up around 54 million years ago; each group started spinning off multiple variations, the vast majority of which went extinct many, many years back.
Over millenia, whale ancestors’ feet morphed into fins (and eventually the back legs disappeared entirely), their bodies elongated and streamlined, and their musculature evolved to propel them through the sea. But despite some very apparent differences, hippos and whales still share some curious features rarely seen in other mammals.
Over several millennia, the foot of a common ancestor developed into fins in the whales
The first is their strangely similar stomachs. Ruminants are defined by multi-chamber stomachs; they regurgitate food at least once, chew the cud, and swallow again, extracting more nutrients. Carnivores, on the other hand, have a single stomach; food is broken down chemically (as opposed to mechanically, as in the ruminants) and excreted in a single process.
Hippo stomachs are a strange hybrid; the animals don’t regurgitate food, but they do have two “preliminary” chambers, their walls roughly textured to help break down the food, which act almost as an internal compost bin; the stomach is so large, and includes so many pockets and chambers, that food is given a chance to ferment before passing to the final, “true” stomach (which resembles the stomach of a carnivore). For years, biologists couldn’t figure out why whales—most of them carnivorous—had multi-chambered “grazer” stomachs, too. The discovery of the link between whale and hippo DNA, points to the whale’s terrestrial history, explains this longtime mystery!
As inexplicable as the whales’ land-lubbing stomachs are the hippo’s undersea-oriented lungs. Most terrestrial animals, including humans, have multi-lobed lungs, but aquatic mammals have single, non-lobed lungs; these can fill with air, or expel it, much more rapidly, allowing animals to surface only briefly in order to “refill.” While hippos do spend plenty of time in the water, it was unclear why a pig descendant would first evolve lobed lungs, then re-evolve single-sac lungs. Much more reasonable is the assumption that an aquatic or semi-aquatic hippo ancestor with non-lobed lungs simply evolved to be even better-suited to the water (in the case of the whales).
Still not convinced? There’s still one very vocal piece of evidence: the hippo’s massive, strangely oriented larynx.
In mammals, the size of the voice-box corresponds to the sounds it can make; bigger larynxes can boom out bass-range sounds, tiny ones are great for treble. In hippos, not only are the larynxes disproportionately large, they’re oriented at a different angle than most terrestrial animals, in a way that matches whales’ voice boxes, and which is thought to help transmit the sounds into the water. Hippos have also been known to produce clicking sounds that resemble those made by orcas; it’s thought that most of their communication in fact occurs underwater.
Considering the weighty evidence (and the similarity in size), maybe the hippopotamus needs a new name. “Cetosedaphos,” a combination of ketos and edafos, sounds about right to us; that would be (approximate) Greek for the “land whale” we’ve come to know as a hippo!