On the Origin of Hippos
(March 17th, 2015) They look like pigs but their genetic make-up puts them closer to whales and dolphins. Now, new fossil finds start to solve the mystery of the evolutionary history of the “river horse”.
Hippos are strange mammals. They lack hairs and sweat glands, and have an unusually thick skin. The only other mammals sharing these features with hippos are whales, but they look nothing alike, except they’re also huge and live in water. Coincidence?
Traditionally, hippos were included in the Suidae (pigs) branch of the mammalian evolutionary tree, but molecular data unambiguously show hippos to be closely related to cetaceans (whales, dolphins and porpoises). This not only sounds unlikely (hippos look much more like pigs than whales), but fossils to test this hypothesis were also lacking. So the origin of hippos remained a mystery. Now, a new fossil discovery by a team of French and Kenyan palaeontologists might have tipped the balance of the hippo evolutionary history.
Fossils of hippo are rare. Every now and then a tooth pops up, but bones are nearly impossible to find. “To make a comparison between whales and hippos we need to find their ancestors. We had the whale ancestor but until now the hippo ancestor was unknown,” says Fabrice Lihoreau, a palaeontologist at the University of Montpellier, in France.
In 2005, Lihoreau and colleagues discovered a mandibula with teeth of unusual morphology in the palaeontological collection of the National Museum of Kenya, in Nairobi. Lihoreau is an expert on anthracotheres, a diverse group of semi-aquatic herbivorous mammals that lived in Africa from around 2 to 40 million years ago. For some time palaeontologists have suspected that anthracotheres could be the ancestor of hippos, but fossil evidence was lacking. “We published many studies suggesting hippo is related to anthracotheres, and not to pigs. This new discovery not only supports that, but it tells us precisely which lineage of anthracotheres hippos originated from,” Lihoreau explains.
The newly found teeth have morphological features of both anthracotheres and hippos. They belonged to a large herbivorous mammal that thrived in Lokona, Kenya, around 28 million years ago. The discovery of this new hippo-like anthracotheres, named Epirigenys lokonensis for ‘hippo’ (Epiri) and ‘origin’ (genys), shows that hippos are definitely not pigs - they originated from an old lineage of antracotheres, the bothriodontines. And not only that, Lihoreau says, “we added a bit more to the history of mammals in saying that hippos are African, they were born in Africa.”
Many African mammals (rhinos, elephants, giraffes…) originated in Eurasia and then migrated to Africa in two large waves of migration, around 35 and 20 million years ago. Because the oldest fossils of a ‘true’ hippo are about 16 million years old, palaeontologists have assumed they crossed into Africa on a land bridge during the second wave of migration. But Epirigenys lived 28 million years ago, so hippos must have originated from their anthracotheres ancestor in Africa. This also explains why fossils of hippo ancestors hadn’t been found before: palaeontologists had been looking in the wrong place.
But are hippos whales? The discovery of Epirigenys doesn’t prove that hippos and whales came from the same ancestor, but it makes any different scenario rather unlikely. “This study is very important because now we have a hippo ancestor. And we know that the ancestors of hippos are from South-East Asia, and the ancestors of whales are also from South-East Asia, from the same period,” Lihoreau says.
Lihoreau and colleagues are now going to focus on searching for the ancestor of anthracotheres in South-East Asia, to then compare it with the ancestor of whales, which is well known. If the team gets lucky, they might find their “holy grail” - the common ancestor of hippos and whales.
Jonathan Geisler, a palaeontologist at the New York Institute of Technology who studies the evolution of dolphins and whales says, “About 15 years ago, there was a big gap between the age of the earliest hippos and the oldest whales. These authors, and their collaborators, have been steadily filling this gap by discovering new fossils, as well as through detailed studies that have moved known fossil species into this gap.”
Many questions remain unresolved. Lihoreau suspects that hippo ancestors hopped into Africa around 30 million years ago alone and… swimming. “This is somewhat speculative but certainly seems possible,” says Geisler. “There is evidence to suggest some anthracotheres were semi-aquatic, and were able to make this crossing.” This hypothesis implies that the hippo-whale ancestor already lacked hairs and sweat glands, which would have “constrained the evolution of the hippo group to get into water,” Lihoreau says. His team is going to collaborate with geologists and geochemists to try and figure out in what sort of environment hippo ancestors were living. This should help us understand what shaped the evolution of hippos towards their semi-aquatic lifestyle, which is very rarely seen for herbivorous mammals (capivaras and beavers are the only other exceptions).