Wallowing in Africa's rivers by day and grazing on their shores by night, hippopotamuses have spurred a long scientific debate about just how they fit into the mammalian family tree.
Some scientists have argued that hippos are most closely related to peccaries and wild pigs. More recent DNA analyses suggested a closer kinship with modern whales; but that proposition left a troubling 40-million-year gap between the oldest known ancestors of whales (55 million years ago), and the earliest hippos (15 million years ago).
Now, a team of paleontologists says they've examined the bones and teeth of 10 living species of hippos, whales and pigs, and 22 of their fossil ancestors, and found evidence that the hippos' closest kin are indeed whales. The findings will appear in Tuesday's issue of the Proceedings of the National Academy of Science.
Jean-Renaud Boisserie, of University of California's Human Evolution Research Center, at Berkeley, with colleagues in France and Chad, argue that skull features quickly eliminated pigs and peccaries as the hippos' closest kin. "That was very clear," Boisserie said.
The team then found striking similarities between hippos and the most hippo-like members of a very diverse animal group called anthracotheres. This extinct family also included pig-like and camel-like members, all of them gone for at least 2.5 million years. The links include constantly growing front teeth and a grooved upper palate exclusive to the two groups.
"The anthracotheres are part of a ... very old family," Boisserie said. They first appear in the fossil record 45 million to 50 million years ago. "That allowed us to make a link with the oldest whales" - four-legged, semi-aquatic animals that first show up in South Asia 55 million to 60 million years ago.
The 40-million-year gap was largely eliminated. But precisely what sort of critter that last common ancestor of whales and early anthracotheres might have been remains unclear.
Since both hippos and many anthracotheres were semi-aquatic, Boisserie surmised, "probably this common ancestor of all of them - this ancestor we don't know because we haven't found it yet - also related with water, and I would say was probably semi-aquatic."
If they're right, the hippos' closest anthracothere ancestors apparently evolved to exploit a riverine environment which hippos eventually shared with the first humans in Africa.
Their sister lineage, meanwhile - the branch leading to modern whales - evolved to exploit the oceans, where in time they came to spend their entire lives.
Finding their common ancestor remains a challenge for paleontologists. But at least they know where to look.
"The oldest anthracotheres and the oldest whales all formed in southern Asia - India, Pakistan, Thailand - and that's probably where I would like to look for the common ancestor of whales and hippos," Boisserie said.