Pop quiz: are you a haplorhine or a streppsirhine? Or to put it another way, are you more like a lemur or a tarsier? If you don’t know, don’t care and don’t understand such crazy talk in the first place, you’re clearly not interested in early mammalian evolution — and you’re probably not alone.
That doesn’t mean the field doesn’t affect you profoundly. We all have an ancestry that goes back much further than Homo erectus and other primitive human species, far further even than the time, between seven and eight million years ago, when we split of from our nearest non-human relatives, the chimps. Go back far enough, and we’re distant cousins, at least, to the tiniest tree shrew and the great blue whale.
But the details of those relationships are far from complete, which is why a paper in the latest Nature is so intriguing. An international team of scientists has found a remarkably complete skeleton of a tiny tree-dwelling mammal that lived some 55 million years ago in what is now China, just 10 million years after most dinosaurs were driven abruptly to extinction. “It’s a remarkable specimen,” said co-author Christopher Beard, of the Carnegie Museum of Natural History, in Pittsburgh, at a press conference.
One reason for that is simply its age. “This is the earliest known primate skeleton,” said lead author Xijun Ni, of the Chinese Academy of Sciences, also at the press conference. And not by a little, either: it’s more than seven million years older than the next oldest primate ever found. “It will tell us a lot of stories about the original primates.”
That’s partly a result of the fact that the fossil, known as Archicebus achilles, is so complete (Archicebus means “ancient monkey,” and Achilles is the surname because the animal has an unusual heel bone). The animal was entombed in lake sediments soon after it died, and was almost perfectly preserved as the sediments hardened into rock. It was easy enough to extract the rock sample from the lake, but extracting the skeleton from the rock might have damaged it, so the scientists didn’t bother. Instead, they used a particle accelerator known as the European Synchrotron Radiation Facility, in Grenoble, France, which produces extremely powerful X-rays, to make high-precision 3-D images. “It’s amazing,” said co-author John Flynn, of the American Museum of Natural History, in an interview. “You can actually study both the outside and the inside of the bones.”
A. achilles will tell paleontologists an enormous amount about a crucial period in primate evolution. The tiny animal — about eight inches long, but most of it tail — lived during an anomalously warm period known as the Paleocene-Eocene Thermal Maximum. “The entire planet was tropical or sub-tropical,” said Beard. “It was a great time to be a primate.” It was during this period that the group known as the haphlorines diverged into two branches, one of which developed into modern tarsiers and the other into the anthropoids — that is, monkeys, apes and humans. (The haphlorines had diverged earlier on from the streppsirhines, which led to modern lemurs. The quiz answer, therefore, is that you’re a haphlorine, and more closely related to tarsiers than to lemurs).
The tarsier-anthropoid split had already happened — but only just —when A. Achilles came along. The new fossil isn’t our direct ancestor, but it brings us a huge step closer to the common ancestor of both lineages. “It really helps constrain the timing of that split,” said Flynn, and along with other recent discoveries, helps fill in key gaps in the history of mammals.
It also confirms the idea that mammals began their evolutionary march to world domination in Asia. Humans first emerged in Africa, but our anthropoid ancestors didn’t arrive there until about 38 million years ago. “We still don’t know how they made it,” said Beard, “because Africa was still an island at that time. It can’t have been easy to cross the water.”
But cross it they clearly did, eventually evolving into a creature that made the final split: chimps on one branch and on the other, a series of increasingly human species, including Ardipithecus, Australopithicus, Homo habilis, Neanderthals — and us.