After 10 years of exhaustive analysis, an international team of researchers has unveiled a tiny fossil skeleton that is among the oldest primates ever discovered. Dubbed Archicebus achilles, the diminutive creature lived 55 million years ago in a tropical forest in what is now central China, where its body was entombed in rock at the bottom of an ancient lakebed. Now, scientists hope that its fossilized bones will help answer some fundamental questions about how, when, and where our earliest primate ancestors evolved.
While most other early primates are represented in the fossil record by a few teeth or a foot bone here and there, A. achilles looks remarkably good for its age. Its hind legs and nearly all the vertebrae in its long tail are strikingly well-preserved, giving scientists a clear picture of the animal's lower half. And with the help of powerful x-rays generated by the European Synchrotron Radiation Facility in Grenoble, France, the researchers even managed to reconstruct key features of its partially crushed skull.
By comparing A. achilles's anatomy with the bodies of all other living and fossil primates, as well as a healthy number of closely related mammals, the team determined that it is most likely a very early ancestor of modern tarsiers, small nocturnal primates that today are found on only a handful of islands in Southeast Asia. With enormous eyes that help them see in the dark and long heel bones that facilitate powerful leaps, "tarsiers are like primates from Mars," says K. Christopher Beard, a vertebrate paleontologist at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, and a co-author of the study. Still, scientists know that these strange creatures share a common ancestor with anthropoids, the group of primates that includes monkeys, apes, and humans. For decades, they've wondered how that common ancestor would have looked and behaved. Was it big like a monkey or small like a tarsier? Was it active during the day or did it lead a nocturnal life? What did it eat? And where did it live?
Because A. achilles "sits at that critical part of the tree right where the tarsier branch is splitting away from the anthropoid branch," Beard says, it can help scientists begin to answer those questions. For one, it was tiny. Weighing in at less than 1 ounce,A. achilles was smaller than any primate alive today, the team reports online today in Nature, adding support to the hypothesis that the earliest primates were diminutive, shrewlike creatures that fed on calorie-rich insects.
And although the existing evidence places the creature just slightly toward the tarsier branch of the primate family tree, A. achilles has some strikingly anthropoidlike features. Its feet, for example, were "a real shocker," Beard says. With relatively short toes and a short heel bone, they look almost exactly like the feet of small South American monkeys such as marmosets—and almost nothing like modern tarsier feet. What's more, its eye sockets were relatively small, suggesting that it hadn't yet evolved the gigantic eyeballs that allow modern tarsiers to see in the dark. Therefore, the team concludes, it must have hunted during the day.
"What this new fossil is telling us is that the common ancestor of tarsiers and anthropoids really was a hybrid," Beard explains. "It would not have been in any way completely monkeylike, but it certainly wasn't completely tarsierlike, either. It had certain features of both lineages already present."
Also significant is where A. achilles was found: China. The location of its discovery supports the once-controversial hypothesis that primates first evolved in Asia. When Beard first proposed that idea in the 1990s, he was "completely ridiculed," he recalls. "Everybody knew that everything in primate and human evolution occurred in Africa." But with a steady stream of early primate fossils being discovered in Asia, the field has gradually accepted that primates probably emerged there and only later migrated to Africa, where some groups eventually evolved to become humans.
Although several experts—including Beard himself—expect debate about the precise position of A. achilles on the primate family tree, they all agree that it is a remarkable specimen. "You don't get these kinds of complete fossils very often," says John Fleagle, a paleontologist at Stony Brook University in New York who was not involved in the research. "It documents an aspect of primate evolution that we didn't have much documentation for."
Paleontologist Richard Kay of Duke University in Durham, North Carolina, who was not involved in the study, is particularly impressed by the team's work on what he calls "probably the most comprehensive phylogenetic analysis that's ever been done on primates." Still, he adds, "I think we have to realize that there's a lot missing here," especially when it comes to the details of A. achilles's crushed skull.
Beard and his team are already working on a second round of analysis of A. achilles. Still, to draw firm conclusions about its role in primate evolution they'll need to find similarly well-preserved fossils of the tiny creature's closest relatives—a daunting task. "There was so little to compare this thing to," Beard says. "We've got this flag that we can plant for [A. achilles]. We need some more flags to plant nearby."