It’s not the missing link between man and apes. But a mouse-sized tarsier that devoured insects in ancient China 55 million years ago could be a long-lost cousin who scampered in the treetops of tropical forests around the time the first primates arose in Asia, according to scientists.
A team of paleontolgists carefully peeled apart layers of sedimentary rock containing the fossil, found in China 10 years ago. Then they took the two complementary sections, each of which had parts of the fossilized primate, and subjected them to a sophisticated X-ray technique at the European Synchrotron Radiation Facility in Grenoble, France.
The X-ray synchrotron tomography ultimately enabled them to assemble the fossil in three dimensions for closer examination. The team compared about 1,000 characteristics across 157 primate species in order to place the creature on an evolutionary tree.
Archicebus achilles, as the specimen is known, came after lemur-like primates branched away from a line that led to anthropoids, such as monkeys, apes and eventually humans. It is a direct ancestor of modern Asian tarsiers, so it is not an anthropoid. But the specimen has a striking mix of features straddling both lineages, according to the researchers, whose work was published Wednesday in the journal Nature.
“We’re quite confident about its position,” said paleontologist Xijun Ni of the Chinese Academy of Sciences, who led the archaeological work in a central Chinese province better known for its trove of fish fossils.
“This is an amazingly complete fossil primate, especially for this time period,” said paleontologist Christopher Beard of the Carnegie Museum of Natural History, lead author of the paper. “There’s nothing else known from the primate fossil record that resembles this fossil.”
A. achilles is about the size of a modern pygmy mouse lemur, with large canine teeth and sharply pointed premolars that suggest an insect diet. The shape of its eye sockets suggests it was active in daylight. It also had feet adapted for grasp-leaping, unlike the more vertical clinging of modern tarsiers.
The "Achilles" name is a sly reference to the human Achilles' tendon. “To be honest, the heel and the foot in general was one of the most shocking parts of the antomy of this fossil when we first saw it,” Beard said. The animal looks like a marmoset, but the foot looks like it belongs to the ape-monkey-human line.
The find confirms a growing body of evidence that primates arose in Asia, though their most well-known progeny migrated to Africa about 35 million years ago, where they flourished and diversified, leading to humans. Tarsier-like primates remained behind.
“The two most specialized primates alive today are humans and tarsiers," Beard said. "Both of those species are completely bizarre anatomically and evolutionarily so we should not be surprised that the common ancestor of those two groups looks neither like a human nor a tarsier.”
Paleontologists who presented their findings cautioned against viewing the animal as a "missing link," although it clearly lies at a "stem" position in evolutionary divergence.
Four years ago, amid media fanfare, Norwegian paleontologist Jorn Hurum presented a similar-looking find, called Darwinius, that was dubbed the missing link in the primate line that led to humans. That claim has since been strongly questioned. Many paleontologists now believe Darwinius was more closely related to lemurs, not anthropoids.
“This analysis was by no means a rush to judgment or a rush into print," Beard said. "We’ve spent a lot of time reconstructing the evidence and pondering it, and I think that our analysis is on about as solid ground as it can be. That being said, it is an analysis.”
Hurum on Wednesday congratulated the team “for a more complete specimen than most early primate fossils except Darwinius.” But, he added, “it is a pity that they do not discuss Darwinius.”