The oldest known fossil primate skeleton, dating to 55 million years ago reveals that one of our earliest ancestors was a scrappy tree dweller with an unusual combination of features.
The discovery, made in central China's Hubei Province and reported in the journal Nature, strengthens the theory that Asia was the center of primate evolution. The new species, Archicebus achilles, also suggests that our earliest ancestors were very small.
"Archicebus was a tiny primate weighing less than 1 ounce," co-author Daniel Gebo of Northern Illinois University told Discovery News. "It would easily fit in the palm of your hand. Its eye orbits were not large, suggesting it was active during the daytime."
He added, "Archicebus likely bounced and climbed around the canopy, being entirely arboreal, looking for food items out on the terminal branches of trees. It had incredibly long legs and was an adept leaper. Think of little lemurs moving through the branches of trees within a rainforest setting."
Analysis, including state-of-the-art Synchrotron CT scanning, determined that the skeleton of Archicebus is about 7 million years older than the oldest fossil primate skeletons known previously, which include Darwinius from Germany and Notharctus from Wyoming.
The tiny primate lived close to the evolutionary divergence between the lineage leading to modern monkeys, apes and humans (collectively known as anthropoids) and the lineage leading to living tarsiers.
Gebo thinks the split might have happened as "each lineage tried to make themselves anatomically and ecologically different to avoid direct competition with each other, since this leads to extinction."
Given its root placement on the primate family tree, Archicebus had a mish-mash of characteristics.
An illustration of an evolutionary tree, showing how Archicebus fits with respect to primate family tree.Mark A. Klingler/Carnegie Museum
"Archicebus is a quite odd creature," lead author Xijun Ni of the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences, told Discovery News. "It has many features that support its tarsiform (like tarsiers) affinity, but also has many features typically seen in anthropoids."
It had the feet of a small monkey, but the arms, legs, skull and teeth of a very primitive primate. The researchers were surprised that it had such small eyes. Modern tarsiers have some of the largest eyes, relative to body size, in the animal kingdom. They allow the tiny primates to see well at night.
Although Archicebus hailed from Asia, the earliest known humans came from Africa.
"This suggests that a primitive anthropoid colonized Africa from Asia, and from these early African anthropoids all later catarrhines (monkeys, apes and humans) evolved," Gebo said.
As for the small size of Archicebus, other mammal lineages often started small and evolved to be bigger over time. The phenomenon is known as "Cope's Rule." No one is entirely sure why this happened among mammals, but the environment must have only supported such a size in terms of climate, food sources and other factors.
Eric Delson is a paleoanthropologist at Lehman College. When sent the study, Delson told Discovery News, "Archicebus is a fantastic new fossil, which preserves more details of its anatomy than anything of a similar age."
Delson expects more very early primates to be found in Asia, particularly in central China.
He said, "It seems likely that when more sites a bit older than this are found in eastern and southern Asia, they might provide hints about the origin and last common ancestor of extant primates."