The ancestral "mother" of all great apes - and yes, that means us humans too - may have been a creature smaller than previously assumed, researchers say.
All living great apes weigh in at between around 65 to 395 pounds. This covers everything from chimpanzees to humans and on up to massive gorillas, so researchers always assumed the original ancestor ape should have been at least a bit hefty.
However, a new fossil - a partial skeleton of a primitive ape believed to be 11.6 million years old - despite having traits linking it to humans and all living apes, may have weighed no more than 10 pounds.
Writing in the journal Science, researchers suggest the new primate, Pliobates cataloniae, existed before the evolutionary divergence of great apes - chimps, orangutans, humans and gorillas - and the so-called lesser apes, much smaller species like gibbons and siamangs.
According to the researchers, that would put P. cataloniae, or at least one of its own ancestors, at the very route of the family trees of all apes.
The fossil is named after a region in Spain called Catalonia, where it was discovered.
As the last common ancestor of both great and lesser apes, its small size had surprised some paleontologists.
"Pliobates indicates that small-bodied taxa might have played a much more significant role than previously thought in the evolution of modern hominoids (great apes), and that their last common ancestor might have been, in some respects, such as skull shape and body size, more gibbon-like than previously thought," says study leader David Alba of the Institut Catala de Paleontologia Miguel Crusafont.
Nevertheless, the new ancestor is more closely related to modern day apes and humans than some previous candidates such as the Proconsul, an extinct genus that lived between 25 and 23 million years ago, he says.
That's when apes and monkeys split from each other, then in another split around 17 millions years ago the apes divided into two groups, the great and lesser apes.
The Pliobates fossil displays some primitive traits that are similar to those of modern day gibbons, including a small, monkey-size brain, small pointed teeth and protruding eye sockets.
However, it also shows some more modern-like traits, such as a short, wide cranium and bones in the elbow and wrist that allowed those joints to rotate form climbing about in trees - traits found in great apes but not in lesser apes.
At just 11.6 millions years old, Pliobates lived too recently to be an actual common ancestor of all great apes, but researchers say it could well be a late-surviving, still-primitive descendant of that ancestor.
It's a case of size doesn't - or didn't - matter, the researchers say, adding it was probably the great apes, not lesser ones, which diverged most from a notably small ancestral body plan.
"We should be careful about discounting small-bodied taxa as the last common ancestor," Alba says.
Another expert, New York University paleoanthropologist Terry Harrison, agrees with that.
"You can't ignore all the little guys," he argues.