From sturdy chimpanzees to massive gorillas to humans themselves, the living great apes are all large-bodied, weighing between 30 and 180 kilograms. So for years most researchers thought the ancestral ape must have tipped the scales as well. But the partial skeleton of an 11.6-million-year-old primitive ape may force scientists to reimagine the ancestor of all living apes and humans. With a muzzle like a gibbon but a large brain for its body size, the ancient primate has traits that link it to all apes and humans—yet it weighed only 4 kg to 5 kg, according to a report today in Science.

The ancient skeleton was found near Barcelona, Spain. If that seems strange, that’s because a bewildering number of extinct apes once roamed far and wide across the forests of Europe, Asia, and Africa during the Miocene Epoch, 5 million to 23 million years ago. After the ancestors of apes and monkeys split into two groups roughly 25 million years ago, apes underwent a remarkable florescence, evolving into more than 30 different types. About 17 million years ago, these early apes diverged into two distinct groups—the “lesser apes,” small-bodied, tree-living creatures represented today by gibbons and siamangs, and the great apes, which include chimpanzees, bonobos, gorillas, orangutans—and humans.

Until recently, most researchers assumed that the fossils of small Miocene apes were the ancestors of gibbons or extinct lineages of little primates, whereas the larger bodied fossil apes were the forebears of greater apes and humans. “For decades, the small stuff was thought to be related to gibbons and the big stuff was thought to be related to great apes,” says paleoanthropologist John Fleagle of the State University of New York at Stony Brook. And many researchers have thought that a large-bodied, 18-million-year-old ape called Proconsul from Kenya offered the best model for the ancestor of all apes.

This neat split is now being challenged by a strange new Miocene ape—Pliobates cataloniae, named for the province of Catalonia in Spain. In January 2011, a team of paleontologists monitoring bulldozers excavating a landfill 50 kilometers northwest of Barcelona found 70 crucial pieces of an ancient primate skeleton: the cranium (the top of the skull), chunks of the upper jaw and muzzle, plus arm, hand, and hind bones, all buried in a layer of sediment reliably dated to 11.6 million years ago.

The bones of the skull were crushed against each other, so rather than trying to break them apart, paleobiologists David Alba and Salvador Moyà-Solà of the Institut Català de Paleontologia Miquel Crusafont in Sabadell, Spain, and their colleagues scanned the entire chunk of skull and rock using computed tomography. Then they created a 3D reconstruction of the skull. They noted that this fossil had primitive traits, such as a monkey-size brain, small teeth with sharp cusps, and eye sockets that telescoped out, similar to those of a gibbon. But it also had more modern or derived traits that link it to great apes, including a short, wide cranium. What really caught the team’s attention were elbow and wrist bones that would have allowed Pliobates to rotate its wrist and forearm for climbing and clambering in trees. Great apes including humans have these traits, but lesser apes do not.

This unusual mosaic of primitive and modern traits suggests that Pliobates didn’t launch itself from branch to branch like gibbon apes, but instead climbed relatively slowly in the trees, moving atop the branches carefully to eat fruit. Given its mix of characteristics, the authors suggest Pliobates was related to both lesser apes and greater apes. “I could imagine something with the face of a gibbon but moving much more slowly than a gibbon, like a slow loris atop the branches … eventually being able to suspend below them,” Alba says.

But Pliobates lived too recently to be the actual common ancestor of all apes and humans, Alba says. Instead, his team proposes that it was a late-surviving, relatively primitive descendant of that ancestor, a creature that provides our closest glimpse yet of the original apes. If so, Pliobates knocks Proconsul from its perch as the previous closest ancestor and suggests “the last common ancestor of great apes and lesser apes looked nothing like chimpanzees or gorillas,” Alba says. That would mean it was the great apes, not the lesser ones, which diverged most from their ancestral petite body plan. “We should be careful about discounting small-bodied taxa as the last common ancestor,” Alba says.

Fleagle agrees: “You can’t ignore all the little guys.” Paleoanthropologist Terry Harrison of New York University in New York City, who has been a lone voice suggesting that the last common ancestor was small, thinks that the Spanish team has made a “compelling” case that Pliobates represents a primitive ancestor of apes.

Not everyone is convinced: Paleoanthropologist David Begun of the University of Toronto in Canada still thinks the relatively large Proconsul is the closer relative, because the new fossil’s teeth and cranial base are so primitive that they “outweigh the derived attributes of the limbs.” Regardless of where Pliobates sits on the primate family tree, it “will shake things up, fuel new debates, and allow us to rethink what we thought we already know,” Harrison says.