A husband-wife fossil-hunting team in Spain has excavated the skeleton of a 13 million-year-old creature they believe is close to the last common ancestor of modern great apes and humans. The discoverers say it bridges the gap between earlier, more primitive creatures and more modern ones that walked upright. 

What looked a little like a monkey, a lot like an ape, and had upright posture? The answer is "Pierolapithecus catalaunicus."   That's the scientific name for the unusually complete fossil anthropologists dug up near Barcelona. In an article published in the journal Science, they argue that it was a link in the line that eventually diverged to become the various two-legged great apes of today, gorillas, chimpanzees, orangutans and people.

One of the discoverers is Salvador Maya-Sola of the Crusafont Institute of Paleontology in Barcelona.

"The [fossil] probably is or is very close to the last common ancestor of great apes and humans," he noted.

The animal shows key traits seen in people and great apes that distinguish them from other primates, like monkeys and gibbons. It had a relatively flat face and flexible wrists, although it retained the more primitive small hands and short fingers and toes of monkeys. Its wide, shallow rib cage and its stiff lower spine show that the animal, which weighed about 35 kilograms, stood upright, although not on the ground because it was a tree dweller.

Professor Maya-Sola says upright movement is what sets great apes apart from all other primates, which walk on all four limbs.

"Our fossil is the oldest one that shows the basic body design that points toward this specific and very peculiar body design. This is the point," he added.

The professor's wife and co-author Meike Kohler calls the specimen a missing link.

"It really fills a gap [in which] we did not find any fossil animal that could fill this gap," she explained.  "We had to expect something there and this is what we found."

The primate fossil record from this period is sparse. The great apes are thought to have split from the lesser apes, a group including today's gibbons, about 11-16 million years ago, and researchers have long been searching for the great ape ancestors that emerged after this division.

A deputy editor for the journal Science, Brooks Hanson, calls the Barcelona discovery a key in this hunt.

"It's an important fossil in primate evolution at a time when the exact nature of the evolution of apes and humans has not been completely clear," said Mr. Hanson.  "Having a much more complete skeleton to work with that shows a variety of characteristics that are both primitive and derived is an important step in understanding this important radiation in evolution."

But because there are few fossils with which to compare the new find, opinions about its place in the ape family tree vary. Harvard University anthropologist David Pilbeam told Science magazine that it could be even more primitive than the discoverers say and not necessarily related to the great apes. In his view, the creature could have acquired its nearly flat face, wide rib cage, stiff lower spine, and wrist features independently long before last the great ape common ancestor lived.

On the other hand, University of Toronto anthropologist David Begun says the fossil might be younger than the Barcelona team suggests. He argues that its facial features indicate it lived after the great apes split into the African branch represented by gorillas, chimps and humans, and the Asian branch represented by orangutans.

"To me it appears that this new specimen may be already on the line leading to the African apes and humans," he said.  "It would have branched off on the line leading to African apes and humans after the orangutans branched off."

No matter where precisely the fossil can be placed on the ape development ladder, Mr. Begun and others describe it as a great discovery.