Scientists have unearthed remains of a primate that could have been ancestral not only to humans but to all great apes, including chimps and gorillas.
By Paul Rincon
BBC News science reporter
The partial skeleton of this 13-million-year-old "missing link" was found by palaeontologists working at a dig site near Barcelona in Spain.
Details of the sensational discovery appear in Science magazine.
The new specimen was probably male, a fruit-eater and was slightly smaller than a chimpanzee, researchers say.
Palaeontologists were just getting started at the dig when a bulldozer churned up a tooth.
Further investigation yielded one of the most complete ape skeletons known from the Miocene Epoch (about 22 to 5.5 million years ago).
Salvador Moyą-Solą of the Miquel Crusafont Institute of Palaeontology in Barcelona and colleagues subsequently found parts of the skull, ribcage, spine, hands and feet, along with other bones.
They have assigned it to an entirely new genus and species: Pierolapithecus catalaunicus.
Great apes are thought - on the basis of genetic and other evidence - to have separated from another primate group known as the lesser apes some time between 11 and 16 million years ago (The lesser apes include gibbons and siamang).
It is fascinating, therefore, for a specimen like Pierolapithecus to turn up right in this window.
Scientists think the creature lived after the lesser apes went their own evolutionary way, but before the great apes began their own diversification into different forms such as orang-utans, gorillas, chimps and, of course, humans.
"Pierolapithecus probably is, or is very close to, the last common ancestor of great apes and humans," said Professor Moyą-Solą.
The new ape's ribcage, lower spine and wrist display signs of specialised climbing abilities that link it with modern great apes, say the researchers.
The fossil has been described as a "missing link"
The overall orthograde - or upright - body design of this animal and modern-day great apes is thought to be an adaptation to vertical climbing and suspending the body from branches.
The Miocene ape fossil record is patchy; so finding such a complete fossil from this time period is unprecedented.
"It's very impressive because of its completeness," David Begun, professor of palaeoanthropology at the University of Toronto, Canada, told the BBC News website.
"I think the authors are right that it fills a gap between the first apes to arrive in Europe and the fossil apes that more closely resemble those living today."
Planet of the apes
Other scientists working on fossil apes were delighted by the discovery. But not all were convinced by the conclusions drawn by the Spanish researchers.
Professor Begun considers it unlikely that Pierolapithecus was ancestral to orang-utans.
"I haven't seen the original fossils. But there are four or five important features of the face, in particular, that seem to be closer to African apes," he explained.
"To me the possibility exists that it is already on the evolutionary line to African apes and humans."
Professor David Pilbeam, director of the Peadbody Museum in Cambridge, US, was even more sceptical about the relationship of Pierolapithecus to modern great apes: "To me it's a very long stretch to link this to any of the living apes," he told the BBC News website.
"I think it's unlikely that you would find relatives of the apes that live today in equatorial Africa and Asia up in Europe.
"But it's interesting in that it appears to show some adaptations towards having a trunk that's upright because it's suspending itself [from branches].
"It also has some features that show quadrupedal (four-legged) behaviour. Not quadrupedal in the way chimps or gorillas are, but more in the way that monkeys are - putting their fingers down flat," he explained.
During the Miocene, Earth really was the planet of the apes.
As many as 100 different ape species roamed the Old World, from France to China in Eurasia and from Kenya to Namibia in Africa.