S.F. scientists theorize about last common ancestor of humans, apes

A chimpanzee looks for its treat after opening a wrapped plastic egg on Easter at the zoo in La Fleche, western France, on April 5, 2015. Researchers now say humans’ last common ancestor linking us to apes was probably more chimp-like than people-like. AFP PHOTO / JEAN-FRANCOIS MONIERJEAN-FRANCOIS MONIER/AFP/Getty Images less A chimpanzee looks for its treat after opening a wrapped plastic egg on Easter at the zoo in La Fleche, western France, on April 5, 2015. Researchers now say humans’ last common ancestor linking us to apes ... more Photo: Jean-francois Monier, AFP / Getty Images Photo: Jean-francois Monier, AFP / Getty Images Image 1 of / 3 Caption Close S.F. scientists theorize about last common ancestor of humans, apes 1 / 3 Back to Gallery

Far back in time, there was a common ancestor in the animal world whose evolutionary lineage led down separate paths to humans and to our remote cousins the apes — but what that creature looked like, no one is sure.

Now a team of scientists contends that our “last common ancestor,” as scientists call it, must have clearly resembled the looks and behavior of today’s chimpanzees much more than humans.

The scientists maintain that the evolution of chimps and other knuckle-walking African apes must have been slow, changing in relatively minor ways during the 6 million to 7 million years since that common ancestor probably walked the Earth. Meanwhile, the human line may have evolved at a much faster pace, from ape-like creatures swinging in the trees to modern people walking erect on two feet, throwing objects, making tools, and reasoning, for better or worse.

Nathan M. Young, an evolutionary biologist and anatomist at UCSF, and Zeresenay Alemseged, the curator of anthropology at the California Academy of Sciences, have just published their conclusions in the Proceedings of the National Academy of Sciences. Their findings are bound to be controversial in the already contentious arenas of fossil studies and evolution.

Alemseged is the scientist who discovered the famed fossil he named Selam, better known as Lucy’s Child, whose bones he found buried in the Ethiopian badlands of Dikika. She had lived about 3.3 million years ago, and was a member of the pre-human hominin species called Australopithecus afarensis.

The equally famed Lucy — no relative — belonged to the same species but lived much later, about 3.18 million years ago, and her fossil bones showed that both she and Selam were already clearly adapted to be upright walkers who could also climb trees.

Alemseged and Young examined images and casts of the fossils’ scapulas — their shoulder blades — and compared them to the intricately detailed forms of other ancient fossils. They also compared them to the scapulas of modern African apes — chimps, bonobos, gorillas and the Sumatran orangutan — and to modern human specimens from all races and eras — more than 200 in all, Alemseged said.

In addition, they examined sample scapulas in 3-D images, using the synchroton X-ray light source at the Max Planck Institute in Germany, where Alemseged was working as a young anthropologist when he first unearthed Selam’s bones.

In two earlier reports, he said, he and his colleagues had reported that in those ancient fossils the evolution of the shoulder bones gave the clearest evidence of how well pre-humans, known as hominins, must have been adapted to upright walking and throwing objects, as well as tree climbing and nesting in trees.

“We found that a sustained shift in scapular shape occurred during hominin evolution from an African ape-like ancestor to a modern human-like form,” the scientists said in their latest report.

Their findings on human evolution, they said, “suggest a long, gradual shift out of the trees and increased reliance on tools as our ancestors became more terrestrial.”

“From our findings it looks as though the chimps haven’t changed a lot since our last common ancestor, compared to the major changes we humans have undergone,” Young said in an interview.

And stirring the controversy pot from UC Berkeley was Tim D. White, a noted paleontologist and leader of the team that discovered Ardi — whose scapula has never been found.

“This approach attempts to infer the common ancestor’s anatomy based on highly evolved modern apes and fossil descendants,” White said in an e-mail. “But in paleontology, such speculations have repeatedly been shown to be inaccurate when the actual bones were eventually found. And that’s why some of us are still looking for the fossils."

Colleagues in Young and Alemseged’s report were Terence Capellini, an evolutionary geneticist at Harvard, and Neil Roach, an anthropologist at the American Museum of Natural History in New York.

David Perlman is The San Francisco Chronicle’s science editor. E-mail: dperlman@sfchronicle.com Twitter: @daveperlman