A fossil hip bone from an ape nicknamed “Gaby” provides new insight into why humans are the only members of our animal family to walk upright.

That’s the conclusion of a new study, accepted for publication in the Journal of Human Evolution, whose lead author is Carol Ward, curator’s professor of pathology and anatomical sciences in the University of Missouri School of Medicine. She is the world’s foremost expert on fossil ape pelvises, and that is what was found at a dig near a Hungarian mining town that gives the ape species its name, Rudapithecus hungaricus.

The section of pelvis, almost certainly from a female, is about 10 million years old. It was found in 2006 along with other fossils from the same individual, including a cranium, jaw, and two femurs.

When David Begun of the University of Toronto, Ward’s advisor in her post-doctorate lab in graduate school, called, he said he found a pelvis and sent some pictures, Ward said.

“It blew my mind because it looked so modern,” Ward said.

Only three fossilized ape pelvises from the late Miocene Epoch, from 11.6- to 5.3 million years ago, are known to science. The evolutionary lineage of humans, chimpanzees and gorillas diverged near the end of the Miocene.

The difference between the pelvis of “Gaby” and modern African apes is its size, Ward said. It was relatively smaller, indicating a longer lower back and more flexibility for standing.

Humans, chimpanzees and gorillas, like Rudapithecus, are the part of the Hominid family. Chimpanzees and gorillas have one fewer vertebra in their lower back than humans and one more in the sacrum, where the hip is attached. That means a stiffer lower back and gorillas and chimpanzees generally walk on all fours, using their knuckles to support their upper body.

Humans, however, have a flexible back, with hips and spines that are very level and very efficient, Ward said.

“You can hold a cup of coffee and not spill it because of the subtle movements of your back,” she said.

Evolutionary scientists have puzzled over how human spines evolved from the spines of chimpanzees and gorillas. Perhaps Rudapithecus shows it was the other way around, Ward said.

“If we evolved from an ancestor more like Rudapithecus, it was a much simpler process,” she said. “You wouldn’t have had to lose the stiff back at the end of the Miocene period, 5- to 8 million years ago.”

The climate Rudapithecus enjoyed in Hungary was wet, humid and forested. As the climate cooled, it is believed the European apes moved south, using land bridges that emerged to retreat to familiar territory. The early ancestors of humans, however, were adapting to a more open landscape.

“If our ancestors were already upright, our ancestors were specialized for that,” she said. “Chimps and gorillas were tied to the trees and forests, and their adaptations were to stabilize their spine by stiffening it.”

"Gaby" was clearly a female, based on the shape of the canine teeth found and its size, study states. She would have been about 50- to 60 pounds, with males of the species up to 100 pounds, Ward said. The scientists who studied her tagged her the nickname, she said.

Scientists who study evolution identify the fossil fragments they find by documenting how it was found and with what other fossils and by comparing it to other specimens and modern relatives. Ward traveled to Hungary to use advanced scanners on the pelvis bone of “Gaby” to gain information about how the muscles attached, how the ball of the femur rested in its socket and the angle of the leg relative to the rest of the body.

Ward was called in for her expertise, Begun wrote in an email.

“Carol is a leading expert in the evolution of the back and pelvis,” he wrote. “She is also a highly respected member of our paleoanthropology community and a good friend.”

The results of the study address a long-standing debate in paleoanthropology, Begun wrote.

“Did human bipedalism evolve from a quadruped, going from all fours to two legs on the ground, or, were humans already pre-adapted to bipedalism and never went through a quadrupedal phase?” he wrote. “Rudapithecus shows us that it had already evolved the preconditions for standing on two legs occasionally while still living primarily in the trees.”

There are only three living species in the hominid family – humans, gorillas and chimpanzees. But there were dozens of ape species in the distant past, including “a few oddball” species, Ward said.

The first remains of a Rudapithecus were found in the 1960s. The coal mines near Rudabanya were about played out when a geologist found the fossils and the first paper describing the find was published in 1967.

Rudapithecus is “one of the ones that appears most similar to African apes and humans,” Ward said.

Rudapithecus went extinct before the last common ancestor of all living Africa apes and humans, Begun wrote. Upright walking is a trait that may have been passed on to that common ancestor or developed in another species that would hold that place.

“It is possible that Rudapithecus is a direct ancestor of the living hominines but we can never really know that,” Begun wrote. “All we can say is that it is a member of this group.”

The recent discovery of an almost complete Australopithecus anamensis skull and face fossil from 3.8 million years ago shows the other changes taking place in the branch that led to modern humans, Ward said. In the more open country with less forest, diet changed from fruits to seeds, nuts and other harder foods, requiring stronger jaws and teeth.

Upright walking gave the ancestors of humans several advantages in the changing landscape.

“When you stand up, you can see predators, you have free hands to carry food back and make tools, and you have better thermoregulation,” Ward said.

The knowledge gained from Rudapithecus is changing the debate about the changes in our ancestors that led to humans, Ward said.

“You don’t have to invoke all these ideas about standing up when we are already upright in the first place,” she said. “What if we were upright to begin with? In science, you never get the right answers until you ask the right questions.”

rkelller@columbiatribune.com

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