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Scientists from Northeast Ohio are helping rewrite the book on human evolution.

In 11 papers published in the journal Science, researchers describe the oldest hominid skeleton discovered to date – a possible human ancestor in the midst of changing from climbing on all fours to walking upright. 

The hominid, named Ardipithecus ramidus, or Ardi for short, is far different from the chimpanzees so often thought to be the model of our forebears.

“It looks, in many ways, more like a modern human,” said paleontologist Scott W. Simpson, associate professor of anatomy at Case Western Reserve University School of Medicine and research associate at the Cleveland Museum of Natural History.

“It’s completely different from what we predicted,” said Simpson, a co-author of three of the papers. “This changes not only the way we think about human ancestors, but chimpanzees and monkeys, too.

“This is one of those quantum leaps in understanding.”

Humans and apes have an older, common ancestor still to be found, but this skeleton shows that chimpanzees and gorillas evolved away from us over the past 5 million years, he said.

Yohannes Haile-Selassie, curator and head of physical anthropology at the Cleveland Museum of Natural History and an adjunct professor in the departments of anthropology, anatomy, and cognitive science at Case Western Reserve, discovered the skeleton in 1994. The fossilized bones were between two layers of volcanic ash in the Middle Awash study area, a site near the Awash River in Ethiopia. They include parts of a skull, pelvis, ribs, forearm and hand, lower leg and foot.

“I knew it was an early hominid remain, but at that point, there was no way I could understand the impact it would have,” Haile-Selassie said. Now, after 15 years of study, the finding “is Amazing. It’s going to open up a lot of discussions and new questions.”

The Middle Awash project was begun by the late J. Desmond Clark, professor of anthropology at University of California at Berkley, and is currently led by Tim White, a paeloeanthropologist at the University of California at Berkley.

At  4.4 million years-old, the skeleton is  1.2 million years older than the famous Australopithecus afarensis skeleton nicknamed “Lucy.” The researchers say Ardi tells us even more about human evolution than Lucy.

Studies of the bones show that Ardi had:

  • Feet with a grasping big toe used for climbing, but the outer portion of the foot had the bone structure and muscle attachments used for upright walking.
  • Hands that differ greatly from modern chimps and gorillas, with a larger and more muscled opposable thumb. The hands are those of an animal that climbed, fed and nested differently from apes, and required little change in structure to enable it to make and use tools.
  • A flexible wrist, different from the stiff wrist chimps and apes rely on for knuckle-walking. On branches, Ardi walked on its palms, hand over hand.
  • Arms and legs more in proportion with monkeys than with the long arms and short legs of modern apes.
  • A mobile lower back with more vertebrae than the stiff backs of African apes.
  • Much smaller, diamond-shaped canine teeth and no structure to hone large canines on lower teeth, as modern chimpanzees and apes have. The modern animals use canines for tearing flesh in battles for dominance.
  • A diet of soft leafs and fruits, invertebrates and maybe small vertebrates. Teeth structure, enamel and wear show Ardi lacked the adaptations seen in Lucy and modern humans to eat abrasive foods.

Ardi was likely a large female, weighing about 110 pounds. The species – parts of others have been found - had not developed the large brain of modern humans. Fossils recovered from the same layers include kudus and numerous monkeys showing that Ardi and her kin lived in closed wooded areas and not the open savannah.

A total of 47 scientists from 10 countries contributed to the papers.

C. Owen Lovejoy, an anthropology professor at Kent State University, is lead author on five papers and a contributor to three more.

Researchers from Case Western Reserve University and the Cleveland  Museum of Natural  History contributed to a total of seven of the papers. The co-authors include Simpson, Haile-Selassie; Bruce Latimer, former executive director of the Cleveland Museum of Natural history and an adjunct professor of anatomy,  anthropology and cognitive science at Case Western Reserve; and Linda Spurlock, director of Human Health at the museum.

The papers, with extensive online documentation, are available on the Science website www.sciencemag.org.


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