An international team of scientists unveiled Thursday the results of 15 years of study of one of the oldest known human ancestors, Ardipithecus ramidus, which they say overturns much of what we know about human evolution.
And surprisingly, it's also rewriting the story of our relation to gorillas and chimpanzees, our closest living relatives.
Yohannes Haile-Selassie, one of the authors involved in the research and the man who discovered the first pieces of the most complete Ardipithecus ramidus specimen, nicknamed Ardi by the researchers, says the findings represent a complete rewrite of what is known about human and ape evolution, and give new insight into how we became bipedal.
"What we are seeing ... is something we never expected to find in the human lineage," he says, his voice buoyant on the phone from the Cleveland Museum of Natural History, where he is curator/head of physical anthropology.
"It's a revelation, and you can imagine how much it's going to change how we think about the earliest parts of our evolution."
The peer-reviewed findings appear today in a special edition of the online journal Science, published by the American Association for the Advancement of Science. They were also announced Thursday morning in simultaneous news conferences in Washington, D.C. and Addis Ababa, Ethiopia.
"I've said it before -- it's like discovering a time capsule to a period and place that we knew nothing about," said co-author Tim White of the University of California, Berkeley, speaking in Washington.
The story of Ardi takes us back 4.4 million years to a corner of northeast Ethiopia that today is a desert where erosion constantly exposes fossils from the dawn of humankind.
In all, scientists have discovered fossilized bones and teeth in the area representing three dozen individual Ardipithecus specimens, including much of Ardi's skull, pelvis, lower arms and feet. There were also 150,000 samples of plant and animal fossils and rocks that together give a detailed picture of Ardi and her environment -- how she lived, what she ate, how she and her fellows would have interacted.
Until now, Haile-Selassie says, much of what we knew about our ancient past derived from comparisons with the other apes, especially chimps, and from Ardi's younger "sister" -- Lucy, the 3.2-million-year-old specimen of another hominid species, Australopithecus afarensis, discovered in 1974, also in Ethiopia.
Lucy's discovery showed that human forebears walked upright that long ago.
But Ardi shows our first erect steps took place more than a million years earlier and that is much closer to the last common ancestor that the human line shares with the ape line, said co-author C. Owen Lovejoy of Kent State University, speaking in Washington. Further, it pushes back the likely date of the split between the two lines to between six and nine million years ago, he said.
"For years, because of the genetic similarity of chimps and humans, it's been presumed that our ancestor would have been chimplike. Ardi tells us that's not the case," Lovejoy said.
Ardi shows that unlike modern apes, which are knuckle-walkers, her species -- and all the ancestors of all apes and humans -- descended from a common ancestor that in turn was not a knuckle-walker.
Through analysing Ardi's teeth, pelvic bones, hands and feet, the researchers determined Ardipithecus had a mixture of primitive traits, shared with its older relatives, and traits that only later hominids -- like Lucy and us -- have.
However, they also found many of those traits do not appear in modern apes, leading to the conclusion that apes have evolved significantly since the split with the last common human-ape ancestor.
In Washington, the Ethiopian ambassador to the U.S. said the discoveries about Ardi show the "interconnected" nature of the human race.
Calling Ethiopia the "cradle of humankind," Samuel Assefa said: "In the wider sense, we are all Ethiopians."
Altogether, 47 different authors, in 11 detailed papers and further summaries contributed to the study of Ardipithecus ramidus and its environment.
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