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, and their development as well.
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 about 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 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 Friday 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, and 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 analyzing 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.
For one thing, says Haile-Selassie, chimps and gorillas developed less flexible wrists that make knuckle-walking and swinging from branches possible only after they split from the line that led to humans.
The researchers say the surprising findings mean chimps and gorillas have specialized greatly since then and are poor models for a common ancestor and for understanding human abilities such as walking.
On that last point, Haile-Selassie says Ardipithecus’s woodland habitat and her ability to walk upright – though not as well as later hominids – also "falsifies" the long-accepted notion that bipedalism originated when our ancestors needed to see further in open grassland – or savannah – in order to avoid predators.
Ardi’s species was already walking before our first ancestors ventured onto the savannah, he says.
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."
Back in 1994, when he found the first fragments of Ardi – half a finger bone and then the other half nearby – Haile-Selassie recalls he "was excited because he knew he had some human forerunner on his hands.
"There’s something that we call `hominid fever,’ " he says with a chuckle. "When you’re in the field you want to find a hominid." But he had no inkling of the significance of his discovery.
The pieces were so fragmented and fragile that it has taken 15 years to dig out, clean, assemble and analyze the partial skeleton of Ardi. "You just touch it, it turns to powder," he says.
That in itself was quite a feat, says David Pilbeam, Harvard University Henry Ford II Professor of Social Sciences and Curator of Paleoanthropology in the Peabody Museum of Archeology and Ethnology.
"The find itself is extraordinary, as were the enormous labours that went into the reconstruction of a skeleton shattered almost beyond repair, and particularly the skull," says Pilbeam, who is not connected with the study team.
Pilbeam doesn’t accept all the authors’ conclusions, but he says the work "is one of the most important discoveries for the study of human evolution."
Altogether, 47 different authors, in 11 detailed papers and further summaries contributed to the study of Ardipithecus ramidus and its environment.
The primary authors include Tim White of the University of California, Berkeley; Berhane Asfaw of Rift Valley Research Service in Addis Ababa; Giday WoldeGabriel of Los Alamos National Laboratory; Gen Suwa of the University of Tokyo and C. Owen Lovejoy of Kent State University.
The studies are available by logging onto http://www.sciencemag.org/ardipithecus
Who was Ardipithecus ramidus?
-They were creatures with long arms, made for climbing, short legs and rigid feet that enabled them to walk upright on the ground – despite having an opposable toe. Yohannes Haile-Selassie, of the Cleveland Museum of Natural History, says they would have probably walked only short distances, given that they had to ‘toe off’ painfully on the second toe, unlike Lucy and all other hominids we know of who pushed off on the big toe.
-The brain was small, similar to a chimp’s. They stood about four feet and weighed about 50 kilograms, with long-fingered, flexible hands and wrists and would have moved about in the trees by walking along branches supporting their weight on their palms.
-The pelvis had features that supported upright walking, as well, and the gluteal muscles allowed it to walk without shifting the centre of mass from side to side, researchers say.
-The males’ canine teeth were similar in size to the females’ – unlike modern apes – and far smaller and blunter than those of the apes From that, researchers infer the males were less aggressive than male chimps and gorillas, who use their large, sharp canines in conflicts.
-Ardipithecus would have been as adept on the ground as in the trees that at that time grew in the Afar Rift’s then temperate climate. (The climate is inferred by the presence of fossilized animals, wood, seeds and other plant materials fromsuch an environment, which also give clues to Ardipithecus’s omnivorous diet.)
According to Haile-Selassie, even 150 years ago, Charles Darwin understood that although chimps are our closest living relatives, we cannot look to them for clues about our evolution, that there had to be a common ancestor.
Ardi isn’t that ancestor but she is the closest to finding it that we have come so far, the research shows.
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