Fossils From Ethiopia May Be Earliest Human Ancestor
San Francisco Chronicle
|July 12, 2001|
A team of scientists led by an anthropologist at the University of California-Berkeley has discovered the fossilized remains of what they believe is humanity's earliest known ancestor, a creature that walked the wooded highlands of East Africa nearly 6 million years ago.
The discovery, which occurred in the Middle Awash River Valley of Ethiopia, is already challenging some existing theories about the ancestral lineage of humans. It is also changing scientific views about the nature of the environment that fostered the evolution of pre-humans as they moved from verdant forests to open grasslands.
The team reporting the discovery in the July 12 issue of the journal Nature was led by two Ethiopian scholars: Yohannes Haile-Selassie, an anthropologist still working on his doctorate at the University of California at Berkeley, and Giday WoldeGabriel, a geologist now at UC's Los Alamos National Laboratory in New Mexico.
The fossils were gathered during four years of demanding expeditions to a harsh and hostile Ethiopian scrubland where lions and cheetahs hunt at night and few people roam the semi-desert wilderness by day.
The remains include a jawbone with teeth, hand bones and foot bones, fragments of arms, and a piece of collarbone. But most important, the bones also included a single toe bone. Its form provides strong evidence that the pre-human creatures walked upright, the scientists said.
The toe bone is a crucial clue to the earliest days of human evolution as it developed soon after the ancestral lines of apes and humans split apart, perhaps 6 million to 8 million years ago.
The fossils in Ethiopia were dated by Paul R. Renne of the Berkeley Geochronology Center. Renne is a co-author of WoldeGabriel's report in Nature.
Another co-author is Tim D. White, a paleoanthropologist at UC-Berkeley who in 1994 discovered a pre-human fossil, named Ardipithecus ramidus, that was then the oldest known, at 4.4 million years.
The latest fossils from Ethiopia vary in age from about 5.2 million to 5.8 million years old, according to Renne. Haile-Selassie has tentatively named the fossils Ardipithecus ramidus kadabba, a subspecies of White's A. ramidus.
In January, a French team headed by Brigitte Senut and Martin Pickford found fossils in Kenya that they dated about 5.8 million years old, from a creature they nicknamed "Millennium Man." Pickford said the newly discovered fossils in Ethiopia are "virtual contemporaries."
It's not yet clear where the fossils of Haile-Selassie and WoldeGabriel belong on the family tree.
The world of paleoanthropology is highly contentious, and scientists have been trying for many decades to sort out the murky ancestry of today's human race by comparing thousands of fossil bones and skulls. But no evidence is certain and no lineages are clear.
Anthropologists call all the species and sub-species of our ancient ancestors hominids, to distinguish them from the ape lineage, which includes chimpanzees. The two branchesapes and hominidsare believed to have separated and evolved from one common ancestor between 6 million and 8 million years ago.
In a telephone interview from Adis Abeba (Addis Ababa), where he is analyzing the fossils, Haile-Selassie said he is being extremely conservative, and the fragments he and WoldeGabriel plucked from the sun-baked ground may represent an entirely new species of pre-human creature.
"It could be the earliest hominid, or it could be a common ancestor, or it gave rise only to the chimpanzee lineage, or it went extinct around 6 million years ago without giving rise to any species," he said.
A major mystery in the story of human evolution is how climate affected the environment where creatures that regularly walked uprightthe hominidsfirst emerged. Now, both sets of recent findsin Ethiopia and Kenyacould help resolve the puzzle.
One widely accepted theory holds that after the ape and hominid lineages split, the earliest human ancestors were forced into the expanding tropical grasslands of the African savanna after the continent's thick forests dwindled as a result of climate change.
But geochemical analysis of the ancient sedimentary soils where Haile-Selassie's Ardipithecus creatures lived shows that the region between 5 million and 6 million years ago was well forested, well watered, and rich in woody plants, according to anthropologist Stanley Ambrose of the University of Illinois, who is also a chemist and a co-author of WoldeGabriel's report in Nature.
The clear inference, according to Haile-Selassie and WoldeGabriel, is that those early human ancestors of the Miocene epoch were already thriving in the forests of a land that was then being shattered by volcanic eruptions, and millions of years later was to become the stony scrubland it is today.
Copyright 2001 The San Francisco Chronicle
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