Paleontologists searching the bleak desert of central Ethiopia have unearthed the fossilized bones of a 5.5 million-year-old creature that appears to be the oldest human ancestor yet discovered.
The discovery, coupled with even older remains reported recently from Kenya, has brought scientists tantalizingly close to determining what the earliest human ancestors looked like at the point in the ancient past -- somewhere between 5 and 10 million years ago -- when apes and humans diverged from a common ancestor to take separate evolutionary paths.
In 1998, researcher Yohannes Haile-Selassie of the University of California searches a site 140 miles northeast of Addis Ababa, Ethiopia, and about 50 miles south of where the fossil "Lucy" was found about three decades ago.
(David L. Brill - Brill Atlanta)
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The Ethiopian creature, dubbed Ardipithecus ramidus kadabba (root-man ancestor), had a toe bone that indicates it walked upright -- a classic characteristic separating humans from apes -- and teeth that appeared to be evolving from apes to later human ancestors, researchers said.
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Scientists have found only 11 bones from at least five different individuals -- including a jawbone with teeth, hand and foot bones, pieces of arm bones, and a piece of a collarbone -- making it impossible to determine the creature's size or appearance.
But tests show the remains are between 5.2 million and 5.8 million years old, making it about 1 million years older than the 4.4 million-year-old Ardipithecus ramidus ramidus, which was found nearby in 1995 and previously was the oldest human ancestor ever discovered.
Paleontologists for generations have been trying to find the fossilized remains of the earliest human ancestors, to get a better understanding of the history of human evolution over the ages. They've had to piece together the picture from a scant collection of skulls, bones and bone fragments.
A detailed examination of the new bones indicates the creature is an older subspecies of Ardipithecus ramidus ramidus in the lineage leading to humans that includes the famous "Lucy," whose 3.5 million-year-old remains were discovered about 50 miles north of the new find. Distinctly human species arose in Africa about 2 million years ago, while modern humans are only about 100,000 years old.
The discovery undermines the view that early human ancestors developed in a savanna-like habitat where they needed to walk upright to cover large distances and where they developed the grinding teeth necessary to crush and digest woody reeds and grasses.
Instead, the creature lived along with ancient elephants, antelopes, horses, monkeys and rhinoceroses in what was then a lush mountain forest periodically destroyed by volcanic eruptions, said Yohannes Haile-Selassie, the University of California graduate student who made the discovery.
"It's not a carnivore, and though some of the teeth are like those of apes, it is not a specialized fruit eater, like all chimpanzees," said Haile-Selassie, who reported his findings in today's issue of the journal Nature. The teeth suggest that the creature dined on a menu of soft leaves and fiber-rich fruit, he said.
Upright walking in this environment was useful, but for another reason. "The way of accessing the food was important," said Brigitte Senut of the Museum National d'Histoire Naturelle in France, who made the recent discovery in Kenya. "They were tree dwellers who needed to come down to the ground and walk to the next tree."
The savanna myth endured because "the preservation of fossils there is very much better than in a forest, and everyone uses the modern visual impression of the area as a reference point," said anthropologist C. Owen Lovejoy of Kent State University.
This image certainly prevailed when Haile-Selassie and geologist Giday WoldeGabriel began exploring the arroyos and escarpments of Ethiopia's Middle Awash region, a boulder-strewn wilderness that had been cracked, crumpled and sunk by millions of years of tectonic upheaval and volcanism. They were working as part of the 20-year, multinational project in the Middle Awash that turned up Ardipithecus ramidus ramidus.
"It was very steep and very rugged, with most of the surfaces" covered by loose rock, said WoldeGabriel, a research geologist at Los Alamos National Laboratory. "The [layers] we were looking for just poked through the surface."
The new Ardipithecus remains were found at an altitude of 3,000 feet, but WoldeGabriel said the region was higher -- perhaps 4,500 feet -- and much greener 5.5 million years ago, with large lakes, forests and plenty of rain.
"In this particular area, most of the volcanoes erupted through lakes and groundwater," WoldeGabriel said. "There's a lot of magma and water interaction. During the eruptions, it was very hostile."
Still, Lovejoy noted that geological time moves slowly, so that "10,000 years after the volcanoes erupt, it wasn't hostile. It was full of the resources that forest apes required to survive."
The discovery of Ardipithecus follows the announcement earlier this year ofthe unearthing of Orrorin tugenensis, or "Original Man from the Tugen Hills," in Kenya. Orrorin is 6 million years old, with teeth that are more apelike than those of Ardipithecus but a thighbone similar to those of later human ancestors and bigger than Lucy's.
Although both discoverers said there is no competition to see which fossil gains stature as the oldest human ancestor, it is clear that the two sides are thinking of one another.
Haile-Selassie said Ardipithecus, with its large molars and upright walking, leans "much more to the hominid [human ancestor] side than the chimpanzee side," while Orrorin has the "canine [tooth] of an ape" and a thighbone that is not "compelling" evidence of a human characteristic.
Senut said that the toe bone cited by Haile-Selassie as evidence of upright walking is "basically a climbing adaption," while Orrorin's teeth are "small and square-shaped, just like modern humans," and that new evidence shows the Orrorin thighbone has definite human characteristics.
In fact, suggested Rick Potts, director of the Smithsonian Institution's Human Origins Program, the evidence recovered from both areas is too sparse to make a definitive judgment. Also, he added, lines blur as scientists reach back in time.
"The question is, 'On which side of the line does it fall?' " Potts said. "As we get closer to that branching point, it's harder and harder to determine whether what you have is a human, a chimpanzee, the common ancestor or an ape on neither lineage that subsequently died out."
