Anthropologists have discovered the remains of the earliest known human ancestor in Ethiopia, dating to between 5.2 and 5.8 million years ago and which predate the previously oldest-known fossils by almost a million years. The previous discovery of the 4.4-million-year-old Ardipithecus ramidus was up to this point the oldest known hominid, the primate zoological family that includes all species on the human side of the evolutionary split with chimpanzees.
The fossil finds, reported in the July 12 issue of Nature, were made by National Science Foundation (NSF)-funded scientists over a four-year period in Ethiopia's Middle Awash study area, about 140 miles northeast of the capital, Addis Ababa. To the team of scientists, the discovery represents more evidence to confirm Darwin's conclusion that the earliest humans, or hominids, arose in Africa.
Yohannes Haile-Selaissie, a paleontologist at the University of California at Berkeley, made the recent fossil discoveries from these earliest creatures. Working under lead researcher and Berkeley colleague Tim D. White, Haile-Selaissie found a jawbone and teeth in December, 1997. More fossils were found, the last a tooth, uncovered in January, 2001.
The area where this hominid discovery took place has been the focus of much recent attention. Eleven hominid specimens have been recovered from five late Miocene localities within the Middle Awash region.
"The new fossils come from the oldest of the patches of exposed sediment at Saitune Dora, Alayla, Aas Koma and Digiba Dora," White said. "These bones and teeth were difficult to find on surfaces that are littered with stones ranging from pebbles to boulders."
The study of the Middle Awash has been ongoing since 1981 under the joint direction of White and Desmond Clark of UC-Berkeley, Giday WoldeGabriel of Los Alamos National Laboratory and Ethiopian researchers Berhane Asfaw and Yonas Beyene.
The researchers explain that about six million years ago, the Middle Awash region was already a well-defined rift valley characterized by intense earth movements, with active volcanoes nearby. "It is hard to imagine life would go on normally under such hostile environmental conditions -- Ardipithecus and the other animals inhabiting the area were real survivors," WoldeGabriel said.
White says that an accurate portrayal of these creatures is not yet possible because an intact skull or limb bones have not yet been found. Researchers estimate the size of the skeletal bones and the lower jaw is roughly the same size as a modern chimpanzee.
"This is an exciting development," says Mark Weiss, program director of physical anthropology at NSF. "Not only are we gaining insights into the anatomy of what may be some of our earliest ancestors, but we are seeing a better picture of the environment in which they lived. I'm really looking forward to further finds by White and his colleagues." White has received close to $1.2 million in NSF funding for his work since 1995.
The age of these newly found fossils was determined by the Berkeley Geochronology Center by employing an argon-argon laser heating method -- a process that determines the time elapsed since volcanic ashes and lavas were erupted by measuring the argon gas trapped in the rock after it cools. "The argon dating results were corroborated by geomagnetic polarity data, then further confirmed by biochronological analysis of the primitive fossil animals found with the human ancestor remains," White explains.
"These fossils are strong evidence that that lines leading to chimpanzees and humans had already split well before five million years ago," Haile-Selaissie concludes.
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