Fossil Of Earliest Hominid Known Found In Ethiopia
Scouring the dry washes encircling an Ethiopian site where scientists seven years ago found fossils of 4.4 million-year-old human ancestors, University of California, Berkeley, graduate student Yohannes Haile-Selassie has found even older fossils that show human ancestors walked on two legs as early as 5.2 million years ago.
The fossils are the earliest hominid known, and date from close to the time when human ancestors are believed to have split off from the chimpanzees on the first steps of their evolutionary trip to modern Homo sapiens.
The fragmentary fossils, which include teeth, a jawbone, hand, arm and collar bones, and one toe bone, appear to be family members of the species discovered in 1994 by an international team led by UC Berkeley paleoanthropologist Tim White. They named that species Ardipithicus ramidus, and concluded that it was the earliest known human ancestor. Haile-Selassie, for now, has designated the new fossils as a subspecies of this earlier find: Ardipithicus ramidus kadabba.
"Its dentition is that of a hominid, its toe bone is like that of a bipedal animal," said Haile-Selassie from Addis Ababa, where he continues to study the fossils at the National Museum of Ethiopia. "It's definitely a hominid, and proves that the earlier 4.4 million-year-old Ardipithecus ramidus was a hominid, not an ape."
Haile-Selassie reported his finds in the July 12 issue of Nature. A second paper in Nature by geologist Giday Woldegabriel of Los Alamos National Laboratory, coauthored by Haile-Selassie, White and others, described the paleoclimate of the area Ardipithecus roamed nearly 6 million years ago.
The publication of these two papers by Haile-Selassie and WoldeGabriel, both Ethiopians, is a major milestone in African paleoanthropology, according to White, a professor of integrative biology.
"These two local scholars have joined the international scientific community at the highest level," he said. "This is a huge development in African paleoanthropology and a welcome change in the conduct of this science.
"UC Berkeley has played a major role in this. Beginning with J. Desmond Clark and Clark Howell, we have had a program for many years at Berkeley of nurturing and training people from developing countries to do paleoanthropology. This is an example of how that has paid off scientifically, for Africa and for Ethiopia."
Clark and Howell are professors emeriti of anthropology at UC Berkeley and among the most respected anthropologists working in Africa during the past century.
Haile-Selassie found the new fossils along the western margin of the Afar rift in the Middle Awash region of Ethiopia, about 140 miles northeast of the capital Addis Ababa and 25 kilometers from the Aramis site where White's team found A. ramidus. The fossils came from sites in four different arroyos draining the margin and one closer to the Aramis discoveries. These sites were identified by first surveying the area with satellite imagery and air photos, then on foot, and finally, after the areas were determined to be possible fossil sites, with an intensive survey to establish their age based on fossils evidence.
"When you don't find anything, an hour is like a day," Haile-Selassie said about discovering the first fossil - a mandible or jawbone with one molar - in 1997. "But when you find good stuff, you don't even want night to come, you want to work 24 hours to find more. That's how exciting it was."
He subsequently found at the five sites a total of 11 fragments representing at least five individuals. Together, he said, these provide evidence that the chimpanzee-sized creature was not an ape but an early ancestor of humans.
"These canine teeth are not of humans, but no chimp has canine teeth like that either," White said. "This argues that these fossils are not from the common ancestor of both chimps and humans, but from very early in our evolution, shortly after our ancestors parted company and before our canines fully reduced."
Another argument that the fossils are from a hominid, not an ape, is that the toe bone shows a slanted surface at the rear joint, which is characteristic of bipedal walking. This is caused by toeing-off - pushing forward by leaving the front part of the foot on the ground and lifting the heel. This anatomy is characteristic of A. ramidus and all later hominids, but not of chimpanzees and other apes, which walk on the outside of their feet.
The toe bone "is consistent with an early form of terrestrial bipedality," Haile-Selassie concluded in his Nature paper.
His continuing search for more fossils from the same period should give a better idea whether the new finds are an early form of A. ramidus or a species ancestral to it.
Haile-Selassie also noted that fossils found in Kenya last year and dated at 6 million years by a team of French paleoanthropologists are ambiguous, even though the researchers claimed they are the oldest human ancestor and named the fossil creature Orrorin tugenensis. Until more bones of this animal are found and more studies conducted, he said, it is impossible to say whether Orrorin is the earliest human ancestor, the earliest chimpanzee, or the common ancestor of both.
Geologist WoldeGabriel, who partnered with Haile-Selassie on week-long forays from the Aramis base camp into the hot, dry margins of the Middle Awash area, found evidence that the ancient hominid species lived in a woodland habitat much different from the savanna environment proposed as the birthplace of human ancestors.
This conclusion is bolstered by chemical analysis of ancient soils accompanying the fossils by Stanley H. Ambrose, associate professor of anthropology at the University of Illinois at Urbana-Champaign.
"The expectation was that we would find hominids in savanna grassland sites that date back to about eight million years ago," Ambrose said. "That hasn't happened. All older hominids have been found in forested environments."
These findings require fundamental reassessment of models that ascribe the origin of hominids to global climatic change or as an adaptation to conditions of a savanna habitat, according to the researchers. They suggest, instead, that all known earliest hominids derived from relatively wet and wooded environments and did not venture into more open savanna settings until after 4.4 million years ago - about the time Australopithecus made its appearance and long after hominids and chimpanzees split from their common ancestor.
Based on analysis of rock types, patterns of volcanic eruptions, animal fossils and ancient soils associated with the hominids, WoldeGabriel and his colleagues paint a vivid picture of the land Ardipithecus roamed. About 6 million years ago, the Middle Awash region was a well-defined rift valley characterized by intense earth movements, with active volcanoes erupting from major fractures and individual centers. Some of these were erupting underwater in local lakes that had been created by subsidence and dams of lava flows. The region was further showered by pulses of thick and hot volcanic ashes from nearby volcanoes.
"It is hard to imagine that life would go on normally under such hostile environmental conditions," WoldeGabriel said. "Ardipithecus and the other animals inhabiting the area were real survivors."
The forested upland where Ardipithecus lived was up to 1,500 meters higher in elevation, and cooler, wetter, and more forested. Fossils of more than 60 mammal species were found associated with the new hominid, including primitive elephants, rhinos, horses, rats and monkeys.
"These hominids always seem to be associated with monkeys and woodland forest antelope, but not with open-country forms," White said.
Today the land is harsh desert.
"The unique thing about the Middle Awash is that here a series of sediments a kilometer deep reaches back to 6 million years, yielding everything from anatomically modern humans spanning the past quarter million years to Australopithecus garhi at 2.5 million years ago, Australopithecus afarensis at 3.4 million years ago, Ardipithecus ramidus at 4.4 million years and now A. ramidus kadabba at nearly 6 million years ago," White said. "This is an incomparable series of fossil snapshots of change through time in one area.
"What does this say about the creationist hypothesis that there was no change and that there were humans all the way back? The evidence shows that it didn't happen that way."
Co-authors with WoldeGabriel on the Nature paper are Berhane Asfaw, PhD, of the Rift Valley Research Service, Addis Ababa; Paul Renne, PhD, director of the Berkeley Geochronology Center and an adjunct professor of geology at UC Berkeley; Grant Heiken, PhD, also of LANL; geologist William K. Hart, PhD, of Miami University; plus Ambrose, White and Haile-Selassie.
The research is sponsored primarily by the National Science Foundation and the Institute of Geophysics and Planetary Physics at Los Alamos National Laboratory. - By Robert L. Sanders
[Contact: Haile-Selassie, Tim White, Robert Sanders]
12-Jul-2001 |
