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.