CHAMPAIGN, Ill. Carbon isotope evidence in almost 6-million-year-old soils suggests that the earliest humans already were evolving in and likely preferred humid forests rather than grasslands, report a team of scientists working in Ethiopia.
The discovery challenges long-held beliefs, beginning with Darwin, that humans did not evolve into upright beings and thrive until expanding tropical grasslands forced our chimpanzee-like ancestors out of dwindling forests about 4 million to 8 million years ago.
Hominid fossil sites from the later Pliocene period (2.5 million to 4.2 million years ago) previously had been found in savanna habitats. Researchers had been confident that the slightly earlier hominids living in the late Miocene also would be found in the savanna.
"The expectation was that we would find hominids in savanna grassland sites that date back to about 8 million years ago. That hasn't happened," said anthropologist Stanley H. Ambrose of the University of Illinois. "All older hominids have been found in forested environments."
The analysis was of fossil soils from paleontological sites in the Middle Awash region of Ethiopia's rift valley, where the remains of a new subspecies of Ardipithecus ramidus have been discovered. They date to the late Miocene period (5.4 million to 5.8 million years ago). Scientists from four institutions report their findings in a pair of papers that appear in the July 12 issue of the journal Nature.
Ambrose collected fossil soil samples from the layers containing the newly found hominids. One of the fossils was found by team member Leslea Hlusko, also a UI professor of anthropology. Ambrose performed geochemical studies on the samples in his UI laboratory.
The region where the fossils were found is now a hot, dry semi-desert occupied by nomadic camel herders. At the time the area formed, it was higher in elevation, cooler, wetter and more forested.
Ambrose's geochemical technique allows for an environmental reconstruction of soils by examining the carbonate nodules (caliche) in the samples. The nodules reflect the types of plants that grew in the soils. Tropical grasses contain more of the heavy isotope of carbon than do trees, shrubs and leafy plants.
The nodules from these late-Miocene hominid fossil sites contain low levels of carbon 13, which is consistent with trees and woody plants. They also contain oxygen isotope ratios that are indicative of a cool, humid climate.
"These hominids were living in the forest, despite the fact that grasslands were available," Ambrose said.
The new findings, he said, require a fundamental reassessment of models that invoke a significant role for global climatic change and/or adaptation to savanna habitats in the origin of hominids.
Ambrose's findings appear in a paper cowritten with seven other researchers: Tim White, Yohammes Haile-Selassie and Paul R. Renne of the University of California at Berkeley; Giday WoldeGabriel and Grant Heiken of the Los Alamos National Laboratory in New Mexico; William K. Hart of Miami University in Oxford, Ohio; and Berhane Asfaw of the Rift Valley Research Service in Addis Adaba, Ethiopa.
The National Science Foundation and the University of Illinois Research Board provided funding for Ambrose's research.