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 hasnt 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 Ethiopias 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.