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.
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Note: This story has been adapted from a news release issued by
University Of Illinois At Urbana-Champaign for journalists and
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link in any citation:
http://www.sciencedaily.com/releases/2001/07/010712080455.htm
