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University of Utah geochemist Thure Cerling is shedding new light on ancient East African landscapes where ancestral humans began walking upright 2 to 6 millions years ago. By analyzing the carbon in fossil-bearing dirt, Cerling and colleagues have determined there was less forest cover much earlier than is widely believed, which could prompt scholars of human evolution to revisit theories explaining the rise of bipedalism.
"It shows there have been open habitats for all of the last 6 million years in the environments in eastern Africa where some of the most significant early human fossils were found," said a news release quoting Cerling, a professor of both geology and biology. His findings, published Wednesday in the journal Nature, touch on the central question in the study of human origins: What prompted our ancestors to evolve an upright posture?
One prevailing view is that ancestral humans, or hominins, began walking on two feet as they adapted to a decline in forest cover resulting from climate change. That adaptation led to other human traits, such as larger brain size, and freed hands for clutching tools.
But the Nature study challenges that view because closed canopy woodlands had already given way to savanna-like grasslands by the time hominins branched off from the tree-dwelling lineages shared with primates 7 million years ago.
The finding "does not suggest a causal relationship, but it means that when they began walking upright the forest was confined to narrow zones along rivers with adjacent open areas of wooded grassland," Cerling wrote in an email Wednesday from a research site in Kenya. "In such an environment it is not possible to get from woody patch to woody patch, or from tree to tree, without putting your feet on the ground. So there is a reason to develop bi-pedality."
Cerling's work for the first time quantifies how much shade covered the lands where humanity first emerged, expanding knowledge of the environmental context associated with early human evolution, said Chris Campisano, an assistant professor of geology at Arizona State University's Institute of Human Origins.
"More open environments have existed for longer than we thought," said Campisano, who was not involved with the study. "These stable isotopes are powerful proxies for climate and vegetation, but they are one of many."
Campisano points out that the findings can't tell us whether early hominins confined themselves to forested areas or ventured away from the safety of trees, a point Cerling concedes.
Over the past 20 years, Cerling has perfected ways to determine what ancient animals ate by analyzing ratios of carbon isotopes in tooth enamel. Enamel bearing more carbon-13 indicates a diet of so-called C4 plants like trees and shrubs. Because of the way these plants use carbon dioxide, they take up rare carbon-13 atoms, which carry one extra neutron. Tropical grasses and other C3 plants that prevail on savannas, on the other hand, prefer the lighter carbon-12 atom.
Now Cerling has applied his technique to ancient dirt, called paleosols, to determine what kind of vegetation has covered now-arid East Africa over the past 7 million years. His team took samples of modern soils from national parks and other fairly pristine locations and generated data to establish benchmarks that correlate carbon isotope ratios to the extent to which the land is covered by forest. More carbon-12 generally indicates less woody cover.
Researchers also gathered 1,300 paleosol samples from sites in Ethiopia and Kenya where ancestral humans first appeared in the fossil record beginning 4.3 million years ago. Carbon isotope analysis found that more than two-thirds of the sites had less than 40 percent woody cover, indicating these places were grasslands with wooded patches.
"For the specific case of Aramis" — the Ethiopian site where the famous 4.4-million-year-old remains of Ardipithecus were recovered — "only one out of about 80 soils even includes the possibility of forest. So the dominant signal is one of disconnected woody canopy," Cerling said.
Cerling's interdisciplinary research team included biologists Samuel Andanje and David Kimutai Korir, of the Kenya Wildlife Service; geologist Michael Bird, of Australia's James Cook University; U. graduate students William Mace, of geology, Anthony Macharia, of geography, and Christopher Remien, of mathematics; and former graduate students Jonathan Wynn, now at the University of South Florida; Naomi Levin, of Johns Hopkins University; and Jay Quade, of the University of Arizona.