The prehistoric tree cover in East African environment goes back to more than 6 million years, where the human ancestors and the ape relatives evolved, says a recent study by the University of Utah.
"Wherever we find human ancestors, we find evidence for open habitats similar to savannas – much more open and savanna-like than forested," said geochemist Thure Cerling, senior author of a study of the new method in the Thursday, Aug. 4, 2011 issue of the journal Nature.
The scientists used chemical isotopes in ancient soil to measure prehistoric tree cover to study the prolonged existence of savannas. The result shows that it’s not just over the past 2 million years that the savannas became more expansive and the human ancestors likely spent time in narrow "gallery forests" along river corridors.
"This is the first method to actually quantify the amount of canopy cover, which is the basis for deciding if something is savanna," Cerling says.
The new method was developed by correlating carbon isotope ratios in 3,000 modern soil samples with satellite photos of tree and vegetation cover at 75 tropical sites worldwide half in Africa, representing everything from closed forest to open grassland. That allowed scientists to determine the percent of tree and woody shrub cover millions of years ago based on carbon isotope ratios in fossil soils known as paleosols.
Cerling used the new method to analyze fossil soils and infer plant cover back to 7.4 million years ago and the analysis of 1,300 fossil soil samples from sites at or near where human ancestors and their relatives evolved shows that more than 70 percent of the sites had less than 40 percent woody cover, meaning they were wooded grasslands or grasslands.
"We conclude there have been open savannas all the time for which we have hominin fossils in the environments where the fossils were found during the past 4.3 million years" – the oldest fossils now accepted as human ancestors, Cerling says.