Adds Kaye Reed of NSF's Division of Behavioral and Cognitive Sciences, "These investigators have done an amazing job of collecting modern comparative isotope and soil temperature samples to compare with paleosol [fossil soil] samples from hominin localities. Their newly developed method for calculating 'paleo-shade' is very innovative."
Cerling does not dispute that East African savannas became more expansive within the past two million years, or that human ancestors and relatives likely spent time in narrow "gallery forests" along river corridors.
But he says the new method shows that grasslands and wooded grasslands--savannas--have prevailed for more than six million years in the cradle of humanity, with tree cover less than about 40 percent at most sites.
By definition, woodland has more than 40 percent tree cover; forest has more than 80 percent tree cover.
"In some periods, it was more bushy, and other times it was less bushy," he says.
"Hardly anything could have been called a dense forest, but we can show some periods where certain environments were consistently more wooded than others.
"We find hominins in both places. How early hominins partitioned their time between 'more open' and 'more closed' habitats is still an open question."
Cerling says that even sparse woody canopy provided hominins with shade, some foods and refuge from predators.
Fossil evidence of hominins dates back 4.3 million years and possibly 6 million years, Cerling says. The new method was used to look for and find savanna up to 7.4 million years ago.
"Currently, many scientists think that before 2 million years ago, the region was forested, and that savanna conditions have been present only for the past 2 million years," Cerling says. "This study shows that during
|Contact: Cheryl Dybas|
National Science Foundation