Scientists using chemical isotopes in ancient soil to measure prehistoric tree cover--in effect, shade--have found that grassy, tree-dotted savannas prevailed at most East African sites where human ancestors and their ape relatives evolved during the past six million years.
"We've been able to quantify how much shade was available in the geological past," says University of Utah geochemist Thure Cerling, lead author of a paper titled "Woody cover and hominin environments in the past 6 million years" on the results in this week's issue of the journal Nature.
"It shows there have been open habitats for the last six million years in the environments in East Africa where some of the most significant early human fossils were found.
"Wherever we find human ancestors, we find evidence for open habitats similar to savannas--much more open and savanna-like than forested."
Scientists have spent a century debating the significance of open, savanna landscape in human evolution, including the development of upright walking, increased brain size and tool use.
Part of the problem has been an imprecise definition of "savanna," which has been used to describe "virtually everything between completely open grasslands and anything except a dense forest," Cerling says.
He adds that the most common usage is a fairly open, grassy environment with many scattered trees--a grassland or wooded grassland.
In the study, Cerling and colleagues developed a new way to quantify the openness of tropical landscapes. This is the first method to quantify the amount of canopy cover, the basis for deciding whether something is savanna.
The National Science Foundation (NSF) and the Leakey Foundation funded the study.
"The development of a paleo-shade proxy for soil temperature and woody cover, and its application to ancient fossil sites, reinforces the long-held theory that the roots of human origins are
|Contact: Cheryl Dybas|
National Science Foundation