June 7, 2010

Forget the forest, scientists say—pre-humans lived in savannahs

Pre-humans in East Africa 4.4 million years ago lived among grassy, tree-studded plains, not in the forests, according to a new study.

The finding by a multi-university team of scientists adds new fuel to the debate over why early humanlike species came down out of the trees and began walking on two legs.

The team’s conclusion that Ardipithecus ramidus lived in a savanna environment directly contradicts that of University of California, Berkeley’s Tim White, whose team found the first A. ramidus fossils in 1992 and 1993.

White argues that A. ramidus’s world—and that of Ardi, an individual female whose skeleton was discovered in Ethiopia in 1994—was dominated by trees.

“Our team examined the data published by White and his colleagues last October,” says Johns Hopkins University earth scientist Naomi Levin, “and found that their data does not support their conclusion that Ardipithecus ramidus lived exclusively in woodlands and forest patches.”

Levin is one of eight geologists and anthropologists from seven universities—led by Thure Cerling of the University of Utah—who recently published their conclusions in the journal Science.

“The White team’s papers stress the wooded nature of A. ramidus’s environment and say specifically that Ardi did not live in a savanna,” Levin says. “Yet, the actual data they present are consistent with exactly that: a savanna environment with a mix of grasses and trees.”

The dispute is an important one because the claim that Ardi lived in woodlands and forest patches has been used to try to debunk a long-standing view of human evolution known as the “savanna hypothesis.”

According to that premise, the expansion of savannas—grassy plains dotted with trees and shrubs—into what had been forest lands prompted apelike pre-humans to descend from trees and begin walking upright. They would have had to do so, proponents of this view believe, to find food more efficiently or to reach other trees for resources or shelter.

Cerling’s team believes that tropical grasses accounted for between 40 percent and 60 percent of the biomass in Ardi’s world. The scientists reached that conclusion using the White team’s own data, collected from ancient soils, plant fossils and other remains in the area known as Aramis in what is now Ethiopia.

Levin, an assistant professor of earth and planetary sciences at Johns Hopkins, says that resolving the issue is important. If scientists are to evaluate accurately the environmental pressures that influenced the course of human evolution, she says, they clearly must understand the environment itself.

It appears from the White team’s years of study of A. ramidus and of Ardi—who stood about four feet tall and had a brain less than a quarter of the size of a modern human’s—that they were “at ease” both walking upright on the ground and moving on all fours through the trees, Levin says.

If Ardi and her contemporaries did walk upright despite living in forests where open grassland was rare, then the savanna hypothesis is wrong, she says.

On the other hand, she says, if the broader habitat of A. ramidus included savannas, “then we cannot rule out the possibility that open environments played an important role in human origins and, in particular, in the origins of upright walking.”