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Savannas Accompanied Human Evolution For 6 Million Years

August 4, 2011

Scientists report they have found that savannas prevailed most of East African sites where human ancestors and their ape relatives evolved for the past 6 million years.

The University of Utah scientists used chemical isotopes in ancient soil to measure prehistoric tree cover in order to make this discovery.

“We’ve been able to quantify how much shade was available in the geological past,” geochemist Thure Cerling, senior author of a study of the new method in the Thursday, Aug. 4, 2011 issue of the journal Nature, said in a press release.

“And 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.”

“Wherever we find human ancestors, we find evidence for open habitats similar to savannas ““ much more open and savanna-like than forested,” adds Cerling, a University of Utah distinguished professor of geology and geophysics, and biology.

Early human fossils have been unearthed in both wooded and open environments in East Africa.  Cerling said that even a 4.3-million-year-old human ancestor known as Ardipithecus, who thrived in the woods, had a small component of tropical grasses in its diet.

“The fact it had some means it was going into the savanna, unless it was eating takeout food,” he says.

Scientists spent a century debating the significance of savanna landscape in human evolution, including the development of upright walking, increased brain size and tool use.

Cerling said that part of the problem has been a fuzzy definition of “savanna,” which has been used to describe “virtually everything between completely open grasslands and anything except a dense forest.”

The common definition of a savanna is a fairly open, grassy environment with a lot of scattered trees.

He and colleagues developed “a new way to quantify the openness of tropical landscapes. This is the first method to actually quantify the amount of canopy cover, which is the basis for deciding if something is savanna.”

“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 (early humans, pre-humans and chimp and gorilla relatives) in both places. How early hominins partitioned their time between ‘more open’ and ‘more closed’ habitats is still an open question.”

The team collected soil samples at Kenyan and Ethiopian sites and used published data on soil samples collected by others during the past decade at the other sites throughout the tropics.

The researchers used a new method to analyze fossil soils and infer plant cover back to 7.4 million years ago, which is a period that includes when human ancestors and apes split from a common ancestor.

Cerling and colleagues found that over 70 percent of the sites had less than 40 percent woody cover, meaning they were wood grasslands.  Less than 1 percent of the samples reflected sites where trees exceeded over 70 percent.

“Therefore, ‘closed’ forests (more than 80 percent woody cover) represent a very small fraction of the environments represented by these paleosols,” the researchers write.

“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.

They found that during the past 7.4 million years, woody cover ranged from 75 percent down to 5 percent or less, but significant areas with woody cover below 50 percent were consistently present.

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