Ardi wasn't human ancestor, says team

Posted by Emma Woollacott

A team of scientists says that habitat evidence shows that Ardipithecus ramidus - claimed last year as the earliest human ancestor - was nothing of the sort.

According to University of Utah geochemist Thure Cerling, there is scant evidence for claims that there were dense woodlands at the African site where the creature lived 4.4 million years ago.

Instead, "There is abundant evidence for open savanna habitats," he says in a critique published as a 'technical comment' in Science.

The criticism is important because the claim that Ardi lived in woodlands was used as an argument against a longstanding theory of human evolution known as the savanna hypothesis.

This holds that an expansion of savannas prompted ape-like ancestors of humans to descend from the trees and start walking upright.

Last October, a team led by Tim White of the University of California published 11 studies in Science which were later named as the 2009 'Breakthrough of the Year'.

In November, Cerling and seven other scientists submitted their critique to Science. But the journal hasn't published it until now – when Cerling and his University of Utah coauthor, geologist Frank Brown, are in the field in Kenya and difficult to reach. Both are pretty annoyed.

The critique concludes that Ardi most likely lived in tree or bush savanna with five to 25 percent of the area covered by trees or shrubs - not the minimum 60 percent that meets the definition of a closed-canopy woodland.

Cerling acknowledges Ardi could have lived in a wooded river corridor, but it was a river that flowed through savanna.

"It wasn’t a pure grassland, and it wasn’t forest either," says Brown.

Cerling and Brown say they aren’t advocating the savanna hypothesis, just noting that White’s own data support it rather than contradict it.
"One of the big, newsworthy items that White and coworkers put forward is that Ardipithecus walked upright on two legs, yet lived in a forested environment," says Brown.

"They then say the savanna hypothesis – which holds that the reduction in forest cover in Africa is one of the reasons for early man becoming bipedal – must be incorrect."

Three of the 2009 Science papers by White and colleagues cited various data they collected from ancient soils, plant fossils and so on to support the notion that Ardipithecus lived in woodlands.

The team based its conclusions on the White team’s own data, including data for ancient soils that are buried and compacted, and phytoliths - tiny grains of silica that form within living plants and later become fossilized in rock.

Cerling says phytoliths from Aramis are inconsistent with a dense woodland environment and instead indicate tropical grasses made up 43 percent to 77 percent of the ecosystem.

The critique concludes that although its authors do not judge the validity of the savanna hypothesis, the connection between human ancestors walking upright and the expansion of grasslands "remains a viable idea."