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Experts debate hominid habitat: discoverers dispute claim that Ardi roamed savannas.

An ancient hominid hung out on grassy savannas, not in forests as first claimed, a new study argues. Whether the species trucked across savannas has major implications for understanding how and why human ancestors began walking upright.

The discoverers of the species Ardipithecus ramidus disagree with the new study and say that other evidence keeps these hominids in the woods.

When a 4.4-million-year-old partial Ardipithecus skeleton was unveiled in October 2009, its owner, dubbed Ardi, was presented as a forest dweller that split time between walking upright and crawling along tree branches (SN: 1/16/10, p. 22). In this scenario, a two-legged gait had evolved to support long-distance foraging by males seeking to impress potential mates. But the new analysis, published in the May 28 Science, supports a long-standing idea that shrinking African forests spawned the evolution of hominids capable of walking across vast savannas.

In Ardi's neck of the woods, at what is now Aramis, Ethiopia, "there is abundant evidence for open savanna habitats," says geologist Thure Cerling of the University of Utah in Salt Lake City. Ardi could have inhabited a grass--and shrub-covered region in a thin wooded strip that bordered a river flowing through a savanna, Cerling's team suggests.

Cerling and colleagues analyzed data from soil and plant fossils collected by Ardi's discoverers. Forms of carbon in fossil-bearing sediments indicate that tropical grasses covered much of Ardi's home area. Microscopic fossils of such grasses found near Ardi's remains also point to a savanna, the researchers say.

Levels of carbon isotopes in teeth from giraffes and other animals found among Ardipithecus fossils resemble those of browsing animals that range today from woods bordering rivers to savannas, the scientists say, noting that aridity and rainfall estimates for Ardi's ancient homeland are compatible with such a habitat.

In a response in the same issue of Science, Ardi's discoverers, including anthropologist Tim White of the University of California, Berkeley, say that no reliable way exists to estimate the extent of savanna in Ardi's corner of East Africa. Fossil and geological evidence indicate that Ardipithecus favored wooded areas over savanna patches, in White's opinion.

Groundwater and springs probably deposited fossil wood, seeds and invertebrates near Ardi's remains, White notes. No evidence of an ancient river or lake has been found at fossil sites in that area.

If Ardi's kind frequented savannas, as Cerling's team proposes, a biological mystery emerges, White says. "What were these large-bodied hominids doing out on an open grassland, besides providing meals to resident predators?"

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Title Annotation:Humans
Author:Bower, Bruce
Publication:Science News
Geographic Code:1USA
Date:Jun 19, 2010
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