Contested evidence pushes Ardi out of the woods

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

The original discoverers of the species Ardipithecus ramidus disagree with the new study and say that a wide array of findings — including evidence not considered in the new investigation — keeps these hominids in the woods.

When a 4.4 million-year-old partial Ardipithecus skeleton, dubbed Ardi, was first unveiled in October 2009, she 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 evolved to support long-distance foraging by males who were seeking to impress potential mates.

In contrast, the new analysis, published in the May 28 Science, supports a longstanding 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.

Cerling’s team concludes that Ardi inhabited a grass- and shrub-covered region. Dense stands of trees covered only between 5 percent and 25 percent of that territory.

Ardi could have lived in a thin wooded strip that bordered a river, the researchers suggest. But that river would have flowed through a savanna.

Cerling and his colleagues analyzed data from ancient soil and plant fossils collected by Ardi’s discovers. Forms of carbon, known as isotopes, in fossil-bearing sediments indicate that tropical grasses covered much of Ardi’s home area, the team reports.

Microscopic fossils of such grasses found near Ardi’s remains also point to a savanna, the researchers hold.

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

In a response published 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 savannas in Ardi’s corner of East Africa. Fossil and geological evidence, including key clues that Cerling’s team didn’t address, indicates that Ardipithecus favored wooded areas over any available savanna patches, in White’s opinion.

Shallow groundwater and springs likely deposited fossil wood, seeds and invertebrates where Ardi’s remains were unearthed, White notes. No evidence of an ancient river or lake has been found at fossil sites in that area.

Abundant fossils of leaf-eating monkeys found at Ardi’s site further reflect a wooded environment, White says. Bones of antelopes known as kudu that were capable of inhabiting forests have also been unearthed. What’s more, Ardipithecus had teeth suited to eating leaves and fruit found in forests. And Ardi’s lower body allowed for regular tree climbing, he asserts.

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