At Home on the Plains, Man's Ancestors Not Forest Dwellers

Ardipithecus ramidus – a purported human ancestor that was dubbed Science magazine's 2009 "Breakthrough of the Year" – is coming under fire from scientists who say there is scant evidence for her discoverers' claims that there were dense woodlands at the African site where the creature lived 4.4 million years ago.

Pre-humans living in East Africa 4.4 million years ago probably inhabited  savannas - grassy plains dotted with trees and shrubs - according to eight geologists and anthropologists from seven universities, including Thure Cerling of the University of Utah and Naomi Levin of The Johns Hopkins University.
A Giraffe Walks Across a Savanna
A Giraffe Walks Across a Savanna

This conclusion is at odds with a theory - which holds that these
early beings lived in a mostly forested environment - put forth by prominent University of California at Berkeley researcher Tim D.
White and his team in a 2009 issue of the journal Science.

The criticism is important because the claim that the 4.4-million-year-old fossil nicknamed Ardi lived in woodlands and forest patches was used as an argument against a longstanding theory of human evolution known as "the savanna hypothesis," which holds that an expansion of savannas – grassy plains dotted with trees and shrubs – prompted ape-like ancestors of humans to descend from the trees and start walking upright to find food more efficiently or to reach other trees for shelter or resources.

In October, 2009, White and his team published 11 studies in Science outlining 17 years of work in which they excavated Ardipithecus fossils – including a partial skeleton of the female nicknamed Ardi – as well as 150,000 animal and plant fossils. They characterized the environment at what is now Aramis, Ethiopia, as woodland to forest patches with a climate cooler and more humid than today.

Science named the research that uncovered Ardipithecus and her environment as the 2009 "Breakthrough of the Year," citing how the fossils were more than a million years older than those of the previously oldest known hominid partial skeleton – that of Lucy, or Australopithecus afarensis.

But a month earlier, in November, the eight scientists submitted their critique to Science. The journal didn't publish it until May 27, 2010.  The critique concludes that Ardi most likely lived in tree or bush savanna with 5 percent to 25 percent of the area covered by trees or shrubs, not the minimum 60 percent to meet the definition of a closed-canopy woodland. 

"Our team examined the data published by White and his colleagues
last October and found that their data does not support their
conclusion that Ardipithecus ramidus lived exclusively in woodlands
and forest patches," said Levin of John Hopkins. "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. Yet, the actual  data they present are consistent with exactly that: a savanna environment with a mix of grasses and trees."

The team of eight scientists found that tropical grasses, in fact, comprised between  40 and 60 percent of the biomass in Ardi's world.

Levin says the conclusion is noteworthy because, if scientists
are to evaluate the environmental pressures that triggered the
evolutionary success of some traits over others, they must clearly
understand the environment itself.
"In their papers and summaries, White and his colleagues emphasize
that A. ramidus had a mix of traits that suggest it was at ease both
walking upright on the ground and moving through the trees on its
palms," Levin explains. "If the habitat of A. ramidus was, in fact, a
woodland with forest patches, where grasses were rare, then it's
unlikely that the increased presence of grassy environments were the
driving force behind the origin of upright walking in early human
ancestors. However, if the habitat of A. ramidus included savannas
where grasses were up to 60 percent of the available biomass, 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. The scientific community and the public should not
accept an exclusively woodland/forested habitat for A. ramidus and
the origins of upright walking, because the data do not support it."

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

The relative abundances of phytoliths from various plants, and stable or non-decaying carbon isotopes in fossil soils both provide evidence of ancient vegetation, letting researchers distinguish whether an area was covered by grasses or woody plants. 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.

White's group examined how much tropical grasses contributed to the carbon composition of soil. Cerling of the University of Utah says 83 of 84 samples from nine sites in the Aramis area indicate a significant amount of tropical grasses were growing when the soils formed, and only one sample could be interpreted as having been from soil formed in a forest with no tropical grasses. For all sites, the median amount of tropical grass by biomass produced was 40 percent to 60 percent – hardly a forest.

Cerling says White contends seven of the nine Aramis sites were forested, yet the carbon isotopes indicate the Aramis site has more tropical grasses than other sites of the same age that other people have studied in East Africa.

"It means Aramis was more open than even the most open of the sites people have looked at," he pointed out.

Carbon-13 levels in tooth enamel from animals such as giraffes and antelopes were interpreted by White and colleagues to indicate closed-canopy forest patches. Cerling says the values are not consistent with modern animals that live in dense forests, but are consistent with browsing animals that range from river-edge woodlands to savanna.

The team that discovered Ardipithecus also used an index of aridity, "and the values they calculate for Aramis are comparable to the driest modern sites that have been examined," says Cerling. Yet they claim "out of the blue" that the area was forested rather than being a savanna with wooded areas along a river.

Cerling sharply criticizes White's team for claiming a forest environment when there are fossils of two species of small grassland rodents.

"What they are saying is all the grassland mice were brought in from somewhere else after they were eaten by owls," he says. "That presupposes it was a forest."

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."

University of Utah
The John Hopkins Office of News and Information