Did Ardipithecus ramidus roam the woods or the grasslands?
Ardi, the 4.4 million-year-old skeleton whose discovery last year upended models of human evolution, has prompted a debate about where she lived, and whether she is indeed an ancestor of modern humans.
The purported human ancestor nicknamed Ardi and unveiled to the world last October was not the woodland creature its discoverers made it out to be, claim another group of researchers.
In a new study, researchers argue that soil samples found alongside Ardipithecus ramidus, a female who lived 4.4 million years ago in what is now Ethiopia, show that the creature lived in a grassy environment of relatively few trees, a type of habitat known as a savanna.
If correct, the argument would undermine the claim by Ardi's discoverers that the fossil contradicted the "savanna hypothesis" – the idea that hominids, or human ancestors, evolved to walk upright after an environmental shift that saw jungle give way to savanna.
"Our conclusion is that much of the evidence that they present should be interpreted as a savanna environment, therefore their rejection of the savanna hypothesis is incorrect," said study researcher Thure Cerling of the University of Utah in a prepared statement.
The researchers who discovered Ardi are not backing down. They contend that the new soil sample analysis, published this week in the journal Science, overlooks other evidence, such as the fact that Ardi's body was adapted for a wooded environment.
Ardi is said by its discoverers to be "the first creature on our side of the family tree." Although some researchers still dispute that contention, Cerling and his colleagues do not.
What Ardi ate
Cerling and his colleagues argue that Ardi's discoverers misinterpreted fossilized soil samples they excavated from nine different sites in Aramis, Ethiopia.
In Africa, trees and grasses represent two different kinds of plants, known as C3 and C4 for the way they carry out photosynthesis. Researchers can try to assess whether ancient plant life was C3 or C4 by analyzing the ratio of two different carbon isotopes [the isotopes are called carbon-12 and carbon-13], or atoms of the same element with different numbers of neutrons.
Cerling and his colleagues point out that in most of the Aramis samples, the soil composition is more than 40 percent C4, which they say is not consistent with wooded habitats. Instead, using modern tropical soils as a comparison, they say the data are compatible with only 5 to 25 percent wooded cover.
"If our interpretation is correct, then a wide variety of environments [was] available to Ardipithecus," Cerling told LiveScience.com. "If and how such environments were used is far more speculative."
But in a rebuttal published in Science, Ardi's discoverers said Cerling's group is overlooking the sum total of evidence, including Ardi herself. Ardi was nearly 4 feet (1.2 meters) tall with a mass of perhaps 110 pounds (50 kilograms). Although her pelvis seems to have been oriented for upright walking, her leg muscles and big toes were specially adapted for climbing.
Based on carbon isotopes in Ardi's teeth and bones, and the way her teeth were worn down, she ate a diet of C3 plants, which must have come from trees, said co-discoverer Tim White of the University of California, Berkeley. And White and his colleagues catalogued other fossilized animals from the site, including tree-dwelling monkeys.
"Here we have a large-bodied climbing primate whose diet is linked to the woodland, found surrounded by woodland birds and animals and snails and fossil wood, and so that's why we concluded that the ... lifestyle of this primate was centered on woodlands instead of open grasslands," White said.
"Their main point is there was grass there, and we agree with that," White said. However, "if Ardi was out in the open grasslands, she wasn't eating the grass."
Cerling acknowledged that "these animals were not eating a significant fraction of C4 biomass," but he said the dental data is more ambiguous than White and his colleagues claim. "The values that they publish are within the range of primates (e.g., baboons) that live in a variety of habitats, from savannas ... to woodlands."
Is Ardi even a hominid?
It's hard to know what Ardipithecus ate and how she moved in her environment without more fossils to go on, said anthropologist Matthew Sponheimer of the University of Colorado, who was not involved in the published exchange. "We don't have Ardipithecus at 20 places," he said. "We would feel much better if we did."
Others take issue with the claim that Ardipithecus is a hominid in the first place. Among them is Esteban Sarmiento of the Human Evolution Foundation in New Brunswick, N.J., the author of a second comment published this week in Science. Sarmiento argues that certain features of Ardipithecus, such as the base of the skull where it meets the jaw, are too primitive for the creature to represent a hominid.
Assuming that Ardi is a hominid, Sponheimer said she's only one part of the picture of early hominid evolution, and for that reason it's too soon to declare the savanna hypothesis dead.
Given the uncertainties, he said a healthy debate is exactly what the field needs. "There are a lot of simple stories out there but we really need nuanced debates," he said. "It's this kind of dialogue that's going to move our field forward."