Dispute Flares Over Ardi

New reports about a 4.4 million-year-old human ancestor claim that the creature lived in a primarily grassy environment and emerged before human and ape lineages diverged. These assertions contrast with reports about the species published last year.

The disagreement concerns Ardipithecus ramidus fossils, which include the skeleton of a female nicknamed Ardi.

In last year's papers, University of California, Berkeley, biologist Tim D. White; Los Alamos National Laboratory geologist Giday WoldeGabriel; and others contended that Ardi emerged after the human/ape split, which they think occurred at least 6 million years ago. Evidence including isotopic data indicates that "Ar. ramidus was a denizen of woodland with small patches of forest ... and likely fed both in trees and on the ground," the researchers wrote. Ardi apparently consumed only small amounts of food from open, less forested areas, "arguing against the idea that an inhabitation of grasslands was the driving force in the origin of upright walking," they added.

Now, Esteban E. Sarmiento of the Human Evolution Foundation, in East Brunswick, N.J., claims that the molecular and anatomical evidence reported in 2009 suggests that, contrary to the original report, Ar. ramidus predated the split between humans and apes, which he thinks occurred about 3 million to 5 million years ago (Science, DOI: 10.1126/science.1184148).

White and colleagues respond in Science that Sarmiento's contentions "rely on inadequate calibration" and "require tortuous evolutionary pathways" (DOI: 10.1126/science.1185462).

Meantime, University of Utah geochemist Thure E. Cerling and coauthors write that the original data, which include ¹³C levels, indicate Ardi's habitat consisted of grassy plains dotted with trees and shrubs (DOI: 10.1126/science.1185274).

White and colleagues counter that this interpretation is "inconsistent with the totality of the fossil, geological, and geochemical evidence" (DOI: 10.1126/science.1185466).

Stanley H. Ambrose, one of White's coauthors and an anthropologist who studies isotope geochemistry at the University of Illinois, Urbana-Champaign, states that Cerling is "incorrectly placing an ape into a more open habitat when its anatomy was only beginning to change from arboreal to terrestrial bipedalism." But he believes the two groups' visions of Ardi's habitat are closer than Cerling's team claims.