ardi human ancestor lineage evolution trees savannaA set of descriptions last year of a 4.4-million-year-old hominid dubbed Ardipithecus ramidus pegged the primate as a human ancestor that lived in ancient African woodlands.

Now some researchers aren't so sure that "Ardi" lived in the forest—or even belongs exclusively in the human line.

Before Ardi, common evolutionary wisdom held that our ancestors began walking upright in large part due to a change in scenery. As African grasslands expanded and new residential opportunities arose, some primates began to explore a new, more efficient method of locomotion, leaving quadrupedal tree-dwelling to the monkeys.

Analysis of the soil, rocks, plants and animals around Ardi, however, appeared to Tim White, one of the October 2009 studies' lead authors and a professor of integrative biology at the University of California, Berkeley, and his colleagues to indicate that she had primarily lived and died in a wooded area. Thus, if she had been an able upright walker (as they also proposed, though other researchers are not convinced it was her preferred way of getting around), the finding meant that our ancestors might have started to walk on two legs in an entirely different setting, for entirely different reasons.

Another group of researchers, however, has proposed that the evidence from Ardi's environment indicates a "tree or bush savanna" that would have had less than a quarter canopy coverage, in a technical comment submitted to Science in November and published online May 27. Thure Cerling, a geochemist at the University of Utah, and his colleagues assessed carbon and oxygen isotopes in tooth enamel, soil samples, rock, aridity levels and other evidence from sites in different regions to conclude that Ardi most likely lived in an environment that was rather grassy and dotted with occasional trees as well as denser vegetation around rivers and streams. Cerling and his coauthors do not say whether they support the idea that early humans started walking in more open savanna areas, but they did conclude that White et al.'s " rejection of the savanna hypothesis is incorrect," Cerling said in a prepared statement.

Such thinking that Ardi and her kind would have "equally exploited all of the habitats available to it" would be "ecologically naïve," White and three colleagues contested in their response, also published online May 27 in Science. They argued that results from enamel analysis "should be used with caution," there was "no local evidence" for a large river channel to produce any riparian vegetation coverage and that the group's definition of Ardi's environment was too broad and should be focused on the immediate surroundings in which the fossils were found.

But what if this primate—forest-dweller or not—was not even on the human limb of the tree of life? White and his colleagues argued in several of the group's 11 papers from October 2009 that Ardi lived after the human-chimp split. "Sufficient support for this claim, however, is lacking," Esteban Sarmiento, of the Human Evolution Foundation, wrote in another technical comment in the same issue of Science. Based on descriptions of some of the teeth, skull, hip bones and foot bones, he argued that rather than suggesting that Ardi's rightful place is in the human lineage, the data "suggest that Ardipithecus belongs to a common human/African ape lineage." And because the species is thought to have lived relatively close to the divergence of human and African apes, such as chimpanzees and gorillas, Sarmiento continued, "it would be difficult to unambiguiously recognize it" as a human-only ancestor.

In a rejoinder from White et al., they noted that in order for many of the characteristics described in Ardi to be ancestral to African apes as well, "a complicated and non-parsimonious sequence of…transformations would be required." Additionally, they noted that although Sarmiento disagreed with their interpretation of many anatomical characteristics, he didn't offer satisfactory alternative explanations of his own. "If Sarmiento wishes to…generate phylogenic conclusions different from ours," White and his colleagues wrote, "then he needs to be explicit as to where, why and how our assessments of these characteristics were in error."

Regardless of where Ardi belongs in the primate tree—or whether she lived in the trees, Sarmiento noted that the description of this species follows a well-worn path in paleoanthropology: "It is curious that in a century-old race for superlative hominid fossils on a continent currently populated with African apes, we consistently unearth nearly complete hominid ancestors and have yet to recognize even a small fragment of a bona fide chimpanzee or gorilla ancestor."

Image courtesy of Wikimedia Commons/T. Michael Keesey