The carbon isotope ratios of the soils indicated that in the time of Ardipithecus the landscape varied from woodland in the western part of the study zone to wooded grassland in the east. None of the Ardipithecus specimens were found in the grassy eastern part of the arc.
"Fossils of many species are common all the way across the landscape," Ambrose said. "But this species is missing in action from the east side of the distribution."
Isotopic analysis of teeth found on the site gave a more complete picture of the habitat of the animals that lived and died there, Ambrose said.
"The distribution of plant carbon isotope ratios conveniently separates out grasslands from forests," he said. "And it also separates out grazing animals, like zebras, from browsing animals that eat the leaves off of trees, like giraffes."
The distribution of the fossil browsers and grazers echoed that of the habitat, he said.
"On the west we find lots of Ardipithecus fossils and they're associated with a lot of woodland and forest animals," he said. "And then there's a break; Ardipithecus and most of the monkeys that live in trees disappear, and grass-eating animals become more abundant."
The carbon isotope ratios of the Ardipithecus teeth also tell the story of a woodland creature, he said.
"The diet of the Ardipithecus is much more on the woodland and forest side," he said. "It's got a little bit more of the grassland ecosystem carbon in its diet than that of a chimpanzee but much less than its fully bipedal savanna-dwelling descendents, the australopithecines."
This evidence, along with the anatomical studies indicating that Ardipithecus could walk upright but also grasped tree limbs with its feet, suggests that this early hominid took its first steps on two legs in the forest long before it ventured very far into the open grassland, Ambrose said.
|Contact: Diana Yates|
University of Illinois at Urbana-Champaign