When Texas A&M University’s Cody Prang was taking his first biological anthropology course as an undergraduate at the University of Delaware in the fall of 2009, the first analyses of Ardipithecus [ramidus] were published. The studies indicated that “Ardipithecus had none of the features that we expected to find in an early human relative close in time to the last common ancestor [of] humans and chimps,” Prang says. The authors of the 2009 papers reported that the species lacked hand structures and proportions needed for vertical climbing and suspension, for example.
[In a new paper, Prang] integrated 26 measurements from the 2009 Ardipithecus data, including the length of finger bones and joint dimensions of the knuckles, into a statistical model that also included corresponding measurements from other primate species, both living and extinct.
They used the model to group species with similar hand morphology. Ardipithecus clustered with orangutans, chimpanzees, spider monkeys, and gibbons—all of which grasp tree branches as they climb vertically and swing through the trees. Ardipithecus hands did not cluster with those of modern humans, Australopithecus—the genus that includes Lucy and is believed to be a direct human ancestor—gorillas, and baboons, all of which climb over the tops of tree branches.