The chimpanzee (Pan troglodytes) is the closest living relative of humans, with the two species diverging about 6 million years ago. One of the best clues about the last common ancestor of people and chimps is the oldest discovered hominin skeleton, that of a human ancestor named Ardipithecus ramidus that lived about 4.4 million years ago.
In previous work, researchers deduced that Ardipithecus moved through the trees over the tops of branches, but didn’t swing from them, meaning that the last common ancestor of chimps and people probably moved in this way as well. But a new analysis of the hand of Ardipithecus, published today (February 24) in Science Advances, indicates that the hominin was likely capable of swinging beneath the branches of trees, as chimps do today.
“This is a nice, complete study of the hand of Ardipithecus, but it’s more than that,” says Campbell Rolian, a biological anthropologist at the University of Calgary who did not participate in the work. “It’s about placing in context what we know about the morphology of the last common ancestor of humans and chimpanzees, which is a long-standing question in evolutionary anthropology.”
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 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. They thus presented Ardipithecus as evidence that humans evolved from an ancestor that was nothing like other living apes in terms of its movements, he adds.
With this latest paper, Prang and his colleagues have used a different strategy to determine how the hands of Ardipithecus were shaped, in order to better understand how the hominin—and likely the shared ancestor of humans and chimps—moved. The researchers 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.
The authors also performed an analysis of curvature in hand bones, which correlates with how animals move, across all the species. The curvature of the Ardipithecus hand bones again pointed to similarities with orangutans, chimpanzees, gibbons, and spider monkeys and differences from people and gorillas.
“Given everything that’s been suggested about the fossils since 2009, we didn’t expect it to look so ape-like, and it was quite a shock that it does,” Prang says. “The fossil evidence suggests that early human ancestors and the last common ancestor of humans and chimps was far more similar to chimpanzees than to any other living primate.”
But not everyone agrees with the new study’s conclusions. In an email to The Scientist, Tim White, who studies human evolution at the University of California, Berkeley, and was one of the leaders of the 2009 studies, questions the ways in which Prang’s team chose and analyzed the data, as well as how they interpreted the results. “The Ardipithecus hand was not specifically chimpanzee-like, aside from having five fingers and the ability to grasp,” he writes. Modern and ancient human hands also have five fingers and the ability to grasp, but don’t swing through the trees. “Their samples and methods fail to capture the functional anatomy so obvious on the actual fossils,” he adds.
Sergio Almécija, a biological anthropologist at the American Museum of Natural History in New York who did not participate in the work, expands on White’s critique. This “study is rightfully capturing that, in its general anatomy, the Ardipithecus hand shares similarities with chimpanzees,” he says in an email to The Scientist. “However, their conclusion that the Ardipithecus hand was specialized for suspension as much as chimpanzees’ (and the chimp-human last common ancestor, by implication) don’t follow their results.”
The authors of the new study use 26 variables “that approximate some aspects of overall digital shape, but not digital elongation specifically,” Almécija continues. The question of whether or not the last common ancestor of humans and chimpanzees had long fingers specifically adapted for suspensory behaviors remains unanswered, he adds. “We need more Miocene ape fossils (pre-dating the human-chimp split) to test fundamental aspects of our last ancestor with apes. Specifically, we need to find the skeletons of ancient chimpanzees and gorillas to check if their hand length proportions were close to us, chimps, or something else.”
Madelaine Böhme, a paleontologist at the University of Tübingen in Germany who was not involved in the work, notes that while the authors “are able to show the suspensory or chimp-like anatomy of Ardi,” it’s still unclear if Ardipithecus is an ancestor of Australopithecus, she says. “It’s an assumption, and this assumption is poorly supported.”
“Fossils are the answer,” says Böhme. “We may well we never have enough, but every new fossil gives us new questions and potentially new answers [to] old questions.”
T.C. Prang et al., “Ardipithecus hand provides evidence that humans and chimpanzees evolved from an ancestor with suspensory adaptations,” Sci Adv, doi:10.1126/sciadv.abf2474, 2021.