Ancestor Lucy Spent Time in Trees

A model of Australopithecus afarensis from a Houston museum. New analysis shows this human ancestor was adapted both to walk on ground and swing from trees. Click to enlarge this image. Zeray Alemseged / Dikika Research Project
Zeray Alemseged / Dikika Research Project

Our closest relative 3.3 million years ago was a half-human, half-ape who could both walk on the ground and swing through trees.

Our closest known relative 3.3 million years ago was a half-human, half-ape species that could both walk on the ground and swing through the trees, suggests a new study.

The research, published in the journal Science, sheds light on Australopithecus afarensis, the species of the well-known "Lucy" skeleton. In this case, remains of a three-year-old A. afarensis girl, named "Selam," were the focus of study. Selam represents the most complete skeleton of her kind to date.

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She and other members of her species were "very human-like from the waist down -- the hip bone, the knee and the foot -- but looked ape-like above the waist -- the torso, long arms, gorilla-like scapula, jutting snout, small brain and a skull with no forehead," co-author Zeresenay Alemseged told Discovery News. "A sketchy depiction of it would be an upright walking ape."

Alemseged, curator of anthropology at the California Academy of Sciences, and David Green, an assistant professor in the Department of Anatomy at Midwestern University, made the determinations after thoroughly examining the well-preserved skeleton of Selam. In 2000, Alemseged unearthed her remains while excavating a site in Dikika, Ethiopia.

The researchers paid attention to Selam's two complete shoulder blades. These tend to be paper-thin, rarely fossilizing, so finding both "completely intact and attached to a skeleton of a known and pivotal species was like hitting the jackpot," Alemseged said.

The analysis of the shape and function of the bones revealed that A. afarensis had ape-like shoulder blades, indicating a partially arboreal lifestyle. Green explained to Discovery News that, during Selam's lifetime, A afarensis was a widespread species across East Africa that occupied a range of habitats, including wooden environments and gallery forests.

"Within this range of environments, Selam and her kin walked upright to move from one place to the other, but also had an arboreal lifestyle that enabled them to nest in trees, evade predators, and provision themselves," Green said. "This was a significant adaptation that enabled this short-statured hominin, with no sophisticated tools, to survive in a dangerous landscape filled with large felines and other carnivores."

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The study reveals how apelike features in our ancestors were not merely evolutionary baggage, he continued. The traits instead reflect adaptations to habitat.

There is no question that Australopithecus afarensis is directly related to humans, according to the researchers.

"A. afarensis is clearly on the human line after the split from apes and eventually became ancestral to the genus Homo," Alemseged said. "It is currently hard to precisely talk about the exact number of species between us and A. afarensis."

Possibilities include A. africanus, Paranthropus, Homo habilis, Homo erectus and many others.

The authors also suspect that A. afarensis was not the first ancestor of humans to walk upright. This could be why Selam's bones indicate she walked with ease, at least when compared to chimpanzees that have been trained to do so. Chimps have a lumbering, awkward upright gait because they lack some important pelvic and lower limb characteristics that Selam possessed.

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Walking upright might have been a breakthrough in our history, leading to a domino effect of other subsequent changes.

"If we look at later hominins, the proliferation of stone tools and the increased exploitation of animal proteins, it is clear that bipedalism enabled hominins to use their forelimbs to fashion and use tools and exploit this new niche (the ground environment)," Green explained. "Environmental change may have prompted this transition, and bipedal exploration of the terrestrial environment likely paved the way for stone tool use, manufacture and hunting."

"It is also plausible that this enabled subsequent expansions in brain volume, which is another hallmark of our lineage," he added. "Other primates may have been able to continue to exploit arboreal or other food sources, which did not require such a dramatic behavioral transition. The ability of early hominins to exploit new resources is probably why bipedalism may be viewed as a successful evolutionary experiment."