Kappelman, however, does not think that this risk caused our primate ancestors to become fully terrestrial.
He said that the arboreal lifestyle "is still a viable niche for lots of animals, including the majority of primates. The first committed terrestrial bipeds (two-legged ground walkers) are probably Homo erectus, but even some modern humans forage in the trees."
He and his colleagues suspect that small-bodied Lucy nested in trees at night to avoid predators, which is what chimps and gorillas do today. This means that, "at a minimum, she climbed up a tree at night, slept there for some hours, and climbed down from that tree in the morning," Kappelman said, adding that she might have sometimes foraged for food in trees too.
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Experts contacted by Discovery News were all intrigued by the new study.
Osbjorn Pearson, an associate professor in the University of New Mexico's Department of Anthropology, said, "The evidence was literally right under the noses of many anthropologists for the last three and a half decades," referring to the time since 1982 that researchers have known of Lucy's remains.
Pearson agrees that Lucy probably nested in trees at night to escape predators, and could have foraged in them every so often, especially to get fruits. He thinks her adaptations for walking on the ground likely meant that "A. afarensis would have been more efficient at bipedal walking than chimpanzees, but perhaps less energetically efficient than modern humans."
John Fleagle, a distinguished professor at Stony Brook University's Department of Anatomical Sciences, said that the new research agrees that the fractures on Lucy's "humerus and other bones show the same pattern that doctors see in people who fall from heights and land on their arms."
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Fleagle added that the research "adds a level of detail to our understanding on the life and death of a fossil that is rarely achieved."
William Jungers, a distinguished professor emeritus also from Stony Brook University and a research associate at Association Vahatra in Madagascar, believes that the new paper presents "a provocative but plausible scenario for the demise of Lucy."
Jungers said that "death from accidental falls from trees is surprisingly common in some human groups, like the Aka pygmies, so why not Lucy too?"