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Little Foot's shoulders hint at how a common human-chimpo ancestor has risen

Little Foot, an almost complete hominid skeleton meticulously carved into the rock inside a South African cave, had a powerful evolutionary charge.

This 3.67 million-year-old adult woman wears the oldest and most complete set of shoulder blades and collarbones of any ancient hominid. Those fossils also provide the best model available for the shoulder appearance of the last common ancestor of humans and chimpanzees, say Kristian Carlson, a paleoanthropologist at the University of Southern California in Los Angeles, and colleagues. Their results provide new insights into how Little Foot and one last human chimpanzee climbed the trees.

Little Foot belonged to the genus Australopithecus, but its species identity is in dispute (SN: 12/12/18). The shape and orientation of their shoulder bones fall among the corresponding measures for humans and modern African apes, but more in line with gorillas, Carlson reported April 27 at the annual virtual meeting of the American Association of Physical Anthropologists. His talk was based on an article published online April 20 in the Journal of Human Evolution.

Little Foot lived about halfway between modern times and the estimated age of a common human chimpanzee ancestor, says paleobiologist David Green of Campbell University in Buies Creek, North Carolina, a member of Carlson's team. If that ancient ancestral creature were the size of a chimpanzee, as many researchers suspect, gorilla-like shoulders would have withstood a slow but competent climb, Green says. Gorillas spend much of their time walking on the ground. These apes climb the trees with all four limbs, reaching even with powerful shoulders and arms to throw themselves.

“Maintaining a gorilla-like shoulder at Little Foot provides clues that climbing remained vital during the early days (hominids),” Green says. It is possible, he added, that Little Foot’s shoulder design represented “evolutionary baggage” among the evolving bodies of the most suitable hominids for walking upright.

The researchers used a digital reconstruction of Little Foot’s almost complete right shoulder blade, shown here, to determine that this ancient hominid climbed more like gorillas than like chimpanzees, orangutans, or humans.K. Carlson

The new analysis makes Little Foot's shoulders "our best candidate for making hypotheses about the appearance of the last common ancestor of the human chimpanzee," says anatomist Susan Larson of New York's Stony Brook University School of Medicine, who did not participate in the research. Ancestral shoulders that had supported tree climbing could provide a basis for the evolution of human shoulders aligned with a two-legged step and chimpanzee shoulders designed to hang and swing from tree branches, he suggested.

In the new study, a digital, three-dimensional reconstruction of Little Foot’s most complete right shoulder blade was compared to the right shoulder blades of chimpanzees, gorillas, orangutans, and current people. Other comparisons were made with partial shoulder blades of 11 ancient hominids. These hominids included four Australian specimens of Australopithecus and East African finds of two members of Lucy's species, A. afarensis, dating to about 3.3 million and 3.6 million years ago (SN: 25/10/12) . Little Foot's collarbones were compared to those of humans, chimpanzees, gorillas, orangutans, and seven ancient hominids.

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Carlson's analysis offers preliminary but still uncertain evidence that Little Foot had the most gorilla-like shoulders of any ancient hominid, says paleoanthropologist Stephanie Melillo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Melillo, who did not participate in the new study, finds it most striking that Little Foot shares many similarities with his shoulders with the other Australopithecus fossils studied by Carlson's team.

Some researchers consider a 4.4 million-year-old skeleton of Ardipithecus ramidus, called Ardi, the best hominid model for a last common ancestor of a human chimpanzee (SN: 31/12/09). Ardi could move slowly in the trees as he held on to the branches above his head, in a different way to any modern ape, they claim. But Ardi's remains lack shoulder blades and collarbones.

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