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Scientists have found new evidence for how our fossil human relatives in South Africa may have used their hands.

Research led by Samar Syeda, postdoctoral researcher at the American Museum of Natural History, together with scientists at the Max Planck Institute for Evolutionary Anthropology, the University of the Witwatersrand, University of Kent, Duke University and the National Geographic Society, investigated variation in finger bone morphology to determine that South African hominins not only may have had different levels of dexterity, but also different climbing abilities.

The work has been published in Science Advances.

This new research focuses on two, almost complete fossil hand skeletons found in South Africa. One is the hand of Australopithecus sediba, first discovered in 2010 at the site of Malapa and dated to approximately two million years old.

The other hand skeleton is from the more recent, but perhaps more enigmatic, Homo naledi, first found in 2015 deep within the Rising Star Cave system and dated to around 250,000 years ago.

Neither hominin has yet been found in direct association with stone tools, but several aspects of their hand and wrist morphology suggest that they had a degree of hand dexterity much more similar to that of humans than to living chimpanzees or gorillas.

"Since stone tools are found in South Africa by at least 2.2 million years ago (and in East Africa by as early as 3.3 million years ago), and many primates are all excellent stone tool users, it is not surprising that A. sediba and H. naledi would be dexterous tool users as well. However, how exactly they used tools and if they manipulated their tools in similar ways is unclear," says senior author, Tracy Kivell.

Moreover, both A. sediba and H. naledi are also found with many other bones of their skeleton that preserve ape-like features, particularly bones of their upper limbs, that would be advantageous for climbing. If these features reflect actual climbing in these individuals or are simply evolutionary holdovers from an ancestor that climbed, is a longstanding debate in paleoanthropology.

Fossil hands reveal ancient human behavior

To help address these questions, Syeda and her colleagues investigated variation in the internal structure—the cortical bone—of the fingers in A. sediba and H. naledi.

Bone is a living tissue that can adapt its structure in response to how we use and load our skeleton during life, getting thicker where loads are higher and thinner where loads are lower. Therefore, variation in the internal cortical thickness can provide new insights in how these two hominin fossils may have actually used their hands during their lifetimes.

"We found that A. sediba and H. naledi show different functional signals in the cortical bone structure of their fingers," says Samar Syeda, lead author of the study.

In A. sediba, the distribution of the cortical bone within the proximal and intermediate phalanges of most of its fingers is like that of apes. However, the bones of its thumb and pinky finger are more like those of humans.

Syeda concludes that "these two digits are more likely to reflect potential signals of manipulation because they are less often used or experience less load during climbing or suspensory locomotion. When we combine these results with the remarkably long, human-like thumb of A. sediba, it suggests that A. sediba used its hand for both tool use and other dexterous behaviors, as well as climbing."

Homo naledi's hands show unique grip pattern

H. naledi, in contrast, is unusual in showing a human-like signal in its proximal phalanges (the bones that articulate with the palm) but an ape-like signal in its intermediate phalanges (the bones within the middle of the fingers).

"This distinct pattern was unexpected and indicates that H. naledi likely used and loaded different regions of its fingers in different ways," says Syeda.

This kind of loading pattern is typical of only certain grip types used today, like crimp grips, often used by rock climbers, where the surface is grasped primarily by just the tips of the fingers.

H. naledi also has unusually highly curved finger bones, particularly for a hominin that lived at the same time as the earliest members of our species, Homo sapiens, which is another indication that it used its hands for locomotion.

While more research is needed to further test if H. naledi may have used crimp-like grips or climbed rocks, it is clear that throughout human evolution there were different ways of combining enhanced dexterity for tool use and food processing with the continued need to use the hands to climb, be it trees or rocks, within the South African paleolandscape.

"This work offers yet more evidence that human evolution is not a single, linear transition from upright walking to increasingly better tool use, but is rather characterized by different 'experiments' that balanced the need to both manipulate and to move within these past environments," says Kivell.