Australopithecus sediba — a small hominin species that lived about 2 million years ago — had a mix of ape-like and human-like features, while Homo naledi — a recently-discovered hominin species that lived between about 335,000 and 236,000 years ago — had a unique pattern of bone thickness, suggesting different loading patterns and possible grip types, according to new research led by Dr. Samar Syeda from the American Museum of Natural History.
Life reconstruction of Australopithecus sediba commissioned by the University of Michigan Museum of Natural History. Image credit: Elisabeth Daynes / S. Entressangle.
Dr. Syeda and her colleagues 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.
They focused on two, almost complete fossil hand skeletons found in South Africa: the two-million-year-old hand of Australopithecus sediba from the site of Malapa and the 250,000-year-old hand skeleton from the Rising Star Cave system.
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 Australopithecus sediba and Homo naledi would be dexterous tool users as well,” said Dr. Tracy Kivell, a researcher at the Max Planck Institute for Evolutionary Anthropology and the University of the Witwatersrand.
“However, how exactly they used tools and if they manipulated their tools in similar ways is unclear.”
Moreover, both Australopithecus sediba and Homo 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 hold-overs from an ancestor that climbed, is a longstanding debate in paleoanthropology.
A reconstruction of Homo naledi’s head by paleoartist John Gurche, who spent some 700 hours recreating the head from bone scans. The find was announced by the University of the Witwatersrand, the National Geographic Society and the South African National Research Foundation and published in the journal eLife. Image credit: John Gurche / Mark Thiessen / National Geographic.
To help address these questions, the researchers investigated variation in the internal structure — the cortical bone — of the fingers in Australopithecus sediba and Homo 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 Australopithecus sediba and Homo naledi show different functional signals in the cortical bone structure of their fingers,” Dr. Syeda said.
In Australopithecus sediba, the distribution of the cortical bone within the proximal and intermediate phalanges of most of its fingers is like that of apes. However, bones of its thumb and pinky finger are more like those of humans.
“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,” Dr. Syeda said.
“When we combine these results with the remarkably long, human-like thumb of Australopithecus sediba, it suggests that Australopithecus sediba used its hand for both tool use and other dexterous behaviors, as well as climbing.”
Homo 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).
The fossil hands of Australopithecus sediba and Homo naledi show that these South African hominins may have had different levels of dexterity, as well as different climbing abilities. Image credit: Tracy Kivell.
“This distinct pattern was unexpected and indicates that Homo naledi likely used and loaded different regions of its fingers in different ways,” Dr. Syeda said.
This kind of loading pattern is typical of only certain grip types used today, like crimp grips, used often by rock climbers, where the surface is grasped primarily by just the tips of the fingers.
Homo 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 Homo 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,” Dr. Kivell said.
The findings appear this week in the journal Science Advances.
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Samar M. Syeda et al. 2025. Phalangeal cortical bone distribution reveals different dexterous and climbing behaviors in Australopithecus sediba and Homo naledi. Science Advances 11 (20); doi: 10.1126/sciadv.adt1201
