Evrim Yazgin
Cosmos science journalist
New research into the hand structures of different ancient human relatives found in modern South Africa reveals varying levels of dexterity and climbing ability, offering new clues into the evolution of humans.
The study, published in the journal Science Advances, focuses on near-complete hand skeletons from 2 ancient hominins – Australopithecus sediba and Homo naledi.
A. sediba was first discovered in 2010 at Malapa, a cave about 45km northwest of Johannesburg. The species lived about 2 million years ago (mya).
H. naledi is a mysterious species first identified in 2015 from fossils found in the Rising Star cave system less than 20km from Malapa. The hand skeleton in the study dates to about 250,000 years ago.
Both caves lie within the 120,000-acre Cradle of Humankind UNESCO World Heritage Site which has the highest concentration of prehistoric human remains in the world.
Neither A. sediba nor H. naledi has been directly linked to the production of stone tools. But the new study suggests they had dexterity closer to that of humans than modern great apes such as gorillas or chimpanzees.
“Since stone tools are found in South Africa by at least 2.2 mya (and in East Africa by as early as 3.3 mya), 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,” says senior author, Tracy Kivell from the Max Planck Institute for Evolutionary Anthropology in Germany.
“However, how exactly they used tools and if they manipulated their tools in similar ways is unclear.”
Both hominin species have preserved ape-like features, particularly in their upper limbs, associated with climbing. Whether these traits are because they were climbers or simply evolutionary holdovers has been subject to debate.
Bones can adapt their structure depending on use through life. Thicker bones indicate higher loads and thinner for lower loads. The researchers studied the internal structure of the cortical bones of the ancient hominin fingers.
“We found that A. sediba and H. naledi show different functional signals in the cortical bone structure of their fingers,” says lead author Samar Syeda from the American Museum of Natural History.
A. sediba’s finger bones were more like those of apes, but its thumb and pinky were more like the bones found in humans.
Syeda says: “These 2 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 behaviours, as well as climbing.”
In contrast, H. naledi has human-like proximal phalanges – the finger bones connected to the palm – and ape-like intermediate phalanges – the bones in 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 pattern parallels certain grip types like crimp grips, used today 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 which is another indication that it used its hands for locomotion.
More research is needed to determine if H. naledi were rock-climbing ancient humans. But the findings point to the variety of ancient hominin lifestyles and adaptations.
“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 characterised by different ‘experiments’ that balanced the need to both manipulate and to move within these past environments,” Kivell says.
