New Insights into Ancient Hominin Hand Function Reveal Complex Evolutionary Adaptations
Recent research into the fossilized hands of two South African hominin species, Australopithecus sediba and Homo naledi, offers compelling evidence that challenges traditional views of human evolutionary pathways. The detailed analysis of finger bone internal structures reveals distinct adaptations in dexterity and climbing ability, illustrating that early hominin hand use was far from a simple linear progression toward modern human tool manipulation.
The significance of these findings lies in the differences observed in the cortical bone distribution of the phalanges—the finger bones—between Australopithecus sediba, dated to approximately two million years ago, and Homo naledi, whose fossils date to around 250,000 years ago. Cortical bone adapts during an individual’s life, thickening in areas subject to higher mechanical loads. By examining these patterns, researchers uncovered behaviorally relevant loading signatures that suggest different habitual grips and hand functions in these two related species.
Australopithecus sediba’s finger bones—especially the proximal and intermediate phalanges—show a mixed morphology. While many fingers display ape-like cortical bone thickness, the thumb and pinky fingers reveal human-like characteristics. These features, alongside A. sediba’s elongated, modern-like thumb, indicate a dual utilization pattern of the hand: capable not only of climbing but also engaged in dexterous manipulation likely related to tool use. This combination supports a nuanced behavioral repertoire that integrates arboreal navigation with ecological demands for precise hand use.
Conversely, Homo naledi exhibits a more complex cortical pattern. Its proximal phalanges demonstrate a human-like cortical bone distribution, indicative of significant loading consistent with manipulative activities. Yet the intermediate phalanges retain an ape-like pattern, implying a different loading regime along the finger length. Such biomechanical disparity is unusual and suggests H. naledi employed specialized grips, possibly akin to the crimp grip popular among modern rock climbers, where fingertip loading predominates. This implies that climbing behaviors remained an important aspect of its lifestyle despite the contemporaneous existence of early Homo sapiens.
Moreover, the pronounced curvature of H. naledi’s finger bones further supports the hypothesis of sustained climbing or locomotion involving footholds requiring strong finger flexion. This curvature contrasts with the straighter fingers found in modern humans and some other hominins, reinforcing the notion that climbing remained a critical functional component within this species’ behavioral repertoire.
These investigations, led by postdoctoral researcher Samar Syeda and senior author Tracy Kivell among an international team, utilized micro-CT scanning and biomechanical modeling to meticulously map cortical bone thickness variations. This approach transcends traditional external morphology assessments, offering a dynamic view of lifetime hand use and functional adaptations in extinct species.
The implications of this study extend beyond these two species, calling into question the simplicity of the “linear evolutionary model” that assumes a straightforward transition from tree-climbing ancestors to fully terrestrial and technologically advanced humans. Instead, the evolutionary landscape appears to be characterized by complex, overlapping adaptations that balanced arboreal activity with increasingly sophisticated manipulative skills.
Distinct from earlier hominins whose hands were generally presumed to transition rapidly toward tool-using specializations, A. sediba and H. naledi demonstrate that climbing-related features persisted well into the late Pleistocene. This persistence emphasizes that hand evolution involved multiple experimental strategies, where natural selection shaped diverse morphologies to fit specific ecological niches and survival challenges.
The research also informs broader debates surrounding the origins of tool use. Neither A. sediba nor H. naledi fossils have been found directly associated with stone tools despite their anatomical capabilities supporting tool manipulation. Given the presence of stone tools in South Africa dating back over two million years, it is plausible that these hominins participated in tool use but perhaps exhibited distinct grips and manipulative techniques compared to modern humans.
A further intriguing aspect relates to how cortical bone internal structure reveals fine-scale differences in finger loading demands, signifying differences in behavior that external bone shape alone cannot elucidate. This highlights the importance of internal bone adaptation studies for reconstructing extinct species’ hand function and, by extension, their ecological and social behaviors.
In sum, the study invites a reevaluation of human hand evolution and posits that adaptations for climbing and dexterity were not mutually exclusive but interwoven in complex ways. Rather than a singular path toward increased manipulation versatility, hominin evolution witnessed a mosaic of functional entanglements that reflected the demanding and varied environments of prehistoric southern Africa.
Ultimately, these findings showcase the intricate evolutionary dance between locomotion, environment, and hand function within our ancient relatives. They provide a vivid reminder that human evolutionary history is rich with biological experimentation, where survival often meant mastering multiple terrains—both arboreal and terrestrial—while refining the hand as both a climbing organ and a tool of innovation.
Subject of Research: Fossil evidence and cortical bone analysis of finger bones in Australopithecus sediba and Homo naledi revealing differential hand function and evolutionary adaptations.
Article Title: Phalangeal cortical bone distribution reveals different dexterous and climbing behaviors in Australopithecus sediba and Homo naledi
News Publication Date: 14-May-2025
Web References: DOI: 10.1126/sciadv.adt1201
Image Credits: © Tracy Kivell
Keywords: Australopithecus sediba, Homo naledi, cortical bone, finger bones, hand dexterity, climbing behavior, hominin evolution, fossil morphology, tool use, paleoanthropology, human evolution, biomechanical adaptation
