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The evolution of the human hand: grasping trees and tools

Final Report Summary - GRASP (The evolution of the human hand: grasping trees and tools)

The GRASP project aimed to improve our understanding of how the unique manipulative abilities of the human hand evolved through the application of new, interdisciplinary methods to fossil human (hominin) hand bones. In particular, this project addressed two fundamental, yet unresolved, questions of human evolution: (1) Did the earliest fossil hominins still use their hands to climb in trees? and (2) When did tool-use evolve and which hominin species were capable of this behaviour. To address these questions, the GRASP project investigated internal bone structure (trabecular and cortical bone), studied living ape hand use in natural and captive environments, conducted experimental biomechanical studies of human and ape hand loading, and created musculoskeletal models of a human and ape fingers.

Outcomes from the GRASP project include the development of a new, open-access segmentation method for the analysis of internal bone structure and the adaptation of several existing biomedical engineering methods to be better suited to high-resolution microCT data and the complexities of fossil preservation. Research outcomes include: (1) gorillas and chimpanzees climb trees in different ways with regards to how they grasp the substrate and position their arms, and have more complex dexterity during tool-use and food processing than previously recognised; (2) that although apes almost always use their thumb when grasping during tree climbing and suspension, the thumb experiences minimal load (pressure) compared with that of the fingers, suggesting that the hominin thumb may have been more “free” to adapt to the selective pressures of manipulation; (3) that the human thumb, index finger and middle finger experience the highest load during tool behaviours, particularly during stone tool-making and bone marrow acquisition, suggesting that these behaviours may have played a key role in shaping human hand morphology; (4) robust experimental and biomechanical testing of how locomotor (e.g. climbing) and manipulative (e.g. tool-making) hand use behaviours are reflected in the variation we find in internal trabecular structure across living humans and apes and that, within this comparative context, we can infer hand use behaviours in fossil species; (5) that the internal bone structure reveals much greater diversity in fossil hominin hand use than previously appreciated, with Australopithecus africanus (3-2 million years ago [Ma]) showing evidence of human-like precision grips much earlier than previously thought, and Australopithecus sediba (2 Ma) showing evidence of human-like thumb use but orangutan-like finger use, and that climbing was still a part of our human evolutionary story at 250,000 years ago in Homo naledi, at the same time as the appearance of the earliest Homo sapiens.