Approaches for assessing whether the human hand evolved by natural selection in adaptation to stone tool use

Resumen

Stone tool use is a key component of the hominin behavioral repertoire. A unique configuration of musculoskeletal traits in the human hand are compatible with effective stone tool use, and the question remains as to how humans acquired this redefined manipulation capability. The most widespread hypothesis is that, once liberated from locomotion, the hominin hand evolved by natural selection to accommodate the functional demands of the use of stone tools. However, there is evidence that the hand could not have this adaptative origin, as derived morphologies were present in the hominin fossil record prior to the systematic use of stone tools. The aim of this thesis was to evaluate whether the human hand evolved by natural selection in adaptation to stone tool use. For this purpose, a comparative sample of hands of extant primates, mainly humans, chimpanzees and gorillas, were analyzed under different methods that relate biological form and function. The first objective was to evaluate the performance of the thumb in terms of stress distribution during simulated hammerstone use. We expected our species to perform better than other primates based on the idea that, unlike apes, the human hand is adapted to the loads from tool-related behaviors. Using a finite element analysis, it is shown that the human first proximal phalanx (modern human and Neanderthal) unevenly distributes stresses and is one of the most fragile compared to apes (i.e., chimpanzee, gorilla and orangutan). These results indicate that great apes can withstand loads exerted during this activity more efficiently than humans. We conclude the human pollical phalanx did not evolve to withstand the stresses associated with hard hammer percussion. Second, we hypothesize that if the human hand evolved to respond to selective pressures of tool-related behaviors, a distinctive pattern for modularity at the hand should have emerged. For this purpose, we evaluate whether humans have a different covariation structure at the wrist than that of chimpanzees and gorillas. Four carpals were analyzed (i.e., scaphoid, lunate, trapezium and capitate) through 3D geometric morphometrics and 15 different modular hypotheses were tested to find the optimal modular model. What sets humans apart from African apes is the degree of codependence between the trapezium and scaphoid, whereas in gorillas and chimpanzees both bones vary independently. This suggests that covariation patterns may be shaping the evolution of the wrist in these primates. It remains to be tested whether function is a factor behind the formation of the wrist modules. Finally, we tested whether entheseal size of ligaments and muscles are reliable for inferring activity patterns in the hand. First, the attachment sites of the ligaments were evaluated holding the flexor digitorum superficialis and profundus muscles at the proximal phalanges 2-5 (flexor ridges). The results indicate that these entheses at phalanges 2-4 are capable of distinguishing between taxa that use their hands for manipulation and locomotion (gorillas and chimpanzees) and taxa that use them exclusively for manipulation (humans). They also indicate that these entheses signal differences in manipulation capabilities across extant hominids at the fifth phalanx. Additionally, results on a cadaveric human sample suggest that the strength of the abductor pollicis longus muscle, but not the opponens pollicis muscle, is related to the size of the insertion sites. This is in line with previous analyses suggesting that the fibrocartilaginous entheses seem to be better correlated with activity levels that fibrous entheses. This result supports previous studies using fibrocartilaginous attachment sites as proxies for stone tool use in hominins and questions the conclusions on thumb dexterity based on the opponens pollicis attachment site. In summary, these studies indicate that stress distribution in hand bones was unlikely to be a selective pressure strong enough to shape the hand of humans; however, the biomechanics of the wrist may have been so. They also indicate that stone tool use may be inferred in hominins by analyzing hand regions where tendons (with fibrocartilaginous entheses) and ligaments attach.