The biomechanical importance of the scaphoid-centrale fusion during simulated knuckle-walking and its implications for human locomotor evolution

Abstract

inferring the locomotor behaviour of the last common ancestor (LcA) of humans and African apes is still a divisive issue. An African great-ape-like ancestor using knuckle-walking is still the most parsimonious hypothesis for the LCA, despite diverse conflicting lines of evidence. Crucial to this hypothesis is the role of the centrale in the hominoid wrist, since the fusion of this bone with the scaphoid is among the clearest morphological synapomorphies of African apes and hominins. However, the exact functional significance of this fusion remains unclear. We address this question by carrying out finite element simulations of the hominoid wrist during knuckle-walking by virtually generating fused and unfused morphologies in a sample of hominoids. finite element analysis was applied to test the hypothesis that a fused scaphoid-centrale better withstands the loads derived from knuckle-walking. the results show that fused morphologies display lower stress values, hence supporting a biomechanical explanation for the fusion as a functional adaptation for knuckle-walking. this functional interpretation for the fusion contrasts with the current inferred positional behaviour of the earliest hominins, thus suggesting that this morphology was probably retained from an LcA that exhibited knuckle-walking as part of its locomotor repertoire and that was probably later exapted for other functions.

Ticks, Hair Loss, and Non-Clinging Babies: A Novel Tick-Based Hypothesis for the Evolutionary Divergence of Humans and Chimpanzees

“…As noted by Thorpe, et al [ 249 ], the LCA is often presumed to have been a quadrupedal primate that converted to bipedalism with the origin of the human lineage. However, there is also support for the hypothesis that the LCA was already bipedal and used a bent-hip, bent-knee (BHBK) gait (i.e., with the hips and knees flexed throughout the gait cycle) as an adaptation for walking on flexible tree branches [ 7 , 81 , 250 , 251 ] (contra, see [ 252 , 253 , 254 ]). LCA arboreal bipedalism is hypothesized to have been either hand-assisted (e.g., in the manner of modern orangutans) [ 7 ] or hand-unassisted (e.g., in the manner of modern gibbons), with the latter hypothesis referred to as the “Hylobatian Model” [ 5 , 255 ].…”

Three-Dimensional Comparative Study of Human Bipartite Scaphoids and the Os Centrale of the Wrist in Neandertals and Non-Human Anthropoid Primates

“…The reason for this fusion in the African ape and human lineage is yet to be clarified as different theories exist [ 5 ]. Some authors argue that an early fusion evolved to facilitate knuckle walking by promoting wrist stability and effective load transmission through the wrist when weight is borne on the dorsum of the middle phalanges [ 4 , 13 , 14 , 15 ]. Analysis of wrist kinematics in apes suggests that scaphoid-centrale fusion likely contributes to the low wrist mobility in extension observed in the African apes [ 16 ].…”

Characterizing the body morphology of the first metacarpal in the Homininae using 3D geometric morphometrics

“…These anatomical traits that set apart humans from the African apes have presumably evolved to cope with the different functional demands experienced by these taxa (i.e., manipulation vs. locomotion) (Almécija, Moyà-Solà, & Alba, 2010; Matarazzo, 2015; Püschel, Marcé-Nogué, Chamberlain, Yoxall, & Sellers, 2020; Richmond & Strait, 2000; Tsegai et al, 2013). The more robust human thumb and greater degree of curvature of the joint surfaces allow our species to produce greater force and to better withstand the stresses of tool-related behaviors (Galletta et al, 2019; Key & Dunmore, 2018; Key & Dunmore, 2015; Rolian, Lieberman, & Zermeno, 2011).…”

“…Even though the great apes use their hand for manipulatory activities, their morphology is likely more related to their locomotion (i.e., knuckle-walking and arborealism) (Almécija, Moyà-Solà, & Alba, 2010; Matarazzo, 2015; Püschel et al, 2020; Richmond & Strait, 2000; Tsegai et al, 2013). It is therefore expected that the selective pressures associated with locomotor behavior in chimpanzees and gorillas will result in an MC1 morphology that varies significantly from that of modern humans.…”