Variation and non-adaptive evolution of the hominin foot.

摘要

The modern human foot is a highly derived, anatomically complex structure that is a critical component of terrestrial bipedalism. However, the human foot was inherited from an ancient, arboreal quadrupedal ancestor. Though the modern human foot is well-adapted to terrestrial bipedalism, variation in living humans often hints at what once was—many contemporary people exhibit pedal skeletal morphology that contributes to a more flexible, compliant biomechanical phenotype. Though all healthy modern humans are terrestrial bipeds, surveys of large samples of modern human foot radiographs reveal that appreciable percentages of living people exhibit both minimal to no longitudinal arch alongside tibial set morphology more commonly found in apes as well as widely divergent (though not functionally opposable) great toes. Analyses of these patterns of variation in living people yield insight into the hominin fossil record of foot evolution; for these characters, the entire hominin range of variation can be found in the feet of living people. This is remarkable given that other structures such as the cranium, dentition, pelvis, and femur present a different pattern of variation in the past and in the present. This research is concordant with the recent work of others describing, in living people, midfoot mobility sufficient to produce a "midtarsal break," a range of joint motion of the midfoot that characterizes apes.;In light of the persistence of remarkable variation in the human foot from the ancient past to the present, I argue that the foot has experienced weak selection in hominin evolution. I suggest the anatomical complexity and resultant biomechanical redundancy of the foot produces a functionally viable phenotype even when specific foot morphologies are `suboptimal' or `atavistic.' That is, selection often simply cannot act upon many given anatomies because they are masked by soft- and hard-tissue compensation elsewhere in the foot. Additionally, I suggest directions for future methods to further evaluate my weak selection hypothesis for the evolution of the human foot.