Evolutionary Processes of Limb Diversification Evidenced Through Comparisons of Carpal and Tarsal Morphology

Abstract

Upright walking is often referred to as the ‘hallmark’ of humanity. The shift from four legs to two may have been facilitated by a decoupling of our hands and feet early in primate evolution, wherein the foot became the primary support during locomotion (e.g., weight support, propulsion) while the hand was free for other tasks. The hands and feet are of special interest in studies of primate locomotor evolution, as they both come into consistent and direct contact with an individual’s environment. This dissertation, thus, tests the hypothesis that increased reliance on the hind limb during locomotion reduced selective pressures on the forelimb for locomotion, allowing selection for other demands to dominant and lead to a greater diversity of hand morphologies, hand uses, and locomotor modes than would have been possible otherwise. In this dissertation, I test this and related hypotheses in two ways. First, using a novel, semi-automated approach to generating 3-D geometric morphometric (shape) data (method explained and tested in Chapter 1), I conduct a large-scale comparison of primate carpal and tarsals to track rates of shape variation across the clade (Chapter 2). Second, I assess the effect of limb dominance on morphological diversity in an independent mammalian clade (Carnivora) that is thought to not have been characterized by hind limb dominance during its early evolution (Chapter 3). Through expansive comparative data collection, this dissertation seeks to better understand the mechanism through which the primate forelimb and hind limb underwent functional and morphological decoupling, eventually leading to modern forms of locomotion seen in extant primates. Specifically, by incorporating carnivorans, a clade that shows different patterns of limb dominance than primates, this dissertation is able to robustly quantify the manner through which two clades do or do not diversify their limbs in the face of limb dominance. Results from this dissertation show that the primate hand is more morphologically variable than the foot in some respects, though the primate foot appears to be under conflicting selection for grasping and weight-bearing, which, in turn, has resulted in a wide variety of functions and morphologies of the primate foot. Further, primates and carnivorans, both, have forelimbs that are more morphologically diverse than their hind limbs, suggesting that, regardless of limb dominance, the forelimb is under a higher selective pressure to functionally and morphologically diversify.