Upper-Limb Evolution and Development

Abstract

The evolution of human forelimb structure is connected with changes in patterns of human behavior. Two complementary approaches—phylogenetic and functional—are used to analyze and understand these connections. The human forelimb is a mosaic of morphological features that reflect the evolutionary history of locomotor and manipulative behaviors. The phylogenetic approach identifies traits shared with and derived from other species. First, the general sequence of appearance of these features can be traced through a comparative process in which traits shared with other living and fossil species are identified. Some are shared exclusively with the closest relatives of humans, the chimpanzees, indicating their relatively recent appearance, whereas others, for example, the forelimb pattern of the humerus, radius, and ulna, are found among mammals, reptiles, amphibians, and certain fossil fish, indicating that the first appearance of the feature was in a common ancestor approximately 380 million years ago. Second, features unique to humans, termed derived features (indicating that they evolved following the divergence of humans from chimpanzees 6 to 7 million years ago) can be analyzed in order to construct a cladogram, or phylogenetic tree, with nodes representing hypothetical common ancestors of humans and living and fossil nonhuman groups, inferred from the shared elements of the human mosaic pattern (Fig. 1). These groups are increasingly inclusive toward the base of the cladogram; human-derived features evolved after node 6. Fig. 1 Cladogram with nodes representing inferred common ancestors of modern humans and fossil (italics) and living taxa with earliest evidence of forelimb features that are seen today in modern humans. mya = million years ago. The functional analytic approach involves linking the morphology of shared and derived traits to behavior. Evolutionary change in limb morphology occurred as locomotor and manipulative behaviors accommodated to changes in the ecology of successive groups of ancestral species. Functional changes in joint-movement …