Origins of homininae and putative selection pressures acting on the early hominins

Abstract

One of the biggest mysteries of human evolution is the divergence of the hominin lineage from the other hominoids. This chapter addresses the problem of the selective forces that might have been behind hominin emergence and that shaped this evolutionary lineage in its early stages. To establish the selection pressures that led to hominin emergence, the following issues will be discussed: (1) The time when the human-chimpanzee split could have taken place according to paleoanthropological and molecular data. (2) The putative traits of the last common ancestor (LCA) of Hominini and extant Panini. The models for the LCA can be constructed only on the basis of the fragmentary fossils of the earliest hominins (ErH) and on the basis of the morphology and behavior of extant apes. (3) The environment in which the ErH lived and could have been exposed to some specific selection pressures. (4) The hypotheses for the selection pressures for bipedality (SPfB), which is the main diagnostic trait of the hominin clade. Some arguments for and against suggested SPfB according to different hypotheses will be presented. (5) The putative selection pressures related to the dental features of ErH. A much more difficult task is inferring behavior (including the social structure) of ErH. Since the fossils of the earliest hominins are so scarce, inferences as to their behavior are possible mainly on the basis of some features of Ardipithecus ramidus (Ar. ramidus) representatives, i.e., their overall body size, sexual dimorphism in body size, and sexual dimorphism of the canines. Finally, the future perspective (e.g., through closer integration of paleoanthropology and genetics) for determining the first appearance of derived hominin traits and the selection pressures that acted upon them is discussed.