Primate evolution: Evidence from the fossil record, comparative morphology, and molecular biology

Abstract

Our understanding of primate evolution is ultimately based on patterns of phyletic relationship and morphological change documented in the fossil record. Stratophenetic interpretation of living and fossil primates yields an objective alternative to the arbitrary scala naturae assumed implicitly in traditional comparative biology. Fossils provide an outline of primate history constraining comparative analyses incorporating taxa and morphological characteristics not represented in the fossil record. Extant taxa without known prehistoric relatives may be interpolated into this outline using deductive cladistic analysis of morphological characteristics and overall molecular similarity. Cladistic analysis provides a method for evaluating the relative strength of stratophenetic links between taxa. The phyletic node connecting Anthropoidea-Adapoidea-Lemuroidea is analyzed here as an example: the link between Eocene Adapoidea and primitive Anthropoidea appears stronger than that between Adapoidea and Lemuroidea because it is based on shared-derived rather than shared-primitive characteristics. Full integration of molecular results with morphological information requires a better understanding of rates of molecular change over geological time. Rates of molecular evolution can be studied using paleontologically documented divergence times for Prosimii-Anthropoidea (ca. 55 m.y.B.P.), Platyrrhini-Catarrhini (ca. 40 m.y.B.P.), and Hominoidea-Cercopithecoidea (ca. 25 m.y.B.P.). Immunological distances combined with these divergence times indicate that primate albumin, widely used as a molecular clock in primatology, has evolved nonlinearly over geological time. A nonlinear albumin clock yields divergence times of about 9 million years before present for humans and chimpanzees, and about 13 million years before present for humans and orangutans (compared with 4 m.y.B.P. and 7 m.y.B.P., respectively, based on a linear albumin clock). Apparent slowing of albumin evolution over time remains to be fully explained. Other proteins and nucleic acids may provide better clocks. Cladistic analysis of morphological characteristics and comparative study of molecular structure, interpreted in the context of the fossil record, promise to contribute to a more complete understanding of primate evolution.