Although much progress has been made in elucidating the phylogeny and systematics of placental mammals, many difficult problems still remain. These problems involve not only the order Primates, which has humans among its members, but also the 17 other orders of placental mammals. For example, one of the most important primate groups is the superfamily Ceboidea comprised of New World monkeys and marmosets, but the pattern of evolution over the last 25 million years that led from a common ancestor to the 16 surviving genera of these neotropical primates is poorly understood. Similarly, a series of phylogenetic branching events that occurred between 90 and 65 million years ago separated the 18 surviving orders of placental mammals from one another, but the exact course of branching, i.e. the evolutionary relationships among these major groups of placental mammals, has yet to be definitively established. Recent technical advances in molecular biology have made it possible to obtain large amounts of DNA nucleotide sequence data from corresponding genes in a wide range of species. Since the sequence of nucleotide in DNA is the genetic or heritable material, such information provides a very precise record of the evolutionary relationships of the species compared. Through utilizing the new molecular methods for generating comparative nucleotide sequence data, DNA information from several different genes will be obtained from the above mentioned mammalian groups. These data will be subjected to rigorous phylogenetic analysis, using computer programs based on theoretical models that best describe the course of genetic change over time. This phylogenetic analysis, when applied to the DNA sequence data from the more rapidly evolving portions of the genes, is expected to provide a highly resolved picture of the evolutionary relationships that exist among the 16 genera of neotropical primates. Similarly, this analysis applied to the slower revolving genic portions should provide a well resolved picture of the evolutionary relationships that exist among the 18 orders of placental mammals. In addition to reconstructing the pattern of phylogenetic branching, rates of DNA change along the branches will be determined. This information on rates, when related to knowledge on the life history strategies of the animals included in the study, will test the hypothesis that life history changes towards longer generation times and life spans cause decreases in DNA mutation rates, whereas life history changes towards shorter generation times and life spans cause increases in DNA mutation rates.
