Jacob K. Javits Fellowship Program

Personal Statement

Reconstructing the Past with Genetic Data - The Evolution of Mitochondrial DNA in the Primates.

Genetic analysis has revolutionized many aspects of physical anthropology over the last 30 years. Questions about human origins and migrations as well as the great sweep of primate inter-relationships that were difficult or impossible to resolve using other methods are made clear with genetic data. Early comparative work established the close relationship of humans to the African apes and later the recent (5 million year old) common ancestor of chimpanzees and modern humans. This has informed the analysis of primate fossils and especially the remains of direct human ancestors. The first genetic research was done with overall DNA and protein similarity but with the advent of easy and inexpensive DNA sequencing methods, much of the analysis has shifted in this direction. Analysis of DNA sequence data has illuminated primate species relationships and the evolution of many important genes and diseases. Mitochondrial DNA (mtDNA) has played a disproportionally large role in these studies due to its unique evolution and inheritance. Unlike nuclear DNA, by far the largest amount of DNA in a cell, mammalian mtDNA is a small circular piece of DNA (a minute fraction of the total cellular DNA) that resides in the mitochondria and is inherited only through females. Mitochondria are cellular organelles that are central to aerobic energy production, and recent research on mitochondrial diseases in humans has shown that the respiratory chain genes that are encoded on mtDNA are vitally important. The rate of evolution of mitochondrial DNA is generally faster than nuclear DNA and is quite variable from gene to gene. This variability allows mtDNA to be used for evolutionary studies of different time depths, a key requirement for reconstructing the past accurately.

Prior to my current graduate school work, I did a study using mtDNA to determine the relationships of various colobine (leaf-eating) Old-World monkeys. These under-studied and, in some cases, threatened primates (there may be as many as 30 species) are endemic to Asia and Africa but how the species are related to each other was not well characterized. An understanding of basic species relationships is required for evolutionary research and important for conservation efforts. This work has resulted in two publications so far and another two are in preparation. In addition to the results indicating species relationships, it became clear that the evolution of the mitochondrial gene I studied was more complex in the primates than originally thought. Surprisingly, different groups of primates evolve at different rates for this gene, and overall, primates display a much higher rate than other mammals. The whole mitochondrial genomes that have been sequenced in the primates indicate that this pattern of increased evolutionary rate holds for all mitochondrial protein-coding genes. An increased rate in itself is not that important but as it involves many genes, it is suggestive of a series of adaptive, co-evolutionary events. So far only the great apes, one gibbon and one baboon have had their mitochondrial genomes entirely sequenced. In order to investigate the evolution of mtDNA in the primates thoroughly, many more whole genome sequences need to be completed.

My proposal is to sequence whole mitochondrial genomes from diverse primate species with the aim of tracing the potentially adaptive changes that have occurred uniquely in this group. Sequencing technology has improved such that this project can be completed in a few years. Tissue samples from primate species will be obtained from zoos and primate centers. (Samples are usually made available when an animal dies, and no animals would be sacrificed for this project.) Standard molecular biological techniques, which I have used on prior projects, of DNA isolation, cloning and sequencing will be used to generate the data. Various computer programs aid in the assembly and analysis of sequences. The new data will be combined with published genomes and analyzed. I have developed unique methods of analysis with my current and former collaborators to compare mitochondrial sequences (manuscript currently in preparation). Certain systematic questions may be answered using these complete mitochondrial sequences that individual genes are unable to resolve, and species will be chosen with this potential in mind. The sequences will also allow a much greater understanding of the molecular evolution of the genes encoded by mtDNA and the overall pattern of mitochondrial DNA change in the primates compared to other mammals.

The results may have implications for the evolution of the anthropoid primates (including humans) as preliminary data indicate that the speed-up began at the time of the anthropoid divergence from other primates. There are many traits that evolved along the lineage leading to, or within the anthropoids such as increased brain size, more elaborate social organization, increased parental care, etc. Since mitochondria and mitochondrial DNA are central to cellular metabolism and they are so well conserved elsewhere in the mammals, the major changes in these genes observed in the anthropoid primates may have had far reaching effects.

When completed, this project would form the core of my dissertation research. More importantly, data generated by this project would further our understanding of the evolution of a key group of primates that includes humans. Complex evolutionary questions are best approached from many angles, and modern anthropology has benefited greatly from the strength of a multi-faceted approach. In this age of genomics, millions of dollars are spent on mega-sequencing projects that are often not informed by evolutionary thinking, leaving our closest primate relatives under-researched. Smaller genetic projects targeted with an evolutionary perspective, such as this one, can provide key information about our own evolution and inform the whole of physical anthropological research.

Copyright 2000.
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