Title: Genomic Memories of the Past: Using Microbial Genomics to Examine the Co-Evolution of Earth and Life. Abstract: Since the origin of life over 4 billion years ago, life has fundamentally altered the habitability of Earth, and the environment has molded the evolutionary trajectory of life itself. Microbial genomes retain a “memory” of this evolution. I will present two examples of how we can use genomics to study the co-evolution of Earth and life in the recent and distant past. To examine evolutionary trends in the more recent past, we have used metagenomics to investigate environmental drivers in the evolution of microbes in deep-sea hydrothermal vents, which are thought to have been important habitats for life’s early evolution. We have shown that microbial populations in a deep, basalt-hosted system appear to be under stronger purifying selection than populations inhabiting a cooler serpentinizing system less than 20 km away, suggesting that environmental context has an important impact on evolutionary trends. However, we can also examine evolutionary trends in Earth’s distant past through comparative genomics. By reconciling phylogenetic trees for microbial species with trees of metabolic genes, we can determine approximately when crucial metabolic genes began to spread across the tree of life through horizontal gene transfer. Using these methods, we conducted an analysis of the relative timing of the spread of nitrogen-metabolizing genes, and have found that genes related to denitrification began to spread across the tree of life after the Great Oxidation Event. In contrast, genes related to nitrogen fixation appear to have spread much earlier, consistent with geochemical evidence. As the sequencing revolution supplies ever more data about the tree of life, studies that couple genomics approaches with environmental context have the potential to reveal important insights into the co-evolution of life and Earth over time.