The present-day environment is the result of billions of years of "coevolution" of life with its surroundings. We try to elucidate this evolutionary history using biogeochemical approaches. For example, we are studying changes in ocean and atmosphere oxygen levels using the Fe and Mo isotope systems as novel paleoredox proxies. These techinques are applied to a range of samples, including pristine drill cores recovered by NASA's Deep Time Drilling Project. In complementary laboratory experiments we assess the consequences for biological evolution by examining how changes in oxygen and metal availability affect microbial metal metabolism and the mechanisms by which microbes acquire metals from minerals. We are also involved in efforts to examine how the origin and early evolution of life were affected by meteorite impacts.