Superimposed on generally warm climates of the early Tertiary Period was an abrupt and extreme warming interval at the end of the Paleocene Epoch some 55 million years ago. This event, the latest Paleocene thermal maximum (LPTM), coincided with a mass extinction of deep-sea organisms and a sudden radiation of terrestrial mammalian orders, including primates. The LPTM also was characterized by a global carbon-isotope excursion that was caused by a massive injection of carbon into the ocean and atmosphere. All of these phenomena were likely related to the escape of large amounts of biogenic methane from gas hydrate deposits on the sea floor. Although many details surrounding the LPTM are poorly known, it is probably the best past analog for understanding long-term effects of future fossil fuel inputs to the atmosphere.