ABSTRACT The earliest Pleistocene fossil forest of Dunarobba (Umbria, Italy) consists of a set of more than 70 tree trunks of an extinct species of sequoia or cypress with original cellulose still preserved. Spectral analyses of tree-ring series (325 and 448 yr in duration) combined with oxygen isotope analyses of the cellulose provide a glimpse into the mean annual temperature and the interannual climate variability that characterized this region at the beginning of the Pleistocene, when the concentration of atmospheric CO 2 was ~400 ppm. The high-frequency variability of the ring width time series shows significant spectral components that are consistent with the influence from the North Atlantic Oscillation, and to a lesser extent, solar cycles and El Niño–Southern Oscillation. The mean annual temperature estimate of ~19 °C, based on a model that combines ring widths and oxygen isotope values, is a full 6 °C warmer than the present-day value for this region. These elevated temperatures are consistent with estimates from pollen analyses and with estimates from higher latitudes.

Science has achieved tremendous success over the centuries, partly because the complexities of the Earth, the physical processes that sustain the planet, and the enormity of life were separated into disparate fields of study—mathematics, physics, chemistry, biology, and geology, to name only a few. Scientific compartmentalization was initially necessary to impart enough focus to make progress on complicated issues. However, as the knowledge base grew, it became more and more difficult to separate life and the history of the Earth, and vice versa. We now understand that to investigate the Earth's surface as an abiologic system is folly: Life and Earth processes are intimately linked. Hence, a new field was born at the interface between biology and geology: geobiology. As a field, geobiology seeks to understand the intersection of life and the rock record across Earth's history: how organisms influence the physical Earth and vice versa, and how the marriage of physical and biological processes have transformed our planet over its long history. The assessment of life's macromolecules of DNA, RNA, polysaccharides, proteins, and lipids, and their potential recalcitrance in an ecosystem, has opened up the field of geobiology to lead us toward a solid explanation of where life came from, how life has altered the planet, what may be possible for life elsewhere, and what represents one of the reasons for the explosion of geobiologic studies today. Here we outline how molecular biology has transformed our understanding of geobiology, describing a few of the essentials needed to understand geobiology and exploring an example of a modern geobiologically relevant system: a living stromatolite from the shore of a geothermal hot spring in Yellowstone National Park, Wyoming, USA.