Early habitability of Earth. The NAI has supported Stephen Mojzsis (University of Colorado team) and others (e.g., Mark Harrison, University of California, Los Angeles, team) to investigate the oldest rocks and to use ancient zircons to characterize the environment of the young Earth. One result is evidence for an ocean and hydrological cycle in the Hadean Eon, the first 500 million years of Earth’s history.^2,3 For additional details see Box 2.1.

The rise of oxygen and Earth’s “middle age.” NAI support was critical in fostering a new interest in the Archean and Proterozoic Eons, the geological periods from approximately 3.9 billion to 2.5 billion and from 2.5 billion to 542 million years ago, respectively. The NAI sponsored collaborative deep-drilling projects and isotopic studies to document the co-evolution of Earth’s biota with the rise of atmospheric oxygen. Findings include new evidence of oxygen before the so-called Great Oxidation Event, improved understanding of the timing of this event, and evidence that this event led to a Proterozoic world unlike what came before or after.^4-8 For additional details see Box 2.2.

Snowball Earth. The NAI supported fieldwork by Paul Hoffman (Harvard University) and his students to provide high-resolution stratigraphic and geochemical data needed to refine the hypothesis that Earth was, at times, completely covered with ice during the period from 850 million to 630 million years ago. Snowball Earth and other extreme events are now considered a natural aspect of Earth’s evolution on long timescales. Other NAI investigators at the California Institute of Technology and Arizona State University have investigated the implications of this period for the evolution of life.^9-12 For additional details see Box 2.3.

Microbial mat ecology. In situ studies, led by the NAI team at NASA’s Ames Research Center, of the Guerrero Negro hypersaline microbial mats (modern representatives of one of Earth’s earliest and most pervasive