Description: Geological time units are the lingua franca of earth sciences: they are a terminological convenience, a vernacular of any geological conversation, and a prerequisite of geo-scientific writing found throughout in earth science dictionaries and textbooks. Time units include terms formalized by stratigraphic committees as well as informal constructs erected ad hoc to communicate more efficiently. With these time terms we partition Earth's history into utilitarian and intuitively understandable time segments that vary in length over seven orders of magnitude: from the 225-year-long Anthropocene (Crutzen and Stoermer, 2000) to the ~4-billion-year-long Precambrian (e.g., Hicks, 1885; Ball, 1906; formalized by De Villiers, 1969). Given the importance of such chronostratigraphic units (sensuZalesiewicz et al., 2004), it is surprising that the key event in the Earth's history, the first appearance of life, is not recognized as a major time boundary. This omission may reflect the relative youth of the field of Precambrian paleobiology. The earliest definitive reports of pre-Ediacaran fossils date to the 1950s (Tyler and Barghoorn, 1954), and only in the last few decades have details of life's early history begun to emerge (e.g., Schopf, 2001; Knoll, 2004). This recent progress in the understanding of early life sets a foundation for augmenting the geologically-derived time units used for the Earth's early history with biological ones, which have already proven so effective when organizing the chronology of the more recent geological past. In recognition of the importance of life in the Earth's history and the efficiency offered by chronostratigraphic terms, we propose to divide the geological time scale into two informal supereons: Pregeozoic (the abiotic supereon) and Geozoic (the biotic supereon)...