If a biosphere consisting entirely of RNA existed, it was relatively quickly displaced by life forms that rely on cooperative activities of RNA, DNA, and a broad assemblage of proteins. According to one view, a diverse assemblage of microbes, including cyanobacteria-like cells, had colonized the seas by ~3.5 BYA (Schopf 1993; Schopf et al. 2002). Some aspects of this interpretation of the fossil record have been questioned (Brasier et al. 2002), but other signs of biological activity have been found in rocks from 3.4-3.8 BYA (Rosing 1999; Fumes et al. 2004; Tice and Lowe 2004), and unambiguous fossils of filamentous organisms deposited around hydrothermal vents have been dated to 3.2 BYA (Rasmussen 2000). Thus, cellular life (as we know it) appears to have emerged from inorganic materials within a window of just a few hundred million years.
