THE EARLY FAINT SUN PARADOX goes as follows: 4 billion years ago the sun (its fusion fire not yet having worked up to present levels) was 25-30% cooler than now. Terrestrial temperatures would have been sub-freezing, precluding liquid water. How then did life form in these early eras? Carl Sagan, in a posthumous paper co-authored by Chris Chyba (Science, 22 May) suggests a possible scenario. Ultraviolet radiation from the sun, they argue, would combine with existing methane to form solid hydrocarbons in the upper atmosphere. This in turn would shield ammonia (otherwise broken up by the UV) long enough for the ammonia to produce a greenhouse warming adequate for liquid water. Sagan and his interest in life in extreme environments was the subject of a session yesterday at the meeting of the American Geophysical Union in Baltimore. According to David Morrison of NASA Ames, there are only two places on Earth where life has not been found---on the Antarctic ice sheet and in the upper atmosphere. Everywhere else, whether in hot springs (even above boiling temperatures) or a kilometer below the surface, life seems to thrive. One speaker, Todd Stevens of the Pacific Northwest Lab, asserted that some subsurface "rock-eating" microbes constituted an ecosystem independent of photosynthesis and that their metabolism (in some cases amounting to a biomass doubling time of millennia) was perhaps the slowest of all life forms.