Impacts of asteroids and comets posed a major hazard to the continuous existence of early life on Mars, as on Earth. The chief danger was presented by globally distributed ejecta, including transient thick rock vapor atmospheres. On Earth, much of the thermal radiation is absorbed by boiling the oceans. Any surviving life is either in deep water or well below the surface. Global thermal excursions are buffered by the heat capacity of the oceans. But when impacts are large enough to vaporize the oceans (>1028 J), thermal buffering serves only to prolong the disaster for thousands of years, while the oceans rain out. Without oceans, thermal buffering does not occur on Mars. Relatively small impacts (1026 J) frequently heat the surface everywhere to the melting point. However, owing to the low Martian escape velocity, the most energetic ejecta (including the rock vapor) more easily escape to space, while massive quantities of less energetic ejecta are globally distributed. Survival in deep subsurface environments is more likely on Mars because (1) Mars' lower background heat flow and lower gravity allow deeper colonies, and (2) the thermal heat pulse from a major impact is briefer. Only thermophile organisms could have survived impacts of asteroids large enough to leave heat or boil the entire terrestrial ocean. Studies of terrestrial microorganisms indicate that the last common ancestor may have been thermophile and the survivor of such a catastrophe. This organism should be distinguished from the first common ancestor. An additional refugium is ejection of rock to space by impacts and their return to a habitable planet. ¿ 1998 American Geophysical Union