*We're getting closer to a "panspermia" idea of galactic microbes being present when the solar system coalesced. If that were the case, then the Earth being occasionally sterilized by heavy asteroid bombardments wouldn't make a lot of difference. The Earth would simply be re-colonized by extremophile organisms already present in the solar system's debris.
*One doesn't see much theoretical work done on the likelihood of living organisms from the Earth leaving the Earth. How many tons of living material have escaped Earth's gravity well, in the past 4 billion years or so? And where would they go, if that happened?
Those improbably ancient things that look like little bugs
(...)
The tornadic microfossils lay in the Pilbara Craton for 3.465 billion years before being separated from their natal rock, packed up in a box and shipped to California. Paleobiologist William Schopf of UCLA published his discovery of the strange squiggles in 1993 and identified 11 distinct microbial taxa in the samples. Critics said the forms could have been made in nonbiological processes, and geologists have argued back and forth in the years since. Last year, Schopf sent a sample to Valley, who is an expert with a super-sensitive instrument for measuring isotope ratios called a secondary ion mass spectrometer.
Valley’s team found that some of the apparent fossils had the same carbon-isotope ratio as modern photosynthetic bacteria. Three other types of fossils had the same ratios as methane-eating or methane-producing microbes. Moreover, the isotope ratios correlate to specific species that had already been identified by Schopf. The locations where these isotope ratios were measured corresponded to the shapes of the microfossils themselves, Valley said, adding they are the oldest samples that look like fossils both physically and chemically....
