The moon is a terrible place for life. Primarily, it has no atmosphere of note, so no form of life currently known could live in any meaningful way there. But there were a few points in the *distant* past when the Moon could have supported a meaningful atmosphere. The possibility exists that very early on (3.5 to 4 billion years ago) it could have had a liquid water over, enough to cover the whole moon 1 kilometer deep; and while the atmosphere would be held to the Moon rather weakly due to the low gravity, and thus be easily blown away by the solar wind, the atmosphere would have been constantly replenished by evaporation from the ocean. A later phase would theoretically have seen a thick atmosphere generated by outgassing from the basalt rocks. The lifespan of this sort of atmosphere would be short on geological timescales… seventy million years or so. But while that is pretty short for biogenesis and natural selection to produce a native ecosystem of any complexity, had terrestrial organisms such as cyanobacteria been blasted off Earth by meteor bombardment, they *could* have been carried to the moon and set up shop there, spending several million years spreading and thriving until the atmosphere did eventually fade away. Additionally, it *seems* that Earth generated primitive cyanobacteria in a very short time once the conditions were right for it… something like ten million years. If the moon had windows of 70 million years, that would have been enough.

Was There an Early Habitability Window for Earth’s Moon?

Had life started off on the moon, I’m not sure we’ll ever know. The place has been blasted to hell and gone by meteor bombardment; any fossils would likely have been quite near the surface, and likely long since turned to powder. I suspect, though, that once we set up shop there and generations of humans start calling the place home, there will be big rewards set up to be the first to find a fossil lunar stromatolite. Long before they find a fossil stromatolite, they’ll probably have to find a fossil shoreline or a fossil river.