New research has shown that the moon was hospitable toward life for a 500 million year period in the distant past. After the moon split off from the earth in a massive collision some 4 billion years ago, large amounts of water vapor were present; enough to form an atmosphere and lunar-surface pools that would have been capable of harboring single-cell lifeforms. At this time the moon experienced heavy volcanic activity, which could have further contributed to available water by pumping trapped liquids in the interior into the atmosphere.
Unicellular phytoplankton, by NEON ja.
The moon was also subject to frequent asteroid bombardment at this time, which could have brought primitive bacteria or at least the necessary ingredients from the inner solar system, including the earth, to the lunar surface. Experiments have shown that abiotic synthesis of organic molecules is quite probable given the right chemicals and environmental conditions, which would have been met during this time period. As it took much longer than 500 million years for multicellular life on earth to develop from single-celled ancestors, this hospitable period was probably too short for any organisms that may have been present on the moon to evolve though.
Lunar Water, ISRO/NASA/JPL-Caltech/USGS/Brown Univ.
Once volcanic activity ceased, the water in the moon's atmosphere and on the surface was evaporated into space, and any organisms present would have ceased to exist. However, there is a great deal of evidence that some of this early water is still trapped deep within lunar rock. Could living organisms still be present there as well?