Not quite as crazy as some might think. The early solar system was very different from the current one:
Life got started on Earth while the planet was still somewhat unstable. It could have got started within 100 million years of Earth’s formation at 4.5 billion years ago or as late as 3.5 billion years ago, depending on who you talk to. Either way, things were much more extreme and much less stable back then. That was true for Venus, Mars, and the Moon as well.
To the extent that the universe appears fine-tuned for life to a dramatic degree, it’s at least reasonable to think it could have survived on, say, the Moon or Mars until conditions there became prohibitive. And if panspermia is a correct assumption (that life spread throughout the galaxy and took root in various places), there would be nothing especially unusual about that.
Astronomer Dirk Schulze-Makuch and planetary scientist Ian Crawford have looked into the possibilities:
There are two possible habitable time periods for the Moon. The first one was very early on, shortly after the Earth and the Moon formed, at a time when early Earth and Moon would be better described as a double-planet system. The Earth-Moon system was likely enshrouded by a common atmosphere. The problem is that the origin of life appeared later (as far as we understand), by about 4.1 billion years ago. However, since the study by Green showed the peak lunar magnetic activity associated with volcanic activity was about 4 billion years ago, this may still have worked for early lunar habitability and life.
The other peak of volcanic activity, which is thought to have provided an atmosphere of about 10 mbar, was timed at about 3.5 billion years ago, based on the NASA scientists Debra Needham and David Kring. By then, life had originated on Earth and was widespread on our planet, and according to Green, the Moon still had a magnetic field. If habitable conditions were present on Earth’s Moon, then life on Earth can be expected to have spread to the Moon via asteroid impacts by ejecting rocks from Earth that later landed on the Moon. And if the Moon was not habitable, remnants of early life on Earth could have still been preserved on the Moon in icy craters. The Moon is very special in this regard because it is the only repository where we might still find evidence from the early evolution of life on Earth.
Dirk Schulze-Makuch, “Life on the Early Moon? An Outrageous Hypothesis Re-Evaluated” at Search for Life in the Universe (March 3, 2022) The paper is open access.
We won’t really know unless we find fossils of microorganisms on the moon. And then we must be sure that they are not “false fossils” because sometimes it’s hard to tell whether an unusual formation is in fact a microfossil or a natural non-living occurrence. But it will certainly be an intriguing hunt.
News, “Is this idea too crazy?: There was life on the early Moon?” at Mind Matters News (March 12, 2022)
Takehome: We won’t know for sure unless NASA’s Artemis project finds microfossils on the Moon. But it’s possible that, as with Mars, early life got started but died out.
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