In 1991 I suggested in one of these notes that if there are a lot of habitable planets out there, yet still we see no evidence of extraterrestrial life, this may indicate that the emergence of life on a habitable planet is more exceptional than we imagined.

It occurs to me that the same argument applies locally. Why has life emerged on Earth just once in billions of years? For I think that is the consensus of biologists: all life on this planet shares the same chemistry and stems from one original event. Experts may investigate details, arguing perhaps that the early Earth had conditions more favorable to appearance of life than the later one, but these are details. Earth is our archetype of a planet favorable for life. If life has appeared here just once, that’s a weighty data point.

[8 November 2025]