Can you run out of Life tokens, and still create life on a planet, in general? I didn't see anything either way on the matter in the Exoplanets rulebook, except that it says that if you upgrade a planet with at least 1 Life token on the beginning of your turn to a Species, you can skip placing any intermediate Life tokens and just replace the existing tokens with a Species.
Pretty sure the answer is 'yes'. I could have sworn I saw it stated explicitly, in addition to the implication in the bit you quoted, but can't find it now. Maybe I was dreaming.
That ended up being the way we played it - which certainly adds some strategic implications to which planets you colonize in the early to mid game!