I don’t think anyone said it was impossible. The extraordinary unlikelihood (is that a word?) of such a necessary set of circumstances does, indeed, boggle the mind; mine anyway.
If you look into the scientific research on the subject of how biological matter can and is found to be preserved for fantastically long periods of time, you will have the opportunity to be more surprised and less boggled. Exobiology is finding extremely difficult to sanitize Terrestrial space probes to avoid contaminating extraterrestrial venues.