concept

Panspermia

The idea that life did not begin where we find it but arrived from elsewhere. Within one planetary system this is plausible: impacts really do throw rock between planets, and spores can survive the trip. Between stars the arithmetic does not survive — transit takes 10^5 to 10^7 years, and the accumulated cosmic-ray dose over that span shatters any genetic polymer. That is why relatedness between biospheres parsecs apart is treated as very hard to believe.