If, for example, Martian microbes arrived on Earth after life independently arose on our planet, the extraterrestrial organisms may not have been able to replace or coexist with the homegrown species.

• 举例而言，如果在地球独力孕育出生命之后，火星微生物才来到地球，这些地外生物便可能无法取代原生物种，或与之共存。