"A hardy microbe that can withstand huge doses of radiation could have evolved this ability on Mars.

That is the conclusion of Russian scientists who say it would take far longer than life has existed here for the bug to evolve that ability in Earth's clement conditions. They suggest the harsher environment of Mars makes it a more likely birthplace."

Earth is ruled out because based on current radiation levels, the microbe would not have had enough exposure to radiation. But if Earth had a past of many global catastrophes, with long periods of radiation, the bug could have evolved solely here on Earth.