[johnandrhonda]

And just how did these microbes live for eons of time in the vacuum of space with temps ranging alternately from hundreds below freezing to hundreds above, and then survive the searing heat of entry thru the earth atmosphere and crash landing? If they were really that tough, they would be living on the moon, too, as well as every other planet.

[James C. Bennett]

Prions are protein molecules with next to no known method for their destruction. They even survive heavy-dose irradiation.

Prions replicate themselves, and that is why Mad Cow Disease is fatal.

[buwaya]

I would not dismiss this on these grounds.

Earth bacteria have been found living INSIDE rocks.

Possibly an alien cell-like organism could be living, yet dormant, inside a carbonaceous chondrite, just one possibility. These have landed on Earth in substantial chunks.

And upon landing in a receptive environment, say the Earth of a billion years ago, or today, some of these could revive and multiply.

On the Moon and elsewhere, lacking the right environment, they would remain dormant.