Thanks very much, Alex. Benton's paper was well worth reading. To my eye he seems to have treated the subject both more rigorously, and without Levin's undeniable bias.

Like Levin's logic chain, Benton's too has factors at opposite ends of the process that work against each other. I noted that larger fragments (boulders >1m) enhance the odds both of complete ejection and survival of microorganisms during transit; while the greater impact velocities at the other end of larger objects tends to decrease the odds of surviving the impact. You lose some, you win some, you lose some again.

It need work but once, however, sometime these last gigayears...

"...look into the seeds of time, and say which grain will grow and which will not" wrote Shakespeare in MacBeth.

Hope has not fled!

--Robert