Elmo's big Mars plans

[Disaffected]

8. That's one of several.

We have come perilously close on more than one occasion ourselves (and still teeter on the brink). It will very likely happen via environmental collapse or nuclear war or perhaps unstoppable contagious disease. Or, maybe some combination of such events.

One other is that the odds of life evolving into "intelligent" life may be vanishingly small. We have no idea what such odds are though, we only know that it is greater than zero and less than one.