[nbashaw]

The universe is so big it seems highly likely

[jacquesm]

It is also so old that intelligence may come and go without ever really overlapping.

[garmaine]

This line of reasoning never made sense to me. The speed of light prevents there from being calamities that can wipe out an entire star-faring civilization at once. Once intelligent life has spread throughout a region of space beyond a few light years, I don't see how extinction is possible.