According to some recent research, modern humans may have been close to extinction 70,000 years ago, reduced to a few as 2,000 individuals.
There's a lot to chew on here, but one thing I think this illustrates is that the Search for Extraterrestrial Intelligence (SETI) may be up against longer odds than may be supposed at first glance.
I think it will turn out that life is pretty common in the universe. Indeed, given the right conditions, it's probably an inevitable natural process.
What is not inevitable is that "intelligent" life will involve. And especially the peculiar kind of intelligence that will result in a "civilization" that might leave detectable traces for outside observers.
This news suggests that Earth came within a hairs-breadth of losing the one species it spawned in 6 billion years capable of such a civilization in a drought 70,000 years ago. Had those few thousand individuals been a little less lucky than they actually were, Earth would, right now, still be thinly populated with roving bands of primitive hominids and some marine mammals with reasonable intelligence but no "civilization." That state of affairs had already lasted many millions of years and could last millions more without change. There's no reason to think that Neanderthals or Homo Erectus populations already existing would have made the same cultural leaps that Homo Sapiens managed since the near-extinction event. There's quite a bit of mystery involved even in the Homo Sapiens cultural explosion that started about 30,000 years ago, and if we ever understand that it may turn out that it turned on some fairly subtle development that was no sure thing, either.
My point is that, while life is common, and "intelligent" life not exceedingly rare, "civilized" life (not meant in any pejorative sense at all) may be very rare indeed. The universe is so vast I think it would be presumptuous to declare that it has happened only once (here) and no where else. But I don't think it's off the mark to suggest that it happens so rarely that the civilizations are too far apart in space and time to ever make contact.