[rbg81]

It is quite possible that there have been many “intelligent” civilizations in our galaxy. However, there is nothing that says that once a species becomes intelligent that it stays intelligent. Its quite possible that civilizations can be destroyed (by natural events or self-destruction). It is also possible that an advanced civilization (even a whole intelligent species) can degenerate back into sub-intelligence (or worse) [HG Wells — “The Time Machine”]. The last possibility is that a species can become so intelligent and cerebral that they care no for their physical selves anymore or evolve into a G-dlike state [”2001 - A Space Odyssey”). Either way, it would not surprise me if intelligent civilizations as we know them have very short lifespans (500-10000 years).

[Louis Foxwell]

The intelligence of our species has apparently lasted just under 10,000 yrs. It is, of course, now defunct. Our little foray out of the jungle has ended badly.

[gargoyle]

...there is nothing that says that once a species becomes intelligent that it stays intelligent.

...Sounds like half the Republic...

[Ken H]

The last possibility is that a species can become so intelligent and cerebral that they care no for their physical selves anymore or evolve into a G-dlike state ["2001 - A Space Odyssey").

Or end up like the Krell in 'Forbidden Planet'.