The dinosaurs came and went. Pfft.
Only they might have evolved further, into birds for examples and thus still be around in effect.
Then there were the Neanderthals. Our ancestors killed them off.
Except a great many people alive today seem to have a sizable portion of Neanderthal DNA.
Previously I wrote about the Fermi Theorem and why intelligent life might be very rare. Many people again think there's some kind of filter wiping out civilizations. There must be aliens with so many worlds in the universe that are likely to support life. We don't see evidence of alien intelligence so they must be gone. It's just conceited to think we're the first born.
Then I read this: http://www.technologyreview.com/view/513781/moores-law-and-the-origin-of-life/
If you don't want to read it, it says that applying mathematical analysis to the evolution of life indicates that according to the rate of evolution life must be about 9 or 10 billion years old. This is odd as the Earth is only 4.5 billion years old.
However we have recently learned this: http://phl.upr.edu/press-releases/kapteyn
Kapteyn's Star is a Halo star and originally part of the Omega Centauri group, a dwarf galaxy that was swept up by the Milky Way. The Omega Centauri stars are about 11 billion years old. Coincidence?
A lot has been said recently about life originating on Mars and being transported to Earth via meteorite.
Let's think more grandly. What if life originated in the Omega Centauri stars and spread to our galaxy? What if we and all the other aliens with funny foreheads we meet are the result of seeding from Omega Centauri? We are all in effect aliens.
Mind this still doesn't answer the question of where the seeding intelligences are now but I'm working on it.