If everyone in Tibet or Earth even for instance quit reproducing then how could a Lama reincarnate when his reincarnation depends on the freewill of the people who are reproducing?
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It's important to keep in mind the true pupose of a reincarnated Teacher: "As long as sentient being remain, may I too remain and dispel the miseries of the world". If there was no one left, then the teacher's job would've been completed and there would be no further need to come back to this world. From a bigger cosmos perspective, there're uncountable other places for sentient existence, so if our planet will no longer be habitable to sentient beings in the future, a great teacher will just simply move on to other places to dispel the miseries of those "other worlds".
These are really heterogeneous questions.
The answer to the body of the question: Bardo is the intermediate step between one life and the next. I don't know of any particular reason why we couldn't image Bardo to last longer if need be.
Also, in the cosmology, it the universe is mind boggling huge and there are multiple "universes", so if you can't be reborn on Earth, you'll be reborn in another universe.
As for what would happen if everyone died in all universes, then I guess that would be the same as the universe ending. The cosmologies say that a new universe will come along, pop back into existence and replace the old one.
The answer to the title, read up on Lopez's The Scientific Buddha. In short, Lopez doesn't think Buddhism has a lot of support for evolution (or vica versa)-- he thinks one is a science, one is a religion (magic, afterlife, abstruse philosophy) and mixing them is bad scholarship.
In Buddhist Biology Barash finds numerous parallels between ideas in Biology and Buddhism, entirely from a secular viewpoint (i.e. Buddhism without reincarnation)