[crail]

How do you suppose they became isolated?
Take your pick: migration, new paths for rivers

If there are isolated tribes of humans say in the Andes, what are the chances they will devolve into monkeys?

Zero. Evolution is not an up-down ladder, but an exploration of the possible forms life may take. Of all the possible forms, the chance you'll see a repeat is essentially zero. Given a long period of time, any reproductively isolated population will drift genetically. Once it can no longer reproduce with the main species, it would be considered a new species.

[mlc9852]

Can an ape and human reproduce?

[muleskinner]

Given a long period of time, any reproductively isolated population will drift genetically. Once it can no longer reproduce with the main species, it would be considered a new species.

Take the Irish, for example...