Yeah, the theory saying that human race came from outter space is not that unbelievable at all... we are the only beings in the whole planet whose brains evolved, and the only race that can actually wonder for its own existance (it's a supposition, there's no way you can know if a chicken wonders or not...)
About the contact between human race and aliens... it has a very very slim change (like E-100000000 %) because of these reasons:
- Universe being so big: Enough said, if we would like to contact some aliens they would have to either live in our galaxy or a galaxy really near to ours or have a LONG lifetime and a "economic" way of travelling (so it doesn't take a lot of hrm... "gas" and they can travel the whole way). Another theory is that black holes can "teleport" stuff from one end to another, in this case, finding a black hole might take us to a galaxy or solar system in which an alien race has evolved...
- Time: Universe is SO old (I don't know exactly but go figure), there might have been millions of races before us but none now, so we are alone... maybe a new race comes before human race is completely extinted... who knows.
- Technology: Maybe they don't have the technology needed to travel to another solar system or galaxy, or maybe they have a very advanced technology that they use for other things (if you could choose between meeting an alien or being immortal and happy forever what would you choose?)
That's my 2 cents...