How could they have survived on Earth? They would've had to evolve on a planet virtually identical.
The same way astronauts survived on the moon.
And the nearest star is so far away it takes light millions of years to get there. And you can't go faster than light. So how did they get here?
A sufficiently advanced society might've thought of a way. There's been a variety of ways that scientists have thought of to bypass the speed of light limit. Of course, we're not advanced enough to see if any of these could actually work in practice.
And even if these things were explained, the question remains: If aliens influenced our past, why haven't they come back? If they visited us in the past, why aren't we hearing from them now?
Who knows? It could be for a variety of reasons. Not that I support the ancient alien theory or anything. I was simply answering the questions as a devil's advocate.
As for me, I believe that there is intelligent life out in space. It would be naive for me to think otherwise.