Use your imagination. Humans can make it into space now, right? This could either be a result of one or two recent generations of humans, or from a civilization thousands of years ago. Other than a lack of surface evidence suggesting that there had been an advanced human civilization before recorded history, I don't think the possibility can yet be discounted. We still have many places, beneath the water and above and below the ground, yet to be explored. And in particular the north and south poles, which had not always been dominated by ice.
And while recent generations don't seem possible, you can't rule it out unless you know the exact effects that zero-gravity would have on the development of things in a womb and such. Which I'm wondering when they'll ever get around to trying... Because they'll need to know if they decide to live for any significant amount of time in space in the future.