We learn and we progress, and while our understanding is limited to what's in front of us, this does not make what seems unlikely, unlikely.
Think through the course of human history and how many things seemed highly unlikely? The idea that the world was round or that man would ever stand on the moon wasn't something anyone could understand let alone comprehend. The idea that you could take out someone's heart and replace it with another, complete lunacy. But all these things, we now look at them as simple, they're not even exciting to us. And in our own arrogance we often view those in the past as small minded. What about in 100 years or 500 years? As we learn and experience more and more will those people not view us as small minded? Think of how big of a deal cellphones have been, no one really thinks about it now since we're so used to them but in years to come we'll look as moronic as a caveman being excited about a wheel. The point - as humans, we have a very limited understanding.