It may be more helpful to think of 'other life forms' instead of 'aliens' which I personally am not keen on.
The universe is a bit like a pond. It's teeming with life in one form or another. Fish of many species...but all fish, newts, frogs, insects etc. etc.
The humaniod type of being seems very common, like the fish, but not all exactly the same.
The main difference here is (I believe) that our 'pond' contains different types of water. Many of the species can swim in all the waters while others are more contained, probably because they haven't yet discovered how to 'swim' in other waters or areas of the 'pond'.
Hope this helps just a little. It's a mammouth subject full of information, and just to make it more difficult...some of the info out there isn't true. Be your own judge.
