Also, I've often wondered if it is possible that lower life forms exist on other planets - like bacteria, photosynthetic creatures, etc. And if there were photosynthetic creatures there'd likely be protozoa that would be more animal-like and would prey upon the photosynthetic creatures to keep them in check. In no time you'd have a whole ecosystem. So if you let one foot in the door for lower life forms it's fairly possible you'd have higher life forms.
Of course the Bible says angels don't marry, and the assumption then is they don't reproduce. And I understand that people have said that human beings are the crown of God's Creation, but even if this is true what about other animal life on other planets?