Darwin:

All of these movies and stories about little green men (or grays) made me wonder about how evolution on a totally different planet could ever end up with a creature that looked anything like us (medially symmetric, bipedal, head configuration etc.). The chances are really infinitesimally small that something would evolve to look so like us. So if these creatures really exist, my only conclusion is that they would have to actually be humans from the far future. That is, they would be time-travellers. Therefore, we should be very afraid of them - they is us!