I was mulling over my post on the Wikipedia list of cryptids when I turned to the list of extra terrestrial species. It is much shorter and simpler. Either people are much more imaginative when it comes to inventing earth creatures than they are spacemen or there are a few interesting cultural abstractions to be made.
It shows an instinctively shrewd understanding of natural selection. Humans are as they are as much because of their posture and their thumbs as their brains. We had pre-adapted large brains, nimble digits and forward facing eyes. Our ancestors moving permanently to two footed locomotion freed their hands to do other things. The ability to look at and manipulate objects in fine detail was the likely foundation for human consciousness. If we do meet intelligent extra-terrestrial life it will be uncannily familiar to us.
We will probably have to search for quite a while before we do find intelligent extra-terrestrial life. We know from the natural history of mammals that flippers and wings have evolved several times on Earth. This is called convergent evolution. Some mammals went back to the ocean, some took to the air. In both cases their found familiar solutions to the ongoing difficulties of locomotion. I looked up the evolution of the eye. Apparently while some of the prerequisites of vision evolved once (which means the rudiments of vision are quite ancient and spread across many species) image forming eyes evolved between 50 to 100 times. What does this mean? It means eyes flippers and wings evolved dozens of times in natural history. Human intelligence, as far as we know, only evolved once.
What do aliens mean? In folklore aliens mean two things. Firstly they are the angels and/or spirits of late capitalism. There is a strong affinity between alien abduction and the incubus/succubus phenomenon, although there’s also clear links with schizophrenia and drug abuse. They are also supermen. Alien encountered are cold, unknowable, superior and in control.