"The iconic Alien is to us what we are to apes:
small, pale, big-headed. and with unfathomable technology.
We even abduct them for medical experiments."
I've had had vividly realistic dreams about encounters with alien spacecraft & extraterrestrial beings, and I wouldn't be surprised if many abduction reports are actually a terrifying combination of lucid dreams & sleep paralysis.
I also suspect that there is also a role to be played by the subconscious re-living the experience of birth. Many of the feelings of helplessness & terror associated with these encounters could be remnants of our feelings as a baby. Perhaps it is a resurfacing of memories of being brought out into the bright lights & clinical setting of a hospital room.
The classic 'grey alien' archetype seems to be like a future, hairless version of human beings- or perhaps it could be a genetically-related species that has evolved parallel to us, but in a divergent way to have major differences from us.
I wouldn't be surprised if our planet was like a zoo, for more advance beings to visit and observe. If such beings were able to exist in other dimensions, their appearance & function might even be beyond our current ability to comprehend.
It's absurd to think that there is no life anywhere else in the Universe. In fact, Life probably exists throughout the cosmos, in an endless array of shapes & sizes. Anyone wondering what other forms living things could take, just need to look at the variety on Earth to see the countless possibilities.
It's conceivable that races from other star systems could have a similar anatomy to us humans. However, I think alien beings could be much more incomprehensible in form than a familiar humanoid shape. There's no reason why they'd have to so closely resemble us.
Whatever they are (if they're real), this is an awesomely realistic rendering of the gray alien by 3d artist Andrew Baker:
by Andrew Baker