This is my take:

When dealing with a stressful situation human brain goes into a fight of flight mode. It quickly scans the memory for any similar situations and draws the decision weather to fight or run according to the experience. For those that have never experienced such a situation "freeze" can happen - a phenomenon when someone who has never been exposed to such stress (or violence) before and the brain is stuck scanning the memory to find an previous event upon which to base a decision on, but can't find any, so you are stuck there frozen just staring at the chaos.

Next, our brain has a way to recognize familiar people (let's say people from your tribe). It's a sense that helps you recognize who is "one of us" and who is an outsider.

Now my idea is that "uncanny valley" is not a sense that humans have developed on purpose, but rather our "familiarity sense" going into freeze or overdrive. You see something that is so foreign and outsider like, but still in some way familiar that you can't say if it's one of you or an complete alien creature. You don't know weather to trust it or go into fight or flight mode.

The more pureblood the jew is, the more likely it will trigger such felling in an Aryan. I remember my first meeting with an racially pure jew, ashkenazi. He was so ugly that part of me was disgusted and wanted to shun him and hate his from the very core of my being, but he was so "friendly and nice" (all fake of course) that is was triggering a sense of pity and guilt inside me that I'm feeling this way towards him. He was a high ranking officer in the military in the unit where I was first posted while I was very young.

During my time there I have grown to furiously hate him. His features and manners have burned into my memory. I can spot jews like him now IRL without even trying. He noticed how much I hate him and "helped" me get out of his unit. I still miss the fear I would see in his eyes every time I would run into him alone in the corridors, or less populated parts of barracks. I would say nothing, just stare. I used to feed on that fear.

Could have been hard-coded in our genes back when there were multiple homo species. Appearances were close enough, but sapien had to have a negative reaction to the others in order to keep their (superior) species pure.

There was once something not quite human which we had to instinctively fear.

It's to keep you from trusting what cannot and should not be trusted.

I figured it was corpse detection as that would be the usual "not quite human" thing we'd see aside from other hominids. I like to think many people's subconscious minds think clowns are zombies.

Just a natural feature of perception of things. Things that straddle the boundary will be weird. It's like one of those optical illusions where your visual system goes back and forth between two interpretations.

I suppose there is some element of the human version with being sure it's a friend or not some trick. There's definitely something similar with deciphering male from female, with regards to sexual attraction.

Same reason we have the Rhesus factor.

When I was a kid I had arachnophobia, like most kids I guess, scared shitless of fucken spiders

And as I grew older, I started to confront my fear, precisely because it was a fear and I wanted to get rid of it/overcome it

Now I understand the value of arachnids, my relationship to arachnids has completely changed since

I rarely if ever remove the webs, especially on my balcony, even when the spider is long dead and you know why? Because I have roller blinds that are always almost closed , just open enough to let sunlight and air in. End result is that fucken mosquitoes get all fucked every time they try to get in, it's quite handy, there's a swamp nearby I let you imagine what that implies sometimes, but I digress

Now it doesn't mean I would tolerate a fucken tarantula anywhere near me