What is the main evidence that humans are at the very least part alien? (not from earth)

 The best evidence is the fact that humans can talk which is unlike any other mammals so far. It's true you can teach apes to sign language and this has been done successfully several times. But, speaking like humans do likely is an off world trait simply because talking can be also an act of lying to others.

So, most beings on earth that want to communicate do it in other ways like telepathy or watching each other's behavior. Dogs are very adept at watching human behavior by the way, especially ones that do well around human beings.

But, the main evidence that humans are at least part alien (not from earth) is the fact that we can talk.

No other primate has this ability but humans.

It's true that Crows for example have a very involved language that they teach their young and the noise that means "Watch Out it's humans they are dangerous" is the one you hear whenever they see a human being which sounds really awful. But, if you can observe them without them seeing you as a human they make very different sounds than the awful sound that means "Watch out humans they are dangerous!"

There are other species that have advanced language ability like Dolphins and whales but they are aquatic creatures not land creatures. So, even though both are mammals they likely went to the ocean when all the larger dinosaurs died as a way to survive at that time 65 million years ago when the asteroid hit earth in the Yucatan Peninsula then.

So, humans being able to speak languages is a sign that that ability didn't actually come from earth but from somewhere else.

My theory is that when Maldek (now the asteroid belt) nuked out 65 million years ago that the alien families that survived that (which also destroyed life pretty much on Mars too) came to earth (the ones not rich enough to go back home to other solar systems and interbred with apes here to result in us. So, we were bred as pets or slaves (your choice) so we could serve them as they began to die from earth diseases because their immune systems didn't do well on earth whereas we are half ape to begin with so through genetic engineering we survived when the aliens didn't as their offspring as half alien half ape humans.