I don't think it's really that wide, but rather that we tend to focus on the difference while ignoring the similarities.
> What are the chances that the intelligence spectrum ends just to the right of our most intelligent geniuses?
Close to zero, I would say. Human brains, even the most intelligent ones, have very significant limitations in terms of number of mental objects that can be taken into account simultaneously in a single thought process.
Artificial intelligence is likely to be at least as superior to us as we are to domestic cats and dogs, probably way beyond that withing a couple of generations.
When my mum came down with Alzheimer's, she forgot how the abstract concept of left worked.
I'd heard of the problem (inability to perceive a side) existing in rare cases before she got ill, but it's such a bizarre thing that I had assumed it had to be misreporting before I finally saw it: she would eat food on the right side of her plate leaving the food on the left untouched, insist the plate was empty, but rotating the plate 180 degrees let her perceive the food again; she liked to draw and paint, so I asked her to draw me, and she gave me only one eye (on her right); I did the standard clock-drawing test, and all the numbers were on the right, with the left side being empty (almost: she got the 7 there, but the 8 was above the 6 and the 9 was between the 4 and 5).
When she got worse and started completely failing the clock drawing test, she also demonstrated in multiple ways that she wasn't able to count past five.