This isn't quite limited to humans, but all of the catarrhine primates (apes and old world monkeys) are missing the alpha gal protein found in other mammals. Alpha gal seems to help mammals remain fertile with age, so losing it would be unlikely to happen by chance. The loss of alpha gal is believed to be the result of an ancient primate virus mimicking it in some way; the only survivors would've been those animals that reacted to the protein as a foreign substance.

You can also become highly allergic to alpha-gal, if you're bitten by a lone star tick. This makes you allergic to the meat of every mammal, except for old world monkeys and apes I guess.