Why do we still have allergies if humans have lived around plants for a long time?
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We haven’t been around ALL plants for ALL of that time.
Many Asian people can’t tolerate milk because they basically don’t have a massive diet of milk. It’s around, they come into contact with it, but they’ve never needed to adapt to it. And every plant is “around”, you don’t come into contact with it, and have no need to adapt to it. So you can form allergies over generations.
Plus, the flora/fauna of one area can be wildly different to that just a few miles away.
Also, most allergies are from *under* or *over* exposure to a substance. Peanut allergies are highly correlated to advice to pregnant women to avoid peanuts. They no longer give that advice, because it literally made babies more likely to become allergic (as they never were exposed to peanuts at a young age or in the womb).
The same works for being swathed in a particular substance all the time, you can still become allergic to something that you’ve been eating or exposed to for years if you are exposed enough.
The body deals with what it needs to. It can’t do everything, it can’t remember everything, and it doesn’t know what things are. New or unusual or rare substances may well trigger an allergic reaction. Many people are allergic to kiwi, because it’s just not the kind of thing that most modern humans are ever exposed to. The body doesn’t know and it takes a best guess and sometimes it gets it “wrong” (but who can blame it, it’s just an organism trying to survive in a hostile environment full of pathogens).
Some people’s immune systems aren’t very clever and get it wrong far more. Some of those people’s systems were damaged by under- or over-exposure in general (e.g. if you lived in a sterile bubble, your immune system wouldn’t develop, so almost anything could be hazardous to the point of fatal when you are then exposed to it, even just a common cold or a simple foodstuff). We gave some people lots of problems by bringing them up in a polluted environment and the body is constantly under attack, so it can overreact to all kinds of things as it’s just trying to survive.
And often such over- or under-reaction is often fatal because the immune system is the only guardian against a knife-edge between you living and dying in a “goldilocks” zone of survival on a rare planet sustaining fragile life in a narrow window of a “goldilocks” zone that supports life, in an otherwise sterile and lifeless universe.
And the immune system, and the body in general, are only trying to survive. If you don’t survive, it really doesn’t matter, long term, in terms of the human race and the billions of organisms that make up your body. If you manage to reproduce before you die, that’s a bonus. If the majority of people manage to live long enough to reproduce and sustain the race, that’s sufficient. That there are a few outliers from the 8-billion-and-growing population of one very specific species, who might not survive against ordinary and mundane things that the others survive, it really doesn’t matter. It doesn’t mean the “design” of the human immune system is flawed, or that you could fix it perfectly forever. Those people are literally in the error margin. The human immune system is still an incredible and highly-successful defence mechanism.
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