Citation(s) Needed: Receding Gums, Bright Colors, and Bloody Urine

This was the end of my short run writing the science column for my high school newspaper. This is the first time I started to find a style I really liked. It’s not exactly credible but it seems plausible enough and I’m happy with how it reads. Sorta like a sillier Cecil Adams, I think. Maybe someday if I have the time I’ll try to revive it but without the excuse of a student newspaper I can’t really justify the sloppy scholarship.

Pretend you’re a primitive monkey. You’re jumping through the trees one fine afternoon, just generally living that primate life. You come across a fruit tree! Oh joy of joys! Sugar, hoorah! Cheap, easy calories, and all for your-simian-self. However, you pause for a second. Weren’t your ancestors mostly nocturnal, ground dwelling scavengers? That is, they spent most of their lives in the dark, in environs where one doesn’t need such bourgeois luxuries like “color vision”. While this was a practical biological budgeting decision for your ancestors, it presents a problem for you. You don’t want to eat fruit that hasn’t fully ripened, but you can’t easily tell ripe fruit from unripe fruit without going by its color.

Fortunately, evolution has got your back. A couple gene duplication events at some point very early in primate evolution when combined with some time, and a whole lot of luck was all it took to give you color vision that’s the envy of the animal kingdom. You’ve got cone cells that’ll let you see in RED, BLUE, GREEN, and well, actually, that’s about it. Varying levels of those 3 colors form everything else. That’s not to say there isn’t some degree of variation among humans. Human females, for example, tend to be much better at discerning colors than human males. There is some evidence to suggest that 2-3% of women can actually see in 4 different colors ( tetrachromacy, as it’s called).

The development of color vision ends up having some bizarre effects on primate biochemistry. The one I want to focus on starts with the reduction in the need for endogenous antioxidants (compounds produced by the body to deal with oxidative damage, among other things). Let’s look at Vitamin C (ascorbic acid). Most mammals can produce ascorbic acid on their own (at a slight metabolic cost), but primates just can’t. Fruits are generally very high in Vitamin C, and because of our excellent color vision, it was wasteful to continue producing ascorbate. This does have a drawback. Take primates away from plants, and you get scurvy (ascorbate actually means “without scurvy”).

This wouldn’t be so bad, except it seems like there was another side effect. Uric acid, a minor component of urine used by the body to dispose of excess nitrogen, also acts as a potent antioxidant, and is naturally produced by the body. In other mammals, the enzyme uricase exists to oxidize it to allantoin, but higher primates can’t do that. Why? Well, some of the things done by endogenous ascorbate just can’t be replaced by dietary ascorbate. So, the body comes up with a way to boost levels of urates to compensate. As a result, primates tend to have higher levels of uric acid. Urates tend to be pretty insoluble in water, so they form crystals. When this happens in our joints, we get gout (effectively unique to primates). When this happens in our bladders, we get kidney stones. So if you ever start urinating blood, just blame it on your fruit crazed monkey ancestors.


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