My Ph.D. advisor Dick Lewontin recently had a triple-bypass operation (it went well and he’s fine), and during my check-in call he described the procedure. Like all bypass operations, veins or arteries from other parts of the body (usually the arm, leg, or chest) are harvested to circumvent the blocked coronary arteries. Dick noted that the body is full of superfluous blood vessels.
“Superfluous?” I said. “Maybe we can live without them, but couldn’t they have evolved as ‘backups’—in case something went wrong with a partner vessel?”
“Oh, Jerry,” said Dick. “Don’t be such a selectionist. Maybe they’re truly redundant: they didn’t have to evolve as backups.” (Dick, remember, was the second author of the famous “Spandrels of San Marco” paper, arguing that much of biological form was not directly adaptive, but present for other reasons.)
Thinking about it, I think there’s a lot of truth in what he said. First of all, it’s unlikely that we have superfluous veins because our ancestors had vein problems and needed backup vessels. What killed them was not thrombosis, but disease, predation, infection, and hard living.
(Caveat: because modern humans can do “fine” without a major artery in the leg doesn’t mean that not having it in our ancestors might have conferred some marginal disadvantage.)
And it’s not just blood vessels that seem superfluous. We have two kidneys, and can do very well without one. Do we have two because one could serve as a backup for the other? And if that’s why we have two kidneys and two lungs and two eyes and two testes/ovaries, why only one liver and one heart?
We are bilaterally symmetrical, bipedal organisms descended from bilaterally symmetrical fishes. In some cases having two of something is useful. Our two eyes give us binocular vision, but we have two not because of that facility, but because our fishy ancestors had two eyes that enabled them to see, nonbinocularly, on both sides of their bodies. It may be the case that when some new organs evolve, like a lung or kidney or ovary, the genes that make it act bilaterally, giving us two organs instead of one. That, and not an evolved redundancy, may explain why we have “superfluous” bits. The fact that one can act as a backup is simply a spandrel–a fortuitous but nonselected byproduct of a developmental imperative.
When we ask, then, why some bits are present in pairs and others not, the answer, then, may not always be because it’s better to have two than one.
Coda: The bypass operation has been one of medicine’s great advances, giving many people many years of added, productive life. I remember the first one, which was only in 1960; and for years afterwards it was a dicey and dangerous procedure. Now it’s more or less routine with a high expectation of success: as Dick said, his hospital was running a “bypass factory.”