[ckdot2]

There are so many unknowns and unknown unknowns in medical sciense, especially regarding the human body. Did evolution really keep two kidneys if you actually just need one? I doubt it. When considering a donation you should also not only think about if you might die. There are more factors like quality of life. In the end, a whole organ will be removed from your body. This will have some effect.

[guenthert]

I was wondering about that, but not enough to do any actual research. I figure that hunter-gatherer's diet consisted of a whole lot of meat (seasonally perhaps) and then two kidneys were of advantage. My doctors recommended against eating big steaks. I wasn't eating much of those before and had no need to adjust.

I can't tell the difference (six years after the procedure), but then, I'm no athlete monitoring his performance closely.

[DoreenMichele]

I remember reading an article about people in poor countries selling a kidney and thereafter being too sick to work as fishermen or whatever. Some charity was trying to discourage poor people from selling body parts as a get-rich-quick scheme.

I'm skeptical this will have little to no impact on quality of life and productivity for the author.

[eru]

Kidneys are relatively close to the surface. So you can injure them mechanically.

Perhaps our ancestors kept their two kidneys around, but we can do with less, because our physical environment isn't as dangerous?

[nyssos]

> Did evolution really keep two kidneys if you actually just need one? I doubt it.

Aside from the nervous system, pairing is the default condition of all mammalian organs which develop from the mesoderm or ectoderm: they can then go on to fuse into a single larger structure, as with the heart, but as far as I know they're never simply lost on one side during normal development.

[jrockway]

The real question is why didn't evolution give us two hearts and two brains?

[XorNot]

Kind of obvious for both of them though: evolving two hearts is putting two pumping systems in parallel on the same fluid reservoir. Given how the pump works (evolution can't handle making gears in general), you'd need a precise synchronization system to keep everything working properly - you're basically much more likely to die from the "high availability" system then a very robust single system (and the usual proviso: evolution doesn't need you alive, it needs you to reproduce and the offspring have some decent chance of survival. Percentage survival rates are just fine.)

Same for the brain but I'd say writ large: easier to make an extra human then try and coordinate a redundant brain.

[Smaug123]

Those ones are much easier to answer: they require very good coordination between each other. "Split-brain" is literally a canonical failure mode in the study of distributed systems, for example. (We kind of do have two brains, right, linked by the corpus callosum; there's a reason they're so close together.)