Question: if someone can cope with one kidney,why do we have two?

Keith Siew answered on 17 Jun 2013:

Now that’s a question I’ve asked myself many a time and there is no straightforward answer.

The kidneys do lots of jobs in the body, but the primary one is filtering the blood. This is actually done by small units called nephrons of which there is anywhere between 800,000 to 1,500,000 per kidney. However as we age we lose nephrons, and the older we get the more we lose. But for the most part we don’t feel any of these effects because there is lots of “redundancy” (excess or spare nephrons) built into the system and we can actually lose up to 75% of our nephrons before seeing the signs or symptoms of disease. It makes sense that as the kidneys are exposed to toxins and likely to suffer damage over the course of our lives that they need these spare nephrons to cope. In terms of evolution if a hypothetical cousin of ours only had one kidney… if it was ever to get diseased or blocked or exposed to toxic damage they would die… so the “spare” kidney would save us but not them.

Although if you think about developmental biology and evolution, its more likely we’ll find our answer. When “advanced” organisms are growing as embryos they form a tube that goes down their midline to form the mouth and anus, eventually becoming the digestive tract. This effectively divides the body in half and the other organs then have to form off either/both sides of this.

But to be honest its still a question that we really don’t have a complete answer for!