Why do humans have extra kidneys, eyes, ears, and other organs?

The abundance of surplus capacity found in the human body is one of its many underrated qualities. In other words, the majority of us do not require the whole reserve of our organs.

It's as though our bodies were built with the possibility that we may require backups in the event of disease or trauma. And presto—if all goes according to plan, we have two kidneys at birth instead of just one!

The kidneys are hardly the only example, of course. Why, then, are we naturally redundant in our design? Which physical part(s) can be removed or fail safely without compromising your health?

Why is there such a reserve of organs in us?

The most plausible explanation is evolution: early humans who had organs with extra functional space due to their genetic composition were more likely to survive, grow, and procreate than those who did not. As a result, genes associated with increased organ capacity—remember, you have two kidneys, not one—were more likely to pass down to succeeding generations.

Conversely, less resilient evolutionary forebears might not have lived long enough to procreate and thus passed on their genes less successfully. Because of the strength of natural selection, modern humans have organs that have had plenty of reserve throughout thousands of years.

Liver, lungs, eyes, and more
The following is only a short sampling of bodily components that have plenty of reserve:

Eyes: One eye can be quite healthy, but you won't be able to enjoy the greater field of vision and depth perception that come with having two. Although blindness can undoubtedly present difficulties and have an impact on one's quality of life, losing both eyes does not necessarily translate into ill health. Furthermore, research indicates that a substantial loss of vision may increase the likelihood of developing Alzheimer's disease.

 
Ears: Losing hearing in one or both ears does not instantly affect general health, even though having two ears enables us to find noises from all directions. However, hearing loss can also lower quality of life, much like vision loss might. Similar to eyesight loss, new research indicates that those who have trouble hearing may also be more vulnerable to cognitive issues.

 
Gut: It is possible to remove a sizable piece of the small and large intestines without significantly affecting your health. In reality, a pancolectomy—the removal of the entire colon—can be performed without reducing a patient's life expectancy, while additional problems such as diarrhea might occur afterwards. While removing a portion of the colon is a fairly routine procedure (for colon cancer, for example), it does not in and of itself cause health problems or limit life expectancy.

 
Kidneys: The majority of people can survive just fine on one kidney. For this reason, kidney donations can be made to those in need. The danger of further kidney failure does rise, though, and the remaining kidney must work harder. Kidney failure can also occur earlier than typical in cases where the surviving kidney is damaged, infected, or suffering from another illness.

 
Lungs: A lung may be removed because of a tumor, but it is also sometimes done because of infection or emphysema. If necessary, a full lung can be removed, and you can rely on the other lung to function rather well.

 
Liver: Due to the liver's capacity for regeneration and the abundance of "reserve" liver tissue, a sizable piece of the liver can be removed (provided the remainder of the liver is healthy).

Does this imply that a large number of body components are actually replaceable?
Perhaps. Many body components could be considered expendable if your main concern is your own survival. You could live without your eyes, hearing, kidneys, lung, spleen, and a large portion of your liver, among other organs.

But it's obvious that there are other things to take into account besides survival, namely quality life. For that reason, no one would advocate for the donation of even the least functional organs.

In summary
It is good that our organs have such a high degree of redundancy because millions of people around the world rely on them for their basic lives. In order to support others in maintaining their quality of life and health, living organ donors may also donate a kidney or a piece of another organ.

Therefore, even though certain parts aren't strictly necessary, it's still nice to know that there is a large reserve. You never know when it could be useful.

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