Spare organs like kidneys, eyes, and ears: Why does the body come with built-in backups?

The human body is equipped with a remarkable level of redundancy, with many organs having more capacity than we actually need to survive. 

This built-in backup system means that if one part fails, another often steps in to carry the load — a marvel of nature often overlooked.

A recent article published by Harvard Health highlights how evolutionary advantages have led to this natural excess, allowing many people to live without certain organs or parts of the body without significantly compromising their health.

The most cited example is the kidneys — humans are born with two, but most people can function perfectly well with just one. This is why kidney donations from living donors are possible, even though the remaining kidney must work harder, and the risk of future failure slightly increases.

But the kidneys are just the beginning.

Built-in reserves: More than just kidneys

The article points out that other parts of the body also have surprising levels of redundancy. 

For example:

Eyes: While having two eyes helps with depth perception and a wider field of vision, a person can live in good health with just one. Even total blindness doesn't directly threaten physical health, although it can affect quality of life and has been linked in studies to an increased risk of Alzheimer's disease.

Ears: Similar to the eyes, having two ears helps locate the direction of sound. However, losing hearing in one or both ears does not immediately harm physical health. Still, studies have shown hearing loss may also be connected to cognitive decline.

Intestines: A large portion of both the small and large intestines can be removed without major health consequences. 

"The entire colon can be removed [an operation called pancolectomy] without shortening a person's life," the article explains. Though digestive symptoms like diarrhoea may follow, the body adapts over time.

Lungs: It's possible to live with just one lung, often the case after surgical removal due to tumours or infections. The remaining lung compensates for the loss.

Liver: One of the most regenerative organs in the human body, the liver can continue to function even if a large part of it is removed, provided the remaining section is healthy.

Why do we have this reserve in the first place?

The answer, scientists suggest, lies in evolution. People whose genes allowed for more resilient, durable organs likely survived longer and had more offspring, passing on those advantageous traits.

"Early humans with a genetic makeup that produced organs with functional space to spare were better able to survive, thrive, and reproduce than others without such a genetic makeup," Harvard Health noted. Over the generations, these traits became more common.

Are some organs expendable?

Technically, yes — at least when it comes to basic survival. It is possible to live without a spleen, a lung, a kidney, or even large parts of the intestines or liver. But that doesn't mean those organs are useless. Removing them often affects a person's quality of life or puts additional stress on remaining organs.

Bottom line

Nature's extra padding has given humanity a second chance in many life-threatening situations. It has made organ transplants possible and allowed people to live long, healthy lives after surgeries or illness.

So, even if some organs may not be essential for survival, their presence — and their backup functionality — is a reminder of just how intricately and wisely the human body is built.