Why Do Some Organs Come In Pairs?

We all have two of certain organs, like kidneys and lungs, but why not multiple hearts, livers or brains?

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About 550 million years ago, a flatworm called an acoel was the first organism to evolve with bilateral symmetry. Bilateralism is the tendency of some lifeforms to be symmetrical, and many organisms on Earth today, especially vertebrates, seem to have inherited this kind of symmetry.

If you were to classify animals by their symmetry, you would have asymmetrical (sponges), radial (starfish), and bilateral. If you drew a line down the middle of bilaterally symmetrical organisms (which would include most primates, birds, fish, crustaceans, amphibians) you'd see a pretty close mirror image. Although scientists don't know exactly why bilateral symmetry became so common, there are some hypotheses. Some speculate a bilateral body can move more easily because the appendages are mirrored, or perhaps they support a central nervous system with a central spinal cord. It's just one of those things we don't know. Since animals with bilateral symmetry have been evolving for 500 million years, it's safe to assume there's a good reason for it.

Evolutionary anthropologists guess that we have two limbs, two lungs and two kidneys because these dual organs gave our ancestral organisms some kind of advantage, not because they're spare parts We can see with one eye (and one-eyed organisms evolved first), but two eyes offer the advantage of depth perception. Because it's possible to survive with only one kidney, people tend to think of kidneys as spares, but a study in the journal Nephrology, Dialysis, Transplantation looked at how people did after getting one removed. Surprisingly, they found that they did better for around 16 years, but at around 24 years later, they showed a decline in renal function.

Don't forget: every Monday, we answer one of your questions. This week, someone named "Dern Sure" wanted to know why we have two of some organs but only one of others. If you have a burning science question, be sure to leave it in the comments section below, and it may end up being a future episode of DNews!

Learn More:
Bilaterial Symmetry (Natural History Museum)
"Humans have body plans that are bilaterally symmetrical. This means that if you draw a line down the centre of the human body, from the middle of the head to the feet, the same features appear on either side. We all have 2 eyes, one on either side of the body, and we also have 2 ears, 2 hands, 2 legs, 2 feet."

Why do you have two lungs but only one heart? (How Stuff Works)
"Your vital organs -- like your lungs, your heart, your pancreas, brain and liver -- are just that, vital. Not only are they vital to life, they are also vital to one another."

Why have one heart but two kidneys? (The Naked Scientists)
"It's not just us that are bilaterally symmetrical. All vertebrates are - be they birds, reptiles, frogs, or fish. In fact, not just vertebrates but almost all other animals are bilaterally symmetrical as well."