#TestTube: Why Do Some Organs Come In Pairs? [VIDEO]

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

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.

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.

*Written by  Jack J. Shamama for #TestTube