The majority of animals on Earth, humans included, are bilaterally symmetrical. It means we can be divided roughly into two mirror-image sides.
Evolutionary biologists believe that it has been like that for at least 300 million years, and because life organised this way survived, so did symmetrical design. Hence, two eyes, two ears, two lungs and two kidneys.
You could argue that even some of our solo organs are really two squished together. The brain has two hemispheres, for example, and the heart is effectively two pumps.
Why did this design endure? Functional advantage. Having two lungs, for example, makes for more efficient gas exchange. It also allows for simple redundancy – you can get by on one kidney if necessary.
In the case of those organs that really are on their own (such as the gastrointestinal tract), they may have originated earlier in evolutionary history and again survived for one simple reason: they worked.
This article is an answer to the question (asked by Simon Bartlett, email) 'Why do we have two of some organs, but only one of others?
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