Our left and right sides are (roughly) mirror images of each other. But not on the inside. Credit: Wikipedia

Q: The outside shape of mammals is symmetrical: limbs, eyes, ears, and nostrils arranged on either side of a central axis. Why are the contents of the abdomen arranged asymmetrically? —Emy Amstein

Cecil replies:

Take a look at a car sometime, Emy. As seen from the sidewalk, nearly all the elements are laid out symmetrically, but pop the hood and it’s a free-for-all in there. And to an overwhelming degree, animal physiology has shaken out the same way. Natural selection doesn’t work from blueprints, of course. In effect, though, the operating principle for human and most other animal bodies seems to be that symmetry prevails where it’s useful, but no further.

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