The most memorable and the most important sights that most of us experience from birth are those of human faces. They identify us and form the basis of first impressions; they are a source of art and social significance in many cultures.
But, there is a tantalising mystery behind the appearance: our faces and our bodies are strikingly symmetrical. Whereas inanimate objects rarely display perfect symmetry, living things almost always possess external right-left symmetry. On the face of it..... this might seem an improbable state of affairs. After all, it requires delicate engineering. By contrast, symmetry is absent in the top-down direction because bodies are adapted to deal with the variation of gravity and weight, with height, and with the need to remain stable under the influence of small perturbations that would otherwise cause them to fall over.
It is rare to find back-front symmetry in animals because it is 'cheaper' to engineer the ability to turn around. Bilateral symmetry is very advantageous for movement - the imbalances caused by any bilateral asymmetry make straight-line motion tricky to engineer and the benefits of symmetry are even greater if motion has to occur off the ground, in air or water.
The classic representation of our human symmetry is displayed in Leonardo's famous drawing of the 'Vitruvian Man', which has been artistically reinterpreted and reproduced on countless occasions, most recently on all Italian 1-euro coins.
Many of our superficial evaluations on human beauty, or attractiveness, focus upon the symmetries of the face and body. The human face displays a very high degree of symmetry, and the evolutionary importance of recognising creatures with faces in crowded fields of vision, between trees and boulders of foliage, has resulted in our sensitivity to lateral symmetry. It is a good rough and ready guide to distinguishing things that are alive from things that are not.
Our bodies however, the bits under the skin, are a squalid muddle. Our hearts are on the left, our brains are laid out in an asymmetrical fashion that reflects the type of cognitive activity being performed. If symmetry persisted under the skin, our vital organs would have to duplicate to maintain it . The brain would use resources in a wasteful way.
Our deep-laid sensitivity to human faces is an evolutionary inheritance that aids our survival and multiplication. The sensitivity fo symmetry that we have inherited from these tendencies shows up in all sorts of other places - in the decorations we like to use in our homes, in the types of maths we like to study (or not), and the sorts of scientific theories that we like to create.... All are residues of the time when we first knew we had faces.
Faces have been a pretty useful too, lets face it! :)
Just for a laugh - listen to a classic 90's tune!
http://www.youtube.com/watch?v=MfDCWD3lhsg
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