The Fascinating Ability of Humans to Walk Upright

We stand upright-after a little practice-on a ship that rolls because we possess an array of sensory nerve cells buried in our muscles, skin, and joints. The function of these sensors is to provide a constant flow of information to the brain about the movements and location in space of the various parts of our bodies, as well as the environmental forces currently acting on them. We also have a pair of balance organs associated with our ears which work like spirit-levels, each having a bubble moving in a fluid medium to record any change in the position of the head; and we have our eyes to scan the horizon and tell us how we stand in relation to it. All this flow of information is processed by the brain, usually at an unconscious level, and is immediately compared with our consciously intended stance at the time. If we have decided to stand level in spite of the ship's motion, perhaps to look at the receding harbour through binoculars, this chosen posture is the reference point used by the brain to compare with departures from it caused by the rolling of the vessel. Thus our sense organs continually inform the brain about our stance, and counter-instructions pass constantly from the brain down the motor nerves to the muscles. As we tip from the vertical, the push-and-pull of these muscles changes, so as continuously to maintain the upright position.

This process of comparing wish with actuality, of sensing error and then correcting it by the precise application of an opposing force enables us to stand erect. Walking or balancing on one leg is more difficult and takes longer to learn; riding a bicycle is even trickier, but this also can become second nature through the same active control process which keeps us upright.

James Lovelock, Gaia: A New Look at Life on Earth (2000-11-23)

• biology
• brain
• evolution
• homo sapiens
• wonder