Ken Livingstone turfed Trafalgar Square last week to draw Londoners’ attention to the green open spaces all around them. The Mayor’s message was reinforced by pictures in the papers and on television of Nelson peering over people sitting in deckchairs and sprawling on the grass, an admirable use of an open space, but not its only use. Equally pleasant, but physically more valuable, is to use our urban parks, as well as the countryside, for walking.

Human beings didn’t evolve their powerful legs and hips to sit at a chair before a computer or television all day. Our ancestors achieved their supremacy in the animal world by developing a supremely cunning intelligence, a memory and the ability to think and plan ahead. However, their brain would have been useless if it hadn’t been accompanied by legs that enabled them to escape a sabre-toothed tiger, and arms that allowed them to clamber up the nearest tree when attacked.

For our ancestors exercise wasn’t an optional extra. If they wanted to survive they had to exercise to eat. Now if we opt to have a sedentary life, we defy nature and our lives may be cut short by cardiovascular disease, the metabolic syndrome, diabetes, obesity and an increased risk of cancer.

Not all exercise improves people’s chances of survival, and some inappropriate exercise increases the likelihood of injury or disease and reduces life expectancy. Exercise shouldn’t be violent unless a person can be certain that he or she can carry it out or take equivalent exercise five days a week.

There are three candidates for the accolade of being the ideal exercise. This must be capable of being carried out daily, or as near daily as is possible. Brisk walking, bicycling or swimming all satisfy this requirement and all are better forms of exercise than tennis, squash, golf, jogging or working out in the gym.

Brisk walking, as the apes discovered when they hopped down from their trees before there were playing fields and tennis courts, is the ideal activity. It exercises muscles and joints without causing arthritis, expands lungs, improves metabolism, keeps heart muscles toned and prevents atherosclerosis, the furring-up of arteries.

Where do I walk briskly? In London I park my car far enough away from the office to provide the statutory 20-minute walk twice a day. My ideal walking for pleasure is at weekends on the paths through the saltings that are hidden behind the hamlet of Burnham Norton on the north Norfolk coast. Here there are sea and marsh birds rather than the urban roses of Regent’s Park.