`To me, the human body is the perfect machine," stated a medical student who once offered me his insights into human anatomy. This a common opinion but regrettably, it does not square with the facts.
From an engineering point of view, the design of the body is not particularly wellmatched to the performance we ask of it. This point is nicely made by S.J. Olshansky, B.A. Carnes and R.N. Butler in Scientific American, March 2001.
Our physical body design is the product of evolution, a process that works through the mechanism of natural selection. Variations arising in a biological population that confer a reproductive advantage on individuals are naturally selected for and these variations eventually spread throughout the population.
In species such as humans, the naturally selected variations must also favour a lifespan whose robust phase is sufficiently long to allow raising the young to the age of independence. A robust phase of 30 years would suffice.
The problem with our bodies is that once we go over the age of 30 or thereabouts, we start to wear out and things go wrong. Had we been selected to operate robustly for longer we would have fewer flaws to harry and debilitate us in later life.
Evolution through natural selection is limited as to what it can do. It introduces novelty and change by working with and modifying what is already there. For example, consider the upright gait in humans. We are descended from animals that walked on "all fours" and our change from the all-four body plan to the upright stance called for a significant modification of the backbone.
The lower vertebrae in the spine have enlarged in order to cope with the extra vertical pressure and the spine has curved to keep us from falling over. But despite modifications, the upright stance also introduced a new range of problems.
Early human-like ancestors benefited from a two-legged stance that allowed the use of tools and facilitated a quantum expansion of intelligence. However many debilitating disorders associated with ageing also stem from the upright stance. When we walk, we place great pressure on our feet, ankles, knees and back. Disks in the lower back experience pressures equivalent to several tons per square inch. This pressure takes its toll over a lifetime.
Other "design flaws" that militate against a long, robust lifespan include bone mineral loss after the age of 30, muscles that lose mass, a tendency towards varicose veins in the legs, joints that wear out, and a short rib-cage that does not fully protect most internal organs.
Bone strength depends on a hard mineral matrix based on calcium. After 30, we start to lose calcium from the bones, making them more susceptible to fracture. Women are several times more susceptible to losing calcium than men. Severe loss can produce osteoporosis, where the bones become very brittle and fragile.
As we use our joints continually over the years, joint lubricant grows thin, allowing the bones to grind against each other. This painful condition can be exacerbated by osteoarthritis.
Muscles tend to waste with ageing, which can interfere with all activities. Hernias can arise when the intestine pokes through weak spots in the abdominal muscle wall. Weakened abdominal muscles can also cause lower back pain. Veins in our legs enlarge with age and small valves can malfunction causing blood to pool. This can lead to swelling, pain and even dangerous blood clots.
How would we look if we were deliberately designed to withstand the long haul? We would be of shorter stature with a lower centre of gravity to help prevent falls and consequent fracture of bones. We would have extra ribs to better hold our organs in place. We would have thicker bones, and extra muscle and fat. Thicker bones would protect against fracture in falls and extra muscle would add weight on the bones, which protects against demineralisation. Extra muscle would also cushion falls.
We would have thicker discs in our vertebrae to better resist pressure, and a forward-tilting upper torso to relieve pressure on the vertebrae. We would have a curved neck with enlarged vertebrae to counterbalance the tilted torso and to keep the head up and face forward. We would have knees capable of bending backwards. In summary we would be shorter, thicker, forward leaning people - less pleasing aesthetically than our present configuration, but a lot longer-lasting.
Of course it is true that a healthy and vigorous lifestyle can slow the gradual deterioration of the body that begins in the 30s. Good nutrition and weight-bearing exercises protect against bone loss and muscle weakening. Vigorous aerobic exercise keeps the heart and circulatory system toned-up. But the machine will wear down eventually.
Despite what many people believe, ageing is not a disease that can be cured or reversed. We can help our bodies to last longer and better, but this does not negate the fact that our bodies are not designed for greatly extended use or perpetual health. The most ideal lifestyle will not prevent any of us from wearing out eventually. And remember Mark Twain's advice: "Be careful about reading health books. You may die of a misprint".
William Reville is a Senior Lecturer in Bio-chemistry and Director of Microscopy