WHY DO WE AGE?

Is it because after producing offspring our bodies are disposable? Or is ageing driven by the ‘overexpression’ of our biological systems?

THEORY 1: THE DISPOSABLE SOMA THEORY

A Greenland shark can live for more than 400 years and appears to remain physically fit and fertile to the end.

However, the average global human life expectancy in 2010-15 was 71½ years (68 years and four months for males and 72 years and eight months for females) according to figures from the United Nations. In 1977, English biologist Tom Kirkwood published his “disposable soma theory of ageing”.

In a nutshell, it goes: for an organism living in the natural world with all its hazards, the biological imperative is that it survives long enough to reproduce and nurture its offspring to independence. Maintenance of cells as they ceaselessly divide is energy intensive and natural selection is concerned only with the survival of the species, not the individual. Hence, says Kirkwood, our body (or soma) is “disposable”.

It ages gradually as a result of a lack of investment in maintenance machinery. Yet organisms with less risk of death by environmental causes, like the Greenland shark, can put energy into cell regeneration for longer.

THEORY 2: THE HYPERFUNCTION THEORY

What if, instead of endorsing the theory that ageing is caused by accumulation of damage and the failure of maintenance systems, the data is pointing to the exact opposite – that ageing is driven not by failure of systems, but by their overexpression? In other words, by natural processes of living that simply run on too long?

This is what is called the ‘ hyperfunction theory’ of ageing, and it was first suggested in 2008 by biologist and cancer specialist Mikhail Blagosklonny.

At its heart is the idea that ageing is driven by the action of normal genes (known as ‘ wild-type’ genes), not mutants – the same genes that drive our development from egg to adult, and throughout reproduction.

These genes continue to work beyond the development programme for which they were selected, but in the post- reproduction period of life they are under ever-weakening evolutionary control and their no longer appropriate activity eventually leads, in time, to disease and death.

Many of the classic pathologies of old age involve runaway growth or over- enlargement of cells rather than decline and decay.

Excess growth is a common cause also of the prostate problems that plague many older men as the tube leading from the bladder gets squeezed.

Even the thinning bones of osteoporosis can be caused by hyperactivity of the cells responsible for breaking down bone in the process of sculpting our skeleton, the osteoclasts, which get out of kilter with the bone-forming cells, the osteoblasts.