Ageing, Sex and Death
Ageing remains one of the greatest mysteries of medical science. As the world's
population grows older than ever before, we need urgently to understand how and
why we age. We know all too well what ageing does to us - how it makes us frail,
more likely to fall ill and eventually to die. But what actually causes ageing
and what, if anything, can we do to slow its advance? And what are we to make of
the gloomy notion, first suggested by Aristotle, that ageing and death are the
price we pay for sex?
"The human brain is a masterpiece of evolution but it complicates our
lives, and nowhere is this more apparent than when we struggle with the big
questions of sex and death," Prof Tom Kirkwood.
Aristotle believed that each sex act had a direct life-shortening effect. While
this may be true for a drone bee mating for the first (and last) time with a
queen bee, or the Pacific salmon which spawns once and dies, is it true of
humans?
Curiously, ageing is linked to sex but not as Aristotle imagined. It appears
that the ultimate reason why we grow old is that during the course of biological
evolution, nature placed a higher premium on reproduction than on the long-term
maintenance of our bodies that might have allowed us to live forever.
Understanding this truth opens up new paths to investigating how genes affect
our length of life.
Prof Tom Kirkwood explains: "there is a profound division of labour between
two principal kinds of cells. On the one hand, there is the germ line - the egg-
or sperm-forming cells of the ovary or testis. These are the cells that, if we
have children, transmit our genes into the next generation. The rest of the
cells - those that make up the other organs of the body - Weismann termed the
soma."
According to Prof Kirkwood, the soma/germ-line distinction is very important for
ageing. The germ-line of cells cannot fail, it cannot permit cumulative damage
to its DNA, for if it did, humans would become extinct rather rapidly. The
germ-line simply cannot be allowed to fail in its duty of keeping going
indefinitely. Some change to germ line DNA does occur, for without this
evolution would be stalled. The damage is however small in comparison to that
which the soma accumulates.
On the other hand, the somatic cells have no corresponding requirement to
maintain their DNA indefinitely. "Life in the natural world is brutish and
short. All that the organism needs from its somatic cells is that they can keep
the soma in good enough shape until an age when the likelihood of still being
alive is negligible. When we factor in the consideration that maintenance and
repair of somatic cells does not come cheap, it makes sense to trim back the
maintenance of the somatic cells and divert the energy thereby saved into
helping with the all-important business of reproduction", according to
Kirkwood.
Kirkwood's controversial theory is that the soma becomes disposable and it is
this that leads to ageing and the gradual failure of the body. Testing this
theory on British aristocratic geanology dating back to the 8th century revealed
a striking pattern - "a predisposition to above-average longevity may be
linked to below-average fertility."
Longevity runs in families and research on the human genome is beginning to
uncover some of the genes that control the length of life. But the same research
shows that genes do not actually programme the ends of our lives - leaving great
scope for the choices we make about our lifestyle. These choices have a big
influence on the health and quality of our later lifestyle. As the global
revolution in longevity transforms societies around the world, new insights from
the world of science are re-affirming the individuality of ageing and rendering
negative stereotypes of older people as scientifically inaccurate as they are
unfair.
What about menopause, that feature unique to female humans? Is menopause linked
to the longer life span of females over males?
Has natural selection woken up to the fact that older women are valuable?
Are the sex hormones the culprits? What is it about testosterone and the Y
chromosome that shortens life by almost 10%?
Professor Tom Kirkwood will shed light on ageing, death and sex, menopause and
hormones in a lecture: "Ageing, Sex and Death" at the Sasol Scifest,
Africa's largest science festival, in Grahamstown, South Africa. For details of
the festival and a comprehensive listing of all other lectures, workshops and
events, visit http://www.scifest.org.za.
Professor Kirkwood is Professor of Medicine at the University of
Newcastle-upon-Tyne, where he heads the Department of Gerontology. Excerpts
taken from his BBC Reith Lectures, 2001.
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