COULD evolution have brought the Grim Reaper into being? Yes, indeed. Not in all his guises, of course – living things have always died because of mishaps such as starvation or injury. But there’s another sort of death in which cells – and perhaps, controversially, even whole organisms – choose annihilation because of the benefits it brings to some greater whole. In other words, death is an evolutionary strategy.
This is most obvious in the many varieties of programmed cell death or apoptosis, a self-destruct mechanism found in every multicellular organism. Your hand has five fingers because the cells that used to live between them died when you were an embryo. Embryos as tiny as 8 to 16 cells – just 3 or 4 cell divisions after the fertilised egg – depend on cell death: block apoptosis and development goes awry. Were it not for death, we would not even be born.
Even as adults we could not live without death. Without apoptosis we would all be overrun by cancer. Your cells are constantly racking up mutations that threaten to make your tightly controlled cell division run amok. But surveillance systems – such as the one involving the p53 protein, called the “guardian of the genome” (New Scientist, 18 December 2004, p 38) – detect almost all such errors and direct the affected cells to commit suicide.
Programmed cell death plays a central role in everyday life too. It ensures a constant turnover of cells in the gut lining and generates our skin’s protective outer layer of dead cells. When the immune system has finished wiping out an infection, the now-redundant white blood cells commit suicide in an orderly fashion to allow the inflammation to wind down. And plants use cell death as part of a scorched-earth defence against pathogens, walling off the infected area and then killing off all the cells within.
It is easy to see how an organism can benefit from sacrificing a few cells. But evolution may also have had a hand in shaping the death of whole organisms. The cells of all higher organisms begin to age, or senesce, after just a few dozen cell divisions, ultimately leading to the death of the organism itself. In part that is one more protection against uncontrolled growth. But one controversial theory suggests this is part of an inbuilt genetic ageing program that sets an upper limit on all our lifespans (New Scientist, 19 April 2004, p 26).
Most evolutionary biologists reject the idea of an innate “death program”. After all, they point out, animals die of old age in many different ways, not by one single route as apoptotic cells do. Instead, they view senescence as a sort of evolutionary junkyard: natural selection has little reason to get rid of flaws that appear late in life, since few individuals are lucky enough to make it to old age. But now that people routinely survive well past reproductive age, we suffer the invention evolution never meant us to find: death by old age.