Humans might learn to stay alive for 150 years, but living beyond that seems impossible despite medical advances
AVERAGE human life expectancy in the West has increased from 50 years in 1900 to 79 years today and the corresponding world average figure has increased from 30 years in 1900 to 69 years today. Human life expectancy continues to steadily increase but there is good reason to think that it will plateau out and that it will never be possible to prolong human lifespan indefinitely. This whole area is elegantly explained by Thomas Kirkwood in September in Scientific American.
When you consider the spectrum of biological species, humans are relatively long-lived, but many species live longer than humans. The figures in brackets in the following examples are the maximum recorded life spans (in years) in the wild for various species: mayfly (1 day), mouse (4), rabbit (13), dog (29), cat (36), chimp (59), horse (62), turtle (80), human (122), lobster (170), whale (211), bristlecone pine (1,000s), hydra (immortal), bacterium (immortal).
Average human lifespan was low in 1900 mainly because infant mortality was high, mass-vaccination was in its infancy and antibiotics were not yet used in medicine. The bio-medical advances made throughout the 20th century were critically important to extending lifespan, but improvements in psychological well-being resulting from enhanced feelings of control over one’s life also had a positive, if unquantifiable, effect. Genetics and environment interact to determine lifespan, but, in practice, we eventually die because our bodies wear out.
A bacterial cell, in principle, can live forever in a congenial environment (sufficient food, temperate temperature, etc). The cell simply grows, divides into two daughters each of whom grows and divides in turn, and so on and on. A bacterium is a single-celled organism – its body function and its reproduction function co-exist in the same cell.
Humans, along with other mortal species, have sacrificed bodily immortality for complexity. We have segregated our reproductive cells (germ cells) from the cells that make our body bulk (the soma). Although we can, in a sense, consider our germ cells to be potentially immortal, our body cells (somatic cells) wear out and die.
The somatic cells make up the various bodily organs and tissues (muscle, heart, kidney, brain, etc) and the germ cells are sperm (in males) and eggs (in females). At conception a sperm cell fuses with an egg cell to produce a new cell, the zygote. This zygote divides in two and this process continues on to eventually produce a baby. When you die, your somatic cells die, but your germ cell genes live on in each of your children. In that sense, germ cells can be considered immortal.
Our cells are controlled by our hereditary material – DNA. This DNA is naturally susceptible to being damaged but it is so important to the organism and to its descendants that mechanisms to repair damaged DNA have evolved. Maintaining DNA in pristine condition in the reproductive cell lines that produce sperm and eggs is of such critical importance that elaborate and energetically expensive repair mechanisms are active in the otherwise relatively quiescent germ cell lines.
On the other hand, it is neither necessary nor feasible to expend huge energies in maintaining the complex and hard-working somatic cells in such pristine condition that they can achieve immortality. It is simply necessary to keep the soma working long enough to allow successful procreation and nurturing of offspring. Because somatic cells do not have to remain in pristine condition it has been possible to evolve them to do highly complex jobs, eg brain cells, jobs they perform for only a limited time before succumbing to the wear and tear that is the unavoidable destiny of every machine composed of matter.
Lifespans continue to lengthen. It is estimated that more than half the UK babies born in 2000 will live to see 100 years of age. Research is shedding much light on the natural mechanisms of wear and tear in our cells that cause ageing and debilitation. Interventions to slow ageing down will inevitably be developed and, combined with new stem-cell therapies, it may eventually be possible to nudge average human lifespans up towards 150 years. But it seems extremely unlikely that it will ever be possible to prolong life indefinitely – the human body is too complex.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC – understandingscience.ucc.ie