We all age. As a child, we love to age and can’t wait to finally become an adult. But when you are in your thirties, growing older is already associated with negative feelings: decline, diseases, loneliness, death. They are phantoms we prefer to put away in dark closets.
Often, we try to counteract the signs of ageing with all kinds of artificial interventions. But in the end, there is no escape. We will age. The years just accumulate. This is a process of physical, psychological and social changes.
Defining people as ‘old’ at the age of 65 no longer fits real-world data. People live longer, healthier lives around the world. Life expectancy of human beings has risen quite a bit in the past decennia.
For thousands of years, our average life expectancy was about forty years. Thanks to better nourishment and advanced medical care, people nowadays live considerably longer. In most developed countries, life expectancy has reached eighty years at the moment and the rise hasn’t come to a stop yet.
But there are also exceptions. In the southern part of Africa for instance, life expectancy has decreased due to the rise of AIDS. And also in Russia, life has become less healthy after the fall of the Soviet Union. For men, life expectancy at the moment does not reach sixty years of age there. The United States and Japan on the contrary, count the largest numbers of centenarians among their inhabitants. Researchers expect half of the baby’s born there since the year 2000 to reach the age of ninety and ten percent will get to one hundred.
The situation is different when you talk about life span. Every organism has a maximum life span. This maximum is more or less fixed. For human beings, the maximum life span is around 125 years of age.
But we are well overtaken by the Galapagos tortoise that can reach 190 years of age, a type of shark and a type of whale that can become 200 years of age, and a type of Japanese fish that amply surpasses those 200 years. A bivalve mollusc tops the list with a maximum life span of 405 to 410 years. How do we know that? By counting the yearly growth lines on their valves, just like in trees.
Mice on the other end of the spectrum only reach the age of four, dogs will not live into their thirties, and cats aren’t going to pass the age of forty. Trees seem to be able to grow for thousands of years and nobody knows whether they then die because of old age.
One organism certainly is not dying of old age: the jellyfish turritopsis nutricula. After this jellyfish has reached maturity and has procreated, it returns to its youthful form. This cycle can go on endlessly. Theoretically this makes this jellyfish immortal, unless it succumbs to a disease or is eaten by a predator, which is almost always the case.
What does it mean, ageing, in the biological sense and why does the maximum life span differ so much for various life forms? The jury is still out on this one. Ageing is about biological changes that gradually take place within an organism after reaching sexual maturity. In the end, ageing leads to the death of an organism although ageing is not a cause of death. For death to occur, always a more direct cause exists, in the form of some disease.
In general, genetic as well as environmental factors contribute to the ageing process. People, who live under extreme circumstances, grow old sooner. This has to do with among other things the fact that their cells suffer a lot.
One of the theories on ageing is about free radicals. Radicals are unstable oxygen molecules that take part in important biological processes. They come into being during normal metabolism in cells. Radicals contain an unpaired electron, which reacts with electrons from neighbouring molecules. This is called oxidation and damages the molecule involved. Free radicals can thus damage DNA, proteins, and cell membranes.
Fortunately, our bodies possess a safety system in the form of antioxidants. In normal situations, these antioxidants are capable of stabilising the radicals and thus render them harmless. Sometimes the safety system fails and it cannot prevent damage caused by radicals from happening.
Radicals are involved in a very extensive range of diseases, among which Parkinson disease, schizophrenia, bipolar disorder, cardiovascular disease, chronic fatigue syndrome, and Alzheimer disease. And they probably also play a part in the ageing process, when the antioxidants’ line of defence is no longer a match for the continuous stream of radicals.
Specialists in evolution theory come up with a more general explanation for the ageing process. According to them, the speed with which different organisms grow older has to do with their genetic make-up. Every organism may have been fitted with a kind of survival program that functions well until the moment an individual becomes superfluous. That moment of superfluity is mainly defined by the maximum age at which an organism is still able to procreate. To state it more clearly: the various body parts will last just long enough to see its children mature.
Genes involved in damage to the body before or during the time of reproduction will automatically become extinct through natural selection. Individuals, who carry these damaging genes, will die young and won’t have the time to reproduce. Thus they won’t pass on the genes concerned to their offspring. Individuals, who do not carry the damaging genes, do procreate. In the course of time, no carriers of the damaging genes will be left.
The situation is very different for the time after the period of reproduction. Genes that could set in motion a chain of decline and disease won’t be eliminated by natural selection. The carriers of these damaging genes have reproduced already and will have passed on the genes to their offspring.
It is also possible these genes that are damaging at a later age may have had useful tasks earlier in life. This could be an explanation for the fact that diseases continue to exist in the course of evolution. According to this theory, ageing is not an inevitable quality of living, but the result of genetic programming.
Various hundreds of genes have already been pinpointed as being involved in the ageing process, but how this involvement goes about is still unclear. Ageing actually is a simultaneous deterioration of various body systems. Genes that play a part in the functioning of each of these systems, carry part of the responsibility for the ageing process.
Something that really is of great importance is the maximum number of times a cell can split itself to form two new copies. Cells are constantly being copied. While copying this large package of information something can easily go wrong. To prevent damage due to the copying process, every DNA molecule is fitted with an irrelevant, extra part at the beginning and the end of the molecule, called telomere.
With every new copy a small piece of telomere is left off. When all the telomere has gone, the cell cannot divide anymore. This almost always leads to the death of the cell. So, the shrinking telomere makes cells grow older.
It appears the length of the telomere determines the maximum life span of an organism. In humans, telomeres have such a length they can easily survive 75 to 90 years. Human cells can divide between forty and sixty times before they have run out of telomere. Cells of mice can be copied about fifteen times, but the cells of the Galapagos tortoise at least ninety times.
The length of the telomeres appears to vary a little between persons. If your parents had long telomeres, you will inherit long telomeres as well. But it certainly does not mean this fact guarantees that you will reach about the same age as your parents did, as was shown in a Swedish investigation.
Environmental factors have a far bigger influence on telomere length. When cells are damaged faster than usual and have to be replaced, more cell divisions take place in a shorter period and telomere runs out quicker. For example, chronic stress or a large quantity of free radicals influences telomere length.
People’s fear of ageing is often greater than necessary. Ageing isn’t synonymous with general decline. This only happens when something is wrong, when some kind of pathological process is going on. Through a healthy lifestyle it is possible to avoid or delay the onset of age-related diseases.
How you think about your age may actually affect how you age. For ten years, researchers in the US followed about five hundred people aged 55 and older. It turned out subjective age has a stronger effect on cognitive abilities than chronological age. Elderly people who felt old experienced more decline. The ones who kept a sense of being younger, showed greater confidence about their cognitive abilities and maintained these. This is good news, as research in Germany showed people over 70 years of age on average tend to feel about thirteen years younger than their chronological age.
A sheep named Dolly
Telomere length may also have to do with the premature death of the famous Dolly, a cloned sheep. Cloning is an artificial reproduction method meant to create identical offspring, a clone. Dolly was the first cloned mammal. She was born in 1996 in Scotland.
To create her, DNA material of one sheep was inserted into an egg cell from which the nucleus was removed, of another sheep. The fertilised egg was placed into the womb of a third sheep and she gave birth to Dolly. Genetically speaking, Dolly had only one parent. She was an identical descendant of the sheep of which the DNA had been taken.
Dolly died in 2003 at the age of six. Normally, sheep of the species Dolly belonged to will live to be eleven or twelve years old. An investigation after Dolly’s death revealed her telomeres were far shorter than those of other sheep of six years old. This may have been caused by the fact that when the cloning took place the telomeres involved were shorter than normal even then. The sheep of which the DNA had been used was six years of age and its telomeres already had been shortened because of previous cell divisions.
Dolly’s creator, Ian Wilmut, contradicts this explanation and maintains Dolly died of a lung disease which is very common in sheep. Of course it could also be that the sheep succumbed to the lung disease, because her cells had aged already and couldn’t handle the disease anymore.
Numbers © mik38 – Fotolia.com
Free radicals © crystaleyestudio – Fotolia.com