Each human unfertilised egg contains one complete human DNA genome in its 23 chromosomes, and each human sperm cell contains a human genome, with an equal chance it will have an X sex chromosome or a Y sex chromosome. On fertilisation there are two genomes, one from each parent, but the females have two X chromsomes and the male has an X and a Y. When the embryo develops, each cell has these two DNA genomes, and this is true of every cell in our body, except for red blood cells. Some cells of the immune system have a slightly altered genome. Also, there is a small amount of DNA in the respiratory mirochondria, which are inherited from the mother.
Each genome has about 3 billion (3 x 109) bits of information, that is, the basic units of information abbreviated to A, T, G & C (see Glossary). The human genome project was the unravelling of the complete sequence of these units. Many people believe we will then know how many different genes we have, but I suspect this will only be partly true, because to count the genes we have a definition of what a gene is, and there may be more types of genes than are at present suspected. What most scientists are usually referring to when they say we will know the number of genes, are the genes that define the structures of all the proteins in the body. It is now known that there about 30,000 of these per genome, somewhat fewer than was originally suspected Although large, such a number only accounts for a fairly small proportion of the total genome. Many scientists believe that much of the rest is just "junk" DNA; others, including myself, think that at least a proportion of this DNA will turn out to have other functions, at present unknown, or poorly understood.
Two major facts are non-controversial, first, that the DNA contains all the instructions for making a human being, and second, genes specify the structure of all the different proteins in our body. Since each mammalian species has a maximum lifespan, preceded by a period of senescence, there must be genes that in some way control ageing and longevity. Some believe that ageing is genetically programmed, or even more strongly, that there must be a programme for ageing. This view is far too simplistic, and I have cited the ageing of teeth as a simple illustration. The size and structure of teeth is genetically determined, but teeth wear down or decay with use. Teeth are indeed programmed to last a lifetime, but if, hypothetically, they were not used they could last much longer than a lifetime. The whole architecture of the body is determined by genes, but this design, as we have seen, is fundamentally incompatible with indefinite survival. Also, the major maintenance mechanisms are all specified by genes. For example, the efficiency of DNA repair depends on the success of enzymes, whose structure is determined by genes, and there are at least 150 repair enzymes. For maintenance mechanisms in toto, several thousands of genes are involved. Ask any immunologist how many genes specify the whole of our immune system, and the number suggested will be a large one, probably more than a thousand Similarly, the numbers of detoxifying enzymes specified by genes is already known to be very large. All these genes, in one way or another, contribute to the determination of lifespan.
This is not to say that there are not important random events which damage molecules and cells and contribute to ageing. There may be rare types of damage in DNA which are not even recognised by repair enzymes, or there could be critical errors in molecules which have lasting effects. It is possible that certain deleterious errors in proteins precipitate further changes of the same type, which leads to progressively damaging effects, for example, in brain cells. We know that there is a strong random or chance element in the development of malignant cancer. One individual can be unlucky, and develop the disease unusually early, whereas others remain completely free of any tumour. Exposure to an infection early in life may trigger a strong immunity, which serves that individual well throughout life. Other individuals, without such protection, may succumb to the same infection when their immune system declines with age.
There are good reasons to believe that random events are an important contribution to ageing. When animals are inbred, the small differences between maternal and paternal sets of genes are ironed out. Normally this is harmful, as hidden or recessive deleterious genes can become expressed. However, scientists have been very successful in inbreeding several strains of mice. These mice have become genetically identical, as similar to each other, in fact, as identical twins in man. Groups of these animals have been used to study their lifespan. Each strain has a characteristic average lifespan as we might expect. What is remarkable, however, is that the lifespan of the animals of any one strain is quite variable (for example, from about 21 to 34 months for male animals of the fairly long-lived inbred CBA strain, leaving aside a few early deaths). It is remarkable because we know all the genes are the same, and the animals are kept in a very uniform environment with a constant and defined supply of food. By a process of elimination, we must conclude that intrinsic chance events are very important in determining lifespan. The same conclusion comes from the study of identical twins. Although their lifespans are more similar than normal sibs, these are commonly substantial differences, which in this case could, of course, be attributed to environmental effects.
In spite of the major conclusion that very large numbers of genes influence in one way or another an animals lifespan, it is commonly believed that there are a few critical "gerontogenes" which really determine lifespan. One reason is that gene mutations have been identified, for example, in a particular small roundworm or nematode, which considerably increase lifespan. Those who have done such experiments believe they have identified single genes that in some way control lifespan. One should be very cautious about such a conclusion, because there are several ways one or a few genes could increase longevity, which I mention below. Another reason that some believe there may be only a small number of genes that control ageing, come from experiments with the fruit fly Drosophila, an organism famous for revealing the secrets of heredity. It has been possible to increase the lifespan, simply by repeatedly selecting the eggs laid by the eldest mothers in a population. This experiment depends on the fact that the flies have two copies of every gene, and these copies are often slightly different from each other. In selecting eggs from the eldest mothers, the experimentalist is exploiting variability in these genes. He is in effect, seeking out those genes (or more strictly, the best of each pair of genes) which have an effect on lifespan. The ultimate result, after about 15 generations of selection, is a significant increase in the lifespan of the whole population. The conclusion is that there are individual genes that have an effect on lifespan, and possibly that the number of such genes is not large. All this is correct, but we need to look more carefully at the question of genes and ageing.
There are in man known heritable traits which result in premature ageing, but luckily they are very rare. One is known as progeria. Young children with this genetic defect are initially normal, but after a few years a series of abnormal features become apparent. These include short stature, loss of hair, skin changes which make the face look adultlike, and several abnormalities in the vascular system which normally cause death in the early teens. Another less severe defect is called Werner's syndrome. Again, the individuals initially appear normal, at least until the second or third decade of life. But then abnormalities set is, including whitening of the hair, progressive skin changes, poor wound-healing, cataracts, diabetes, increased risk of cancer and heart disease. The average lifespan is 46 years. It has been known for a long time, that Werner's syndrome is due to a single recessive inherited defect, that is, the defective gene must be inherited from both parents. Many scientists working on this gene believed that if its function could be identified, a great deal would be learned about the process of ageing. Some even believed it might be a gerontogene which would be the key to understanding ageing. The gene has now been isolated and shown to be like one determining the structure of a protein important for unwinding the double helix of DNA, known as a helicase. How can a single defective gene like this have multiple and diverse effects on the individual? One simple explanation is to think of the defective gene product as a "spanner in the works". Any complex machine can grind to a halt if there is a single defective component. Or there may be a single essential component which does not last as long as it should, so the whole machine's lifespan is reduced. With regard to Werner's syndrome, the likelihood is that the defective helicase is one of several helicases. Initially these are able to fulfill all normal functions, but as time goes on DNA repair and maintenance is not quite all it should be, so a wide range of body features become progressively affected. So, unfortunately, knowledge of the defective gene causing Werner's syndrome has not provided as much insight into the cause of ageing as might have been hoped, because there are multiple causes of ageing and multiple genes involved.
Before the human genome was sequenced, a large number of human genes had already been identified from the study of human genetics. Human populations are very large and our medical screening programmes are very efficient. A very wide variety of medical defects were known to be inherited from the study of family pedigrees, and a high proportion of these defects are due to mutations in single genes. In many cases, the defect at the biochemical level has been identified. Some years ago, the pathologist and gerontologist, George Martin, went through the whole catalogue of human genetic defects, and listed all those that had some connection with ageing or age-associated disease. The proportion of these genes out of the total number known at that time was surprisingly large, and now more and more are being identified, many being necessary for the maintenance mechanisms discussed in Chapter 3. Thus, the conclusion is that many genes, in one way or another, have something to do with the changes that occur during ageing. This supports the view that ageing is multicausal or multifactorial, and is contrary to the notion that there are just a few "critical genes" which determine lifespan.
What about the genes in worms or flies which increase longevity? At first sight, they seem to play a critical role in the ageing process, because changing one gene can increase lifespan by as much as 50%. This is very much like taking a complex machine, changing one component, and then finding the machine lasts twice as long as before. With regard to organisms, all sorts of possibilities exist. For instance, we know that reducing calorie intake extends lifespan in rodents. It is fairly easy to imagine a mutation which simply reduces the efficiency of digestion, thereby decreasing the intake of calories, and thereby having the long term effect of increasing lifespan. Alternatively, we can easily envisage genes which reduce fertility, and allow more resources to be diverted to maintenance. There could be genes that slow down metabolic rate, reduce the production of ROS, and as a consequence there is less damage inflicted on proteins and DNA. All these possible changes, and probably many others, are important and are relevant to ageing. However, from a biological point of view, they are in a sense experimental artifacts, because one could predict that the mutants produced in a laboratory environment, would not be advantageous in a natural one. In all cases, the mutants would probably be selected against and be eliminated because they were less fertile, less active or mobile, or whatever. Of course, much the same could be said about the artificial selection of animals. Many domestic animals, selected for particular desirable traits, would do badly in a natural environment in competition with the "wild type" animal from which it was derived. In fact, it is known that domesticated animals introduced into a natural environment (such as the pigs introduced by Captain Cook into New Zealand) soon change by natural selection back to the original form of the species. Feral cats in Australia have often become much larger than the domestic variety.
The genes identified by scientists which affect the longevity of experimental animals probably have less to do with the genetic control of ageing than they would like to think. Nevertheless, during evolution genes must have been selected which affect longevity, because we know in different instances that the longevity of certain animals decreased during their evolution, whereas that of others increased (as we shall see in the next chapter). There are two ways these changes could occur. First, we know that there is intrinsic variability in any animal population, affecting a wide variety of animal features. We know that genes inherited from the maternal parent are often slightly different form that those inherited from the paternal parent (They have slightly different DNA sequences). Artificial selection for almost any feature of the animal acts on this variability and over a few generations significant changes are seen. For example, selecting for larger or smaller size in dogs will produce very significant changes in weight and dimensions. The experiment previously mentioned, in which the eldest egg-laying female fruit flies were repeatedly selected, depends on the intrinsic variability in genes. The selection acts on new combinations of genes, to produce in a few generations flies with significantly increased lifespan. Human populations have a similar intrinsic variability affecting lifespan. It is well known that some families tend to be long-lived and others shorter lived. It is joked that if one wants to have a long lifespan, one should choose long-lived parents and grandparents. There are a few individuals who have lived to 115-120 years of age, and it is probable that these, by chance, have combinations of genes which delay senescence and ageing. They exist in human populations, because such populations are extremely large, so only a minute proportion of individuals have these combinations of genes. (I refer here to populations where the birth date of the very elderly individual is properly documented. There are of course innumerable claims of extreme age, where there is no authentic documentation of birth). The second type of variability which can affect a given feature of an animal depends on new mutations in genes. Thus, the huge differences in size between different breeds of dogs, from the Great Dane to the Chiwawa, is due both to new gene mutations, as well as intrinsic variability, both of which are selected for artificially. We can confidently expect that there are mutations which will increase lifespan, as well as those that reduce it. The ways these new mutations operate is clearly a very complex problem, especially as I have stressed that ageing has multiple causes. A mutation which reduced the rate of collagen cross-linking would have little or no effect on the ageing of the brain, the eye and so on. However, evolution takes place over a very long period of time and it is not impossible to envisage genetic changes, each of which have a very small effect, adding up to a very significant increase, or decrease, in the overall rate of ageing. Moreover, there could be mutations which have multiple effects on cells and tissues of the body. For example, a mutation which reduced the production of ROS might have multiple effects. An improved defence against ROS would have a similar outcome. We can easily envisage changes in the efficiency of DNA repair. Suppose, for example, a gene producing a DNA repair enzyme is duplicated. This could be followed by the evolutionary divergence of the two genes to produce repair enzymes with related but overlapping functions. We know that DNA repair is more efficient in human cells than in mouse cells. This could be due to the greater efficiency of a particular pathway of repair, or more likely, it could be due to more alternative pathways. The duplication of particular functions is well known in evolution; if one fails for any reason, there is very commonly a back-up mechanism. With regard to the many maintenance mechanisms previously discussed, the prediction would be that long-lived species have many more back-up mechanisms than short-lived ones.
Another type of mutation may also be very important in evolution. We know that there is some relationship between the rate of development of an animal to its maximum lifespan. In general, long-lived species become sexually mature more slowly than short-lived ones. Therefore, if a mutation acts to slow down the rate of development it is likely to also increase the lifespan as well. The mutation acts to stretch out the whole life cycle between generations, including the period of gestation, growth from infant to adult, period of active reproduction, and the final stage of senescence and death. The domestic cat and lion are quite closely related, but all these stages happen much more quickly in the domestic cat than the lion. For cats and lions the actual values are respectively: gestation period, 65 days and about 110 days; time to reproduction, 10-12 months and 3-4 years; inter litter interval 6 months and 1.5-2 years; longevity 15 years and 30 years. These are all quantitative changes which could certainly be caused by a fairly small number of mutations. It is even said that the mew of a cat when recorded and slowed down, sounds much like the roar of a lion, which is again another fairly simple quantitative change.
There is a special evolutionary mechanism, known in several zoological contexts, where the adult of an evolved species resembles the not-yet adult of form of an earlier species. It is known as neoteny, and it depends on the later species becoming sexually precocious in a body which, anatomically speaking, is not yet fully mature. A strong case can be made for the occurrence of neoteny in the evolution of man, because anatomically our adult bodies resemble more strongly the infants of great apes than adult forms. Mutations which have quantitative effects on development of the body and the development of sexual maturity would be responsible for neoteny, and it does not need much imagination to envisage an effect on lifespan as well. The novelist Aldous Huxley exploited this theme in his novel After Many a Summer. A particular diet induced great longevity in two individuals, but unfortunately these individuals also lost their normal human characteristics and developed ape-like features when they became very old.
There is an argument put forward by the gerontologist Richard Cutler that longevity evolved very rapidly during the evolution of man from higher primates. Curiously, Cutler took the maximum longevity of human to be 120 years, and that of the great apes as 60 years. Now it is true that gorillas, chimpanzees and orangutans live for up to 60 years in the well protected environment of a zoo. However, the number of animals which have actually been kept from early age for such a long period in any zoo is quite small, probably less than a dozen. In contrast, the number of recorded ages of humans in countries with proper records is vast. Amongst these are a few who have lived for 120 years. Now, imagine a dozen human individuals selected at random and kept in zoo-like conditions. How long would they live? My guess is that it would be close to the average human expectation of life in Westernised societies, say 70-80 years. So the increase in lifespan during human evolution is not a doubling, as Cutler assumes, but an increase of about 30%. Such a change occurred over a period of about five million years (the time the chimpanzee and human lineages diverged), perhaps around 200,000 generations. This is quite a long time for natural selection to act, first on intrinsic genetic variability, second on new mutations affecting maintenance, or having quantitative effects on the human life-cycle. The reasons why human beings have such a long lifespan is an interesting and important one which will be outlined in the following chapter.
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When over eighty years of age, the poet Bryant said that he had added more than ten years to his life by taking a simple exercise while dressing in the morning. Those who knew Bryant and the facts of his life never doubted the truth of this statement.