Why do we grow old until death?

There are different animals have short life, the life of Drosophila melanogaster only a few weeks of life but, turtle up to hundred years. Plants, many plants are a year old, in a growing season is complete growth, knot, death of life history, but also many plants can live for a thousand years, and every year to knot. Growth in the United States California pallidiflora pine (Pinus aristata) is a plant of longevity, calculated from its rings, life has more than 4500 years, but showed no signs of aging.

Aging of diversity, is also part of the biological diversity. Darwin first pointed out that natural selection is the primary mechanism for biological diversity. Biological evolution is a fact that, since biological variation of individual existence, no two individuals are identical, their body size, disease resistance, running speed, vision or weak, and so there is a difference. If an individual has some traits suited to survival in the environment, it will be better than other individuals to live longer, leaving future generations with more. On behalf of these characters hereditary gene would continue, and over time, gradually spread. Longevity means that there can be multiple birth, may leave more offspring. But longevity also means that it takes more energy to maintain one's own body and does not necessarily better than those who though short-lived, but put all their energy into the fruited species have an advantage. The main purpose of the organism is reproduction, but their survival and reproduction of high or low depending on its environment. If a suitable environment, a species of life is too short, natural selection will let them get longer. On the contrary, in a harsh environment, longevity, will be eliminated. Therefore it is not difficult to understand why there will be different aging mode. But fundamentally speaking, decrepitude is an undesirable trait, it reduces the ability of living organisms to survive, but also reduces their ability to reproduce. So why natural selection does not take this kind of out of bad behavior, but in the outdoors so common?

The origin of aging is how and why we are aging until death? this is the date of the birth of mankind, to haunt us. There are also various national myth, legend has it that attempt to explain. Only to the 19th century, began to have scientists try to make a scientific explanation. Some say this is the law of nature, sickness, irresistible. This is a problem that does not give the real cause. But this "natural law" does not generally established. Those through cell division of bio-clonal propagation, for example, sea anemones, although the will because of illness or accident and death, but they can not naturally die of old age. If you listened carefully, they can always resented the youth. Die but sexual reproduction of biological characteristics. Some people say you can avoid aging death becomes more crowded world. This is a teleology, to accept it, we need to believe that the world has a mysterious forces do this ingenious arrangements. Death makes the world a less crowded, this is the death of additional significance, but not the cause of death. Some people say that aging is a genetic program. Indeed we have now seen and aging-related genes, researchers also often said that "being programmed cell death". But it was demonstrating how aging, does not tell us how the origin of aging, but also that those "aging gene" will come from.

Obviously, we need to find out the evolutionary factors of aging that will ultimately answer the question. A way of saying that the natural death is the result of natural selection. 19 century Germany great biologists Vaisman just holding this view. He believes that longevity is an unreasonable luxury, the body has been overwhelmed by life suffering individuals continue to live on their species not only useless but also harmful, because they are in and the younger generation for life resources. Old age and death frees up for future generations to survive, save a life resources, contribute to the survival of future generations, while natural selection results, not just leave more offspring? but this specious argument is contrary to the principle of natural selection. A group of older generation's death, it is beneficial to the survival of future generations as a whole; however, the death of a single individual, but does not directly benefit the survival of its descendants. Natural selection is not possible for the Group's long-term interests at the expense of the interests of individual and direct. Natural selection can only be on the individual and their descendants have direct advantage to work, and aging in relation to the individual and their descendants, and there is no advantage.

United Kingdom great biologists Holstein Board (J.B.S.Haldane) in the 1940s first pointed out that the old age and death is not the result of natural selection, on the contrary, is a natural selection has no effect. Another United Kingdom biologist m Dawa (Peter Medawar) in 1952, further elaborates on this point. Natural selection is done by different individuals have different reproductive capacity and performance, therefore, those individuals lose the ability to reproduce works of before, it was natural genes. A lethal gene in adolescence is expressed, the patient is likely to leave future generations before death, so that gene will be eliminated by natural selection, genetic, diffusion is difficult. On the contrary, those hidden, and so the only expression of fatal in young gene, it's carrier in diseased offspring before leaving, this gene is cannot be eliminated by natural selection, genetic persist indefinitely. Natural selection will enable to survival, reproduction of the gene expression of time getting pushed. Over time, will have more and more only in old time expressed in fatal genes in groups, non-cumulative. In each person's body is more or less with this deadly genes, therefore, also inevitably aging, death. The asexual creature, they always retain their reproductive capacity, natural selection has been in effect, forcing them to remain healthy, and therefore do not grow old.

1957, United States biologist George · Williams (George Williams) on aging theory made important complement. He noted that many genes have a multi-effect

Sex in the organism of different developmental stages may have a different role in pointing out those late in life, harmful gene in the life of early and not always to remain silent, but they were able to play a useful role. In other words, those early in life develop normal function of the gene, late in life, even if no mutation, also can cause injury. This gene is favored by natural selection, because natural selection focus more on the benefits of early in life, and the disregard for the life of late emerging harm. This hypothesis and subsequent discovery of molecular genetics. Many causes cancer in old age, of genes and regulation of cell growth, differentiation, in early development, play an important role.

Obviously, a biological group, the number of elderly individuals and young is inversely proportional to the individual mortality rates. If young individuals rarely as natural enemies, harsh climate, food shortages, infectious disease, death, so there will be many individuals can enter into old age. On the other hand, if many young individuals died due to a few individuals could live to old age. Therefore, if the cause of aging is due to natural selection cannot clean the bad genes responsible for aging processes, if the higher mortality of young individuals, bad genes will be no opportunity to say that natural selection, will in future generations in piling up. We can predict that those living in the harsh environment of the group, will generally aging sooner, faster. United States University of Idaho's Stephen · Ostrov Dade (Steven Austad) on the Virginia opossum of subtly verify this prediction. Virginia opossum lifespan for a very short period of mammals, a study of mammals aging of useful tools. The opossum lifespan most less than two years. In its second year of life, all of the Virginia opossum has signs of aging, such as weight reduction, eyeball appear cataract, female genital atrophy, fecundity declines. Osvaldo Dade is the life to the Georgia coast of Virginia opossum. There's Island and the Mainland is distributed with Possum group, however, life in the island of little Possum natural enemies, and on the Mainland is very easy to become a variety of natural food. Therefore, the continent of opossum face greater pressure to survive, according to our forecast, they are also aging faster, shorter life expectancy.

Osborne, found that the Mainland opossum is indeed life is short, but this is not due to aging? in order to answer this question, Paranhos da de determination the from field in different age groups of Possum tail extracted the collagen fiber strength. Collagen cross-linking between each other to the fiber. With the years, more and more cross-linked collagen, collagen protein composition of organizations lose elasticity. Paranhos da de the collagen fibers in urea solution, and then pull the collagen fibers. Urea is able to break down the collagen fibers of cross-linked Collagen fibers, reducing the mechanical strength, until they can't afford to pull and breakage. Therefore, the number of cross-linked collagen, the more they can bear in urea solution provides longer it. Paranhos da de discovered not only the number of collagen cross-linking with Possum age increases, and continental opossum than Island opossum earlier this aging signal indicating that their aging faster.

United Kingdom biologist Hamilton (w.d.hamilton) in the 1960s for aging theory mathematical model has been established. Since the 1970s, this model has been validated by many experiments. This old theory predicts that if we let sexual reproduction biology of reproduction period ahead, then after several generations, their life will be shortened. United States University of Illinois at Chicago's David · Metz (David Mertz) with bread, insect inspection of this forecast. Bread of life for about a year, in which about 270 days of adulthood can reproduce. Metz on every generation of bread, simply select those in the adult after 10 days before the egg production. His hypothesis is that these earlier egg producing offspring than advanced descendants of egg production are more natural selection pressure will tend to lay their eggs much earlier. After 12 generations later, bread from early fecundity increased about 10%, for the first month of spawning from 417 to 460, but their life is reduced accordingly, male life expectancy fell from 271 days to 231 days, female of life from 228 days down to 207 days.

In turn, this theory predicts that if delaying sexual reproduction of biological breeding period, then, after several generations back can extend its life. There are several laboratories to Drosophila materials check this forecast, the results are similar. Drosophila grow to about two weeks to begin to lay eggs. In one experiment, researchers will these young fruit production of eggs for hatching all thrown away, only those who are elderly Drosophila (at least six weeks old) of eggs. This ten generations later, the life of the fruit fly Drosophila melanogaster is common for two or three times, and they are more full of youthful vitality. They are more resistant to hunger, the more resistance to dry, walking ability and flight ability is also stronger.

We can imagine that if we conduct similar to humans in Drosophila, forcing females, and their descendants illness without medical technology rescue and let it out of natural selection, then the 10 generations later, human nature will significantly extend the life span. Of course we could not have done this brutal experiments. To prolong life, we can find another way. Since aging is due to natural selection does not work, accumulated too many harmful gene results, we can find ways to eliminate these harmful gene?

For thousands of years, mankind has passed the artificial selection of solutions according to our need to develop numerous animal and plant species. Similar artificial selection and natural selection, but also a removal bad genes, the process of accumulation of good genes, gene is good or bad, is recognized by the who. In the beginning of the 20th century, when just establish genetics, many people also thought to be a human artificial selection, in Western countries, a "eugenics" movement. The campaign since then Nazi Germany as a genocide Holocaust excuses, and notorious. But still there are many

People think that using humane methods for engaging in "eugenics" is fine. So we have no possible through eugenics to enable mankind to become healthier?

Eugenics is a way to bring the "bad" gene of human birth control or sterilization, thus eliminating the "worst" gene. However, aging and not by the individual's "bad" gene, genetic factors responsible for aging processes in all those who are present, it is not possible in this way will extinguish. And we said before, some of the gene responsible for aging processes in the life of early necessary functionality with normal, less likely to be cleaned. Eugenics in another solution is to bring the "good" people who have more genes that increase the "good" gene frequency in the crowd. Some people are very long life, this may and they carry a "anti-ageing" longevity genes. But the special long life people represent only a very small human scale, to improve their reproductive rate, to a crowd of almost longevity gene frequency will not be affected. Moreover, it is difficult to predict whether someone will live longer, until a person as is famous for its long life, he has been a growth period, no way to keep him more offspring.

Molecular Genetics of development as we conquer the aging brings new hope. When the human genome project to a close, some biologists even claimed that in 20 years humanity passes through the Centre for genetic engineering and live forever. Indeed, we have developed and are testing a variety of genetic engineering techniques, such as gene therapy has been a certain degree of success in a gene defect, we can bring to the body has a normal function of the gene to make up for it. Perhaps one day, we can also use similar gene therapy approach, in due time replacement of aging that lead off the gene. But according to our aging theory described above, aging is long-term accumulation of many lethal gene expression results. These genes may have hundreds or thousands, and the human body, the vast majority of the biochemical function and expression at different times. We want to conquer the aging, it is first necessary to understand all the mechanisms of aging, which means that you want to find all of these come with aging-related genes, and to understand their role. This is an extremely complex and difficult work. Even if you find a variety of aging-related genes with, or is not in fact have the means and ability to displace it. This is a non-optimistic hope.