In the past few hundred years, the average life expectancy of human beings has been steadily extended in modern society, but the highest life expectancy has not. Hundreds of years ago, some people lived to be 1 15 years old, and today this record has not been broken. All medical miracles and advances in public health facilities have failed to raise this limit. If old age is a disease, it seems incurable.
Technically, we point out that the problem of old age is not the increase of age from birth, but the deterioration and deterioration of physical condition in old age. Aging is not an isolated process, but the susceptibility to various diseases increases gradually, and the ability to repair injuries decreases as a whole.
Aging has damaged our health so badly, so why didn't natural selection wash it away? This possibility is abnormal, because aging is an inevitable part of our experience.
In addition, our bodies have strong maintenance ability: skin and blood cells need to be updated once every few weeks, and teeth need to be replaced once in a lifetime-but why not replace them six times in a lifetime like elephants? Damaged liver tissue may be renewed quickly, and most wounds can heal quickly. The fracture can be healed again, and the lost small pieces of skin, bone and liver tissue can be supplemented; But some tissues, such as the heart and brain, cannot regenerate. Some creatures, such as lizards, begin to grow new ones as soon as their tails are cut off. Our bodies also have some ability to repair damage and replace missing parts, but this ability is limited. The body cannot sustain itself indefinitely. Why not?
For most of us, one day after the age of 45, we suddenly found that we can't read the words on the book unless we straighten our hands with the book. At this time, I have lost a lot of hair or have a lot of white hair and wrinkles on my face, but these changes are easier to deny than the difficulty of holding a book with my hand straight. 50th birthday parties are often unpleasant. Some people who enthusiastically recommend mineral water will mention some sensitive issues, such as memory loss, facial flushing, impotence and so on. We know too well what it is, but few people know that the aging process begins very early. Aging does not begin at 40 or 50, but a gradual change, which begins as early as shortly after puberty. If you are an athlete, you don't have to be 40 to get past the age of best performance. Adults' early activity ability is the best, and then it decreases at a gradually accelerated rate. This decline is a sign of aging. It is true that many people run very fast at 40, but not as fast as he does at 30. That is to say, whether chasing elk or avoiding tigers is unfavorable, it is a relatively unfavorable factor.
Aging is the first mysterious thing in evolution. Any explanation must involve the phenomenon we have just discussed.
Some people think that aging must be good for species. In order to leave living space for the new generation, aging is necessary, so that evolution can maintain the adaptability of species to ecological changes. This view has fallen behind the position of weismann, a Darwinist in the19th century. He wrote in 188 1: "Eliminating some individuals is not only worthless to the species, but even harmful, because they replace better individuals. In this way, through natural selection, our hypothetical immortal individuals will decline because they are replaced by many useless individuals in this species. "
After learning that natural selection is not for the benefit of the race, but usually only for the benefit of the individual, he gave up this wrong assumption. So he needs another explanation. When he knew that the collaborators of the University of Michigan's "Evolution and Human Behavior Project" had long sought the evolutionary explanation of aging, they smiled and asked why they didn't know the article on aging written by biologist george williams at 1957.
Williams' article profoundly expounds why natural selection proposed by biologists Haldane and Medava leaves genes that lead to aging. As early as 1942, Haldane made it clear that genes that have harmful effects after the highest reproductive age will not be eliminated by natural selection. This is a great progress, but it doesn't explain why reproduction should stop. By 1946, Medawa further explained that the power of natural selection gradually weakened in the later period of life, because many individuals had died for reasons other than aging.
It is easy to imagine that genetic selection will give priority to young animals, which is definitely unfavorable compared with their older animals, and it is also unfavorable for these young animals when they grow up. One or more genes that promote aging will spread to the population under many conditions that can be explained by numbers. One simple reason is that it prefers younger animals. As a group, it has made relatively great contributions to the ancestors of future groups.
Williams extended these ideas to the nutrition theory of aging genes. If a gene has more than one function, it is multifunctional. Suppose there is a gene that can change calcium metabolism, promote calcium absorption and deposition, and make fracture heal faster; But the same gene will slowly precipitate calcium and deposit it on the arterial wall. Such a gene is probably left by natural selection. Because many people benefit from it when they are young, only a few people can live long enough to understand its shortcomings of causing arterial diseases in old age. Even if this gene causes everyone to die at the age of 100, it will spread even if it is less beneficial to youth. This theory is untenable because of aging. Many other causes of death-accidents, pneumonia, etc. -are enough to greatly reduce the number of elderly people. This theory is also different from Haldane's theory of stopping dependence on reproduction.
Recently, anthropologists Hill and Rogers (Kim Hill &; AlanRogers) challenged this explanation of menopause. However, this hypothesis is an example, and the choice of relatives may explain the seemingly useless biological characteristics.
Not all genes that can lead to aging are necessarily beneficial in the early stage. The existence of some aging genes simply means that there is no chance to be eliminated by natural selection. Because only a few people can live to the age when this gene has adverse consequences. AlexoCmfort is a famous biologist in some interdisciplinary fields. He has two classic textbooks: Biology of Aging and Sexual Pleasure. If Comforte is right, aging is hardly the cause of wildlife death. He observed that old and weak animals are rarely seen in nature, so he concluded that aging is not one of the causes of wildlife death. But don't forget the marathon record mentioned above. If older animals just run a little slower, they may be caught by predators more easily than younger competitors, so they will age and die long before the obvious signs of aging that we can see appear.
Smart readers should look at examples of genes that are beneficial in the early stage and lead to aging in the later stage. It is known that there are multiple genes, and one gene has multiple functions. For example, the gene that causes phenylketonuria can make hair lighter and mental development stagnate. Now we are interested in whether there is a gene that is beneficial to young people and has a role that will pay the price in old age. In an article from 65438 to 0988, Dr. albin of the University of Michigan listed several diseases caused by this gene. One is siderophore cirrhosis, which is characterized by excessive iron absorption and death in mid-adulthood. The cause of death was liver iron deposition, which destroyed the liver. In the early stage of life, attracting too much iron can prevent patients from iron deficiency anemia, a beneficial feature that exceeds the subsequent loss. Dr albin noticed that the frequency of this gene was as high as 10% in the population, and there was another explanation for heterozygosity. The survival of this gene may also be the result of gender antagonistic selection: it is beneficial to women, who need iron to supplement menstrual loss, but it is harmful to middle-aged men, who just accumulate too much iron.
In another example, Dr. albin noticed that some people have a gene that can lead to the overproduction of a substance called pepsinogen I ... These people are more likely to develop and die of gastric ulcers when they are older. However, in life, because these people have more stomach acid, it is possible to provide additional protection to prevent infection. So far, as far as we know, no one has tested albin's statement to see if more pepsinogen I can fight gastrointestinal infections, such as tuberculosis and cholera.
PailTurke, an evolutionary anthropologist and expert in aging research, became a Darwinian doctor of medicine after graduating from medical school. He reminded us that the whole immune system is biased by age. The immune system releases destructive chemicals to protect us from infection, which will inevitably damage tissues and eventually lead to aging and cancer.
Genes that become prerequisites for Alzheimer's Harmo's disease (also known as Alzheimer's Harmo's disease) may also be selected for their early interests. This is one of the most common causes of devastating intellectual impairment, which affects about 5% people at the age of 60 and 20% people at the age of 80. It has long been considered as a disease affected by genetic factors, because many familial venereal diseases and people with three copies of chromosome 2 1 have the highest frequency. 1993, Duke University psychiatrists found that a gene on chromosome 19 produced E4 apolipoprotein, which is especially common in patients with Alzheimer's disease. The probability that the heterozygote of this gene will suffer from this disease at the age of 80 is 40%. As far as we know, no one has discovered the benefits of genes that later developed Alzheimer's disease in early life. Laborde of the National Institute on Aging proposed a related explanation. He noted that Alzheimer's disease is characterized by abnormalities in the last region of brain evolution, which does not exist in other primates. This leads him to mention that the gene that caused the rapid increase of human brain volume in four million years may cause Alzheimer's disease in some people, or have side effects that are not mediated by other genetic changes. It will be a very important discovery if we can confirm that those people with higher intelligence or larger brains have the necessary genes for Alzheimer's disease.
There is considerable laboratory evidence that genes with early benefits can lead to aging. RobertSokal, a group biologist, keeps a group of houseflies (one of the most common kitchen pests) to breed the earliest batch in its life cycle. After 40 generations, the selected houseflies did produce more offspring in the early stage, but they also aged quickly and died quickly. It is possible to choose genes that are beneficial in the early stage of life and unfavorable in the later stage. Biologists Ross and Charles Weiss (Michael ross &; BrainCharlesworth) experimented from another angle to cultivate fruit flies that reproduce in the late life cycle. This fruit fly not only reproduces more in the later life, but also has a longer life span, but also has fewer offspring. As a result of this experiment, the expected artificial selection eliminated the genes that were profitable in the early stage but needed to pay the later cost.
More and more evidence shows that these genes are the cause of the aging of wild animals. For many years, gerontologists have accepted Comforte's erroneous conclusion that wild animals will not age. In their observation of the expected results, a typical mistake is that these scientists who study wildlife groups did not even think about the age difference between the mortality rates of old animals and young animals. They assume that the death rate will remain the same throughout their lives. Now, gerontologists begin to analyze all kinds of observation data. For many biological species, aging reduces the success rate of reproduction more strongly than all other options combined. Although this does not prove the role of pluripotent genes in aging, it certainly challenges the theory that natural selection has no chance to eliminate aging genes.