Following on from Judit Al's question Longevity of women compared to men Nir asks what is the evolutionary logic in living long after the period of fertility is over?
The answers to this question concern our very uniqueness as humans and the way in which evolution shaped these characteristics. Every multicellular animal and man in general is programmed to die. Repairing DNA damage and maintaining the body are expensive tasks and many creatures have chosen to devote their resources to rapid reproduction and having multiple children: the genes are conserved and duplicated efficiently when they are not, they bother too much with the maintenance of the temporary body which may be eaten at any moment anyway but direct the animal to transfer them as quickly as possible to a large number As much as possible of fresh bodies.
A small number of animals chose a different strategy: building a large body that requires a lot of investment from the parents and a large brain that is only useful after a long period of study. Such a lifestyle does not allow for a large number of births and the investment is worthwhile only if the body lasts long enough to return the investment. Man has taken this strategy to an extreme: he lives the longest of all warm-blooded land animals and his brain is the largest and most wasteful. The difference is not only quantitative but also qualitative: the human female is the only one that lives long after the cessation of fertility. Chimpanzees in the wild live up to the age of 45-50 and the female only has a few years of old age after the ovulation cycles have stopped. Also in other long-lived mammals, the female is fertile until old age: elephants may conceive and give birth in their seventh decade, and with whales, pregnancies of females over 80 years old have been reported.
There is apparently no evolutionary logic in the preservation of body cells after the reproductive system has ceased to function: long old age means that an animal has wasted resources on the preservation of body cells that have no value in the distribution of genes. Therefore, an explanation is required for the strange human disconnection between the reproductive system, which ages at roughly the same rate as other monkeys, and the body that has added two or three more decades of barrenness to itself. A fascinating theory attributes human longevity to grandmothers. The human child requires a greater and longer investment than any other puppy: monitoring of hunter-gatherer tribes revealed that it is only around the age of 18 that the young man produces food equal in caloric value to what he consumes: by then he has burdened his family, which is required to provide him with about 14 million calories. For comparison, the young chimpanzee reaches this point of balance already at the age of 5. Between the ages of 5 and 11, humans go through a developmental phase that has no parallel in the life of any other animal: body growth slows down (this is the happiest time when you can let the child wear the sweater from last year) and what develops is mainly the brain. During this period of childhood we acquire physical and social skills and abilities that are the basis of human culture. Of course, someone has to pay the bill - to provide the child with food and shelter for many years. The investment in a long childhood pays off of course: while the adult chimpanzee manages to obtain at most 400-500 calories more than he needs at the peak of his life, in the hunter-gatherer tribes a skilled hunter manages to bring about 3000 excess calories to the camp every day and he persists in doing so for about three to four decades. A significant part of this success is the ability of human hunters to communicate: to design and jointly produce tools and techniques using the language acquired and perfected in long childhood. Perhaps it is no coincidence that the creature closest to man in terms of social intelligence and communication ability: the dolphin is also the only animal in which grandmothers have been observed actively involved in the care of young people. Apparently the long childhood was supposed to reduce the fertility of the human female, but strangely it turns out that women in hunting and gathering societies produce more offspring than chimpanzee females. Mysteriously, we extended the child's period of dependence 3-4 times compared to our ape ancestors and at the same time we shortened the time between births so that in the same period of time between the last menstruation (a characteristic that probably remained unchanged in evolution) the human female (until the last generations) gave birth more times than a female the chimpanzee. The grandmother theory explains this paradox. Observations of the Hadza tribe in Tanzania revealed that the main burden of caring for the child falls on the grandmother: the main source of plant food is roots and tubers that require great strength, coordination and great skill to find, extract and use. The children, therefore, lack the ability to obtain food at all and the grandmothers look after the children who have already been weaned and provide them with food while the fathers are engaged in hunting and the mothers are taking care of the babies. At the time when the first members of our species (Homo) evolved about two million years ago, the climate in the plains where they lived became more arid and juicy fruits became a rare commodity - plants adapt to dryness by protecting the fruits with hard skins and storing the starch in underground roots and tubers. Man is forced to rely on rich and concentrated food but hard to obtain (similar to the ecology in which the Hadza live). Those who lived longer and maintained physical and mental ability to help her family raise her children gained an important evolutionary advantage: more grandchildren. Among the species that are evolutionarily close to us, the mother is the main caretaker of the offspring, but there is diversity in the degree of support she receives from the members of the band around her. Transferring part of the burden to family members carries great advantages but requires the mother's trust in the babysitter whose interest and accordingly the degree of his commitment to the job may be difficult to predict. A grandmother is a rational choice because the interest of preserving her genes justifies investing in a grandchild, especially when she has no small children of her own as competitors. The advantage of long-lived women was twofold: they increased the grandchild's chance of reaching adulthood and allowed the mother to shorten the time until the birth of a young brother or sister.
In hunter-gatherer societies today, it is found that women whose mothers are alive manage to bring more children to adulthood. Of course, there were those who claimed that in societies where life expectancy was shorter than 30-40 years, it was impossible to rely on grandparents, but the statistics, as usual, lie. The low life expectancy in primitive societies is mainly due to high mortality of babies and children and not of adults. A woman who had the privilege of celebrating her 45th birthday and the natural end of her menstrual cycles can look forward to another 20 years of life, which means a lot of babysitting shifts for grandchildren. When you examine not the total life expectancy but the chances of mortality in each age group, it becomes clear that low mortality between the ages of 10 and 60 is a typical "fingerprint" of Homo sapiens and appears in every human society.
The records of births and deaths in the church books in the villages of Europe make it possible to examine the importance of grandmothers in traditional societies until the 18th and 19th centuries - before kindergarten and pediatric medicine. It turns out that with European farmers, just like with hunter-gatherers in Africa or South America, it would be good for a child to have a grandmother around. More than that: it turns out that the benefit of the grandmother is clear when it comes to a mother's mother - the motivation is higher to help raise a child who is undoubtedly your descendant. When you chart the life course of several generations of women, an interesting and unique picture emerges for the human race: the age at which menstruation stops roughly corresponds to the age at which girls begin to become fertile, and the increase in the chances of mortality around the age of 70 roughly corresponds to the age at which the last grandchildren are born and the girls reach the threshold of their own grandmothers. It seems as if evolution took care of maintaining physical and mental health long enough for the woman to be an effective grandmother but not a great grandmother. In men, the decrease in physical fitness makes them less effective hunters with age and at the age of 60 in hunting and gathering tribes, he again reaches the balance point where he provides food only for himself and later he is a burden for his family members. Natural selection had a stronger motivation, therefore, to invest in mechanisms for repairing damage and maintaining the female body.
As expected, the grandmother theory has alternatives: some argue that the duration of fertility in human females is similar to that of related species and the life period that follows is simply a by-product of extending the general lifespan, and there are those who claim that the advantage of this life period was the possibility of caring for the young offspring and not the grandchildren. Other critics claim that the grandmother theory underestimates the father's contribution to raising the children and requires that the woman stays with her relatives after finding a partner. Despite the reservations, both anthropological and historical observations and models that try to calculate the contribution of various factors to success in gene inheritance tend to support the idea that the grandmother has a significant role in our evolutionary success. The crucial role of the ancestral grandmothers in satisfying the child's needs even when the father is around is explained by some researchers by the fact that the value of the father's investment in the child decreases as his certainty of paternity decreases so that the maternal grandmother is the closest figure who is sure of her genetic proximity to the offspring.
The beauty of the grandmother theory is that it offers one explanation for the 2 unique characteristics of a person: childhood and old age. Everything we created, discovered and invented originated in childhood and those who allow us to be children are grandmothers. Perhaps it was precisely the period of life that earned the insulting nickname "procrastination" that enabled the birth of human civilization.
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More of the topic in Hayadan:
2 תגובות
Cute theory. But even with the elephants, the grandmothers are involved in raising the grandchildren...
I am 48 years old and this week I lost my grandmother.
Indeed, as in the article, she helped my family grow. and the education of the children.
I won.