A study of women in the 18th and 19th centuries reveals that the longer a woman lived past the age of 50, the more grandchildren she had. Is this why women live decades after they lose their fertility?
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Nature is devoid of sentiments. In the famous words of Herbert Spencer, it is a question of survival of the fittest. You are born, if you are lucky you reproduce, then you die. In fact, when you can no longer reproduce you are evolutionarily dead. This, at least, is the common position. It is therefore not clear why women, who normally lose their fertility in their 40s, with the cessation of menstruation, often live many more years. The life expectancy of women in almost all societies exceeds that of men, but most men remain fertile into old age.
The supposed explanation relies on a more complex understanding of the concept of "competence". It is not enough to survive and produce offspring. Your lineage should continue through your grandchildren. In this context, the survival of women beyond the age of menopause makes sense if it translates into the birth of more grandchildren. However, this assumption is difficult to prove. But Mirka Ladanfra, of the University of Turku in Finland, and her colleagues believe they have succeeded in doing so. Their work, published in the journal Nature, relies on records from the 18th and 19th centuries in Finland and Canada. The results are impressive. In both countries, women gained two more grandchildren for every decade they lived beyond the age of 50.
The survival of humans beyond the age of childbearing - and especially of women - is indeed unusual. Among many species, a kind of infertile helpers work, helping the individuals that reproduce. But usually these are young animals, and not members of previous generations. The adults who are no longer fertile simply die. Chimpanzees, for example, have a fertility pattern similar to that of humans. The female chimpanzee's fertility peaks in her late 20s and more or less disappears by her mid-40s. However, in chimpanzees the mortality rate increases as fertility decreases.
Despite the increase in life expectancy in the rich world in recent centuries, life extending beyond childbearing age is not a modern phenomenon. Until recently, most of the increase in life expectancy was attributed to better survival in childhood and adolescence, and not to the extension of middle age into old age. Even in pre-industrial societies about a third of women were over the age of 45.
All these data indicate that the prolonged old age, especially of women, is indeed a developmental phenomenon and not the result of improved living conditions. And the quality of the records used by Dr. Ladanfra allowed her to examine in detail how this happens.
First, the possibility was ruled out that women who lived longer had more offspring, and therefore, depending on the number of children, also more grandchildren. Nor was any visible effect of the socio-economic status of the aforementioned women discovered. Rich or poor, raised from the people or not - the same trend was observed in all of them. And contrary to what some field researchers believed, both the women's sons and daughters benefited from their long lives, and not just the daughters.
The increase in life expectancy was not caused by one reason. Dr. Landenfra managed to isolate a range of beneficial effects, the combination of which produced an increase in the number of grandchildren. People whose mothers were still alive both produced more offspring and raised a greater percentage of those offspring to adulthood. They also had their first child at a younger age than people whose mothers had died, and their children were born less apart. All these things contributed to greater productivity.
Moreover, the physical presence of the mother-grandmother was essential. Children who lived more than 20 kilometers from their mothers had significantly fewer offspring than those who lived in the same village as their mothers. The finding indicates that the cause of the increase in the number of grandchildren was not genetic, but actual help or advice given by grandmothers.
However, the most important finding from an evolutionary point of view is the age at which the study subjects died. The life expectancy of the Finnish women who lived past the age of menopause was 68. In Canada it was 74. These ages correspond to the age at which the children of the mothers-grandmothers themselves stopped having children. At this point, the grandmothers' competence ends and this is also the limit of their life span.
They know evolution - the rise of man
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