Grandmother hypothesis

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Neva Morris (August 3, 1895 - April 6, 2010), aged 110; she lived more years after menopause than before, measured against the average

The grandmother hypothesis is a hypothesis from biological anthropology . She tries to explain why in the course of evolution in women the menopause has evolved. They also tried to explain why women after age-related infertility, many can achieve years of life , while this is not the case with most other female mammals, including among the great apes .

Evolutionary background

According to the evolutionary theory of Darwin and Wallace , those characteristics of a species are retained which give the characteristic-bearing individuals the greatest reproductive success. In the course of the selection process for most mammals, it has become established that the female population members are fertile until shortly before the end of their life expectancy , because their higher reproductive rate in relation to less fertile conspecifics has proven to be advantageous in the selection process. In this context, evolutionary biologists ask why, in the course of hominization , infertility developed so early in relation to maximum life expectancy in women, because according to the principles of selection, they would have to deal with conspecifics who may have developed no or later menopause , be at a disadvantage due to the lower reproductive rate.

The menopause as an adaptation

The fitness in the selection process depends not only on the number of newborns, but especially by their probability of survival from. George C. Williams brought the explanation into play that older women who are no longer interested in their own reproduction can have a positive effect on the survival rate of their grandchildren and that menopause is thus an adaptation of today's people. According to Williams, it is therefore more advantageous to avoid the expense of further reproduction and instead use your skills to support the offspring of the next generation but one. In the course of the selection process, according to this hypothesis, statistically those individuals who had a helping, long-lived grandmother are favored. Thus, according to the grandmother hypothesis, the characteristic of early infertility with a simultaneously high life expectancy of women could prevail. This is also the result of stochastic modeling , which was created in 2014 by Kristen Hawkes ( University of Utah ).

In relation to other mammals, humans are also highly dependent on supply from adults. The later a woman becomes a mother, the higher the risk of dying an age-related death before the child is able to self-care, which would greatly reduce the chances of their own offspring surviving. Accordingly, the characteristics of those individuals are favored who, in addition to giving birth to offspring, can also ensure their long-term survival and the survival of their offspring.

Subsequent births increase the risk of genome transmission errors in the egg cell during cell division . This also increases the likelihood of stillbirths, miscarriages and hereditary diseases. This connection is cited as further evidence for the plausibility of the grandmother hypothesis.

Empirical studies on the grandmother hypothesis

Several studies of historical data confirm a link between a woman's longevity and the number of grandchildren surviving. An examination of church registers in the Krummhörn landscape (western East Frisia ) from the years 1720–1874 showed that this connection was limited to maternal grandmothers, while the grandchildren's probability of survival decreased in the case of paternal grandmothers. Other studies have also come to the conclusion that the presence of grandmothers in the family is much more beneficial for the offspring of daughters than of sons. This is explained by the constant uncertainty of the extent to which a man is the father and thus his mother is the grandmother of a child, i.e. the probability of a genetic relationship is always lower.

criticism

Testing the hypothesis using empirical data is extremely difficult because it describes a process of adaptation over several thousand generations, but today's studies can only cover a few generations. The evaluation of historical sources, however, is confronted with the problem of the reliability of the information.

The conception of menopause as an evolutionary adaptation is criticized that it does not necessarily have to be a direct adaptation, but could also be an exaptation, i.e. an indirect adaptation of a characteristic through the selection of a related characteristic.

In addition to humans, short-finned pilot whales and possibly some species of the African elephant are known to have a longer maximum life expectancy after becoming sterile. All these mammals have in common that the female members of the population develop their supply of egg cells before birth and this is used up after around 50 years at the latest. Menopause could therefore neither appropriateness still be a secondary exaptation, but an inevitable result of the evolutionary established structure and thus a part of the evolutionary constraints ( engl. For evolutionary constraints') which have resulted from the physique of the ancestors and not more are reversible. The high life expectancy of women could not be explained by the fitness advantage of the offspring of the next generation but the average menopause between the ages of 50 and 51 would be an “architectural” limitation that cannot be overcome.

further reading

Individual evidence

  1. ↑ GC Williams: Pleiotropy, natural selection, and the evolution of senescence. Evolution 1957 (No. 11), pp. 398-411.
  2. ↑ Kristen Hawkes, Peter S. Kim, John S. McQueen, James E. Coxworth: Grandmothering drives the evolution of longevity in a probabilistic model Journal of Theoretical Biology , Volume 353, July 21, 2014, Pages 84-94, accessed 8 May 2014. September 2015
  3. ^ A b Lynette E. Leidy: Menopause in Evolutionary Perspective in Wanda R. Trevathan (Ed.): Evolutionary Medicine , Oxford University Press, New York (1999), pp. 414, 417 at Google books (English)
  4. ^ Kristen Hawkes: Human longevity: The grandmother effect. Editorial, Nature 2004 (428), pp. 128-129. doi : 10.1038 / 428128a
  5. ↑ Eckart Voland , Jan Beise: Balance sheets of old age - or: What do East Frisian church registers teach us about the evolution of grandmothers? Historical Social Research, Vol. 30 (2005 - No. 3), pp. 205–218 on the website ( memento from September 29, 2013 in the Internet Archive ) (PDF; 394 kB) of the University of Giessen
  6. ↑ a b Eckart Voland, Athanasios Chasiotis, Wulf Schiefenhövel : The paradox of the second half of life: Why are there grandmothers? In: Biology of Our Time. 34th year 2004 (No. 6), p. 369 and 367, full text