We are a species with exceptional longevity. Compared to the great primates, which are our closest living relatives in the animal world, we humans live much longer. We surpassed in a couple of decades the life expectancy of chimpanzees, orangutans and gorillas. While Homo sapiens can aspire to reach 85 years, chimpanzees will hardly exceed 53 years, gorillas 54 and orangutans 58. We could think, and rightly so, that the longer a species lives, the more time it has to have children, so that longevity would be a factor in favor of reproductive success. However, if we take a closer look at our life cycle and that of the great primates, we are in for some surprises. Despite living longer, the age at which our species has its first child is quite late, 19.5 years, compared to 10-15 years for our cousins and, as if that were not enough, we do not compensate for this by having offspring until later, but we have the last offspring around the same age, 42-45 years. We could say that the time that, as a species, we dedicate ourselves to having children (approximately 25.5) is lower than the average hominoid (approximately 29) despite living much longer. In summary, our species has increased its longevity by precisely extending the periods in which not we are reproductive How do you eat that? Has natural selection gone mad?
Despite everything we complained about in the previous chapter when we talked about senescence, the reality is that the rate of aging in humans is much slower than in the rest of the primates. At the age of 35, chimpanzees already show obvious signs of senescence, slowness of movement, muscle weakness, weight loss and loss of agility, among other things wonders. Based on external signs of deterioration, the famous primatologist Jane Goodall, thanks to whom we have come to know much about the biology and behavior of chimpanzees, directly classified as “old” those who reached the age of 33, the time from which they entered a rampant deterioration in less than a decade. Are we then complaining of vice? We live many more years and also in a physical state that a chimpanzee would describe as enviable. But what is the reason for what seems like a concession on the part of nature? From the evolutionary point of view we can begin to suspect that the old age hides something that natural selection has decided to bet on.
If we analyze the data, we verify that, despite the fact that Homo sapiens live longer, the length of the fertile period in human females and the vast majority of primate species is similar. In some way it could be said that in the case of humans there is a gap between somatic senescence (from the Greek word soma, which means “body”) and the reproductive. While in other animals the senescence of the reproductive system is gradual and is accompanied by the decline of other systems, in women it is abrupt and apparently very early if we take into account the total duration of life and our general physical state. It is as if the onset of menopause were subject to particular mechanisms different from the wear associated with age.
Mammalian females are born with a stock o Fixed number of oocytes, which are the cells that will mature and give rise to the ovules. From the moment a female begins to ovulate, that store of oocytes will be progressively emptied. At each ovulation, a mature oocyte or ovum will be released which, if fertilized, will give rise to a pregnancy and, if not, will be expelled in what we know as menstruation or, colloquially, a period. For an ovulation cycle to occur, it is necessary for the oocyte store to send an endocrinological or hormonal signal to our nervous system, specifically to the hypothalamic-pituitary-ovarian axis. As the oocyte store empties, this signal weakens, and menstrual cycles begin to become irregular until they cease. In principle, all menstruating species can experience menopause if they live long enough. However, except in humans, reproductive senescence corresponds to somatic senescence, and few species live long enough to completely empty their oocyte reservoir and thus experience menopause. In contrast, the postmenopausal period in human females is characterized by surprising physical vigor.
The data is conclusive. Among hunter-gatherer populations, having or not having a grandmother can mean up to 40% less survival of grandchildren
In light of these singularities, the American anthropologists James O’Connell and Kristen Hawkes elaborated what is known as grandma hypothesis. These investigators emphasized the benefit of relatively early cessation of fertility in females in our species. Thus, instead of continuing to have more children, with the risks that surviving childbirth also entails, they would dedicate a greater effort to guaranteeing the survival of the children they had already had, thus ensuring the transmission of their genes not only through the children, but also grandchildren. Beyond the theoretical development of this explanation, O’Connell and Hawkes provided an extensive body of data collected from their first-person experience living, as one more, with some of the most emblematic hunter-gatherer populations in Africa. Hunter-gatherer populations are like authentic living redoubts of the way of life that has characterized more than 90% of the people who have inhabited the Earth. Fewer and fewer, and in real danger of extinction, there are still tribes in the world such as the Hadza or the !Kung, in Africa, whose way of subsistence is close to that of our ancestors. This allows us to make inferences about the behavior of hominids during the Pleistocene and about that of our own species before agriculture and sedentary lifestyles became the main axes of society. We cannot, of course, make direct equivalences between the lifestyles of current hunter-gatherer populations and those of extinct Pleistocene populations, but they are the best living example we have to try to approximate that subsistence regime.
I met James O’Connell in 2018, as a result of an invitation we made him to give a conference at the CENIEH, in Burgos, precisely on the grandma hypothesis. Five days before we met in Tenerife, where he was participating in a scientific outreach conference and where I was lucky enough to soak up his knowledge and his experiences with hunter-gatherer populations under the imposing setting of Mount Teide.
The data is conclusive. Among hunter-gatherer populations, having or not having a grandmother can mean up to 40% less survival of grandchildren. This difference, moreover, is not limited only to the first years of life, but is maintained and even accentuated in adolescence. The evidence seems overwhelming to me. Throughout our evolution, the possibility of having a part of the population that is not directly involved in reproduction, but dedicates effective time to reducing infant and juvenile mortality, has been a transcendental benefit for the species. Natural selection would have favored the premature cessation of fertility in women, allowing a more proactive role in the subsistence of the group and the perpetuation of their clan. Menopause in our lineage would be read not so much as a sign of senescence or deterioration, but as an adaptive strategy that implied clear benefits for the success of the group.
Menopause in our lineage would be read not so much as a sign of senescence or deterioration, but as an adaptive strategy that implied clear benefits for the success of the group
The impact that grandmothers had on the survival of succeeding generations was manifest. I confess that I was amazed at how James O’Connell talked about them. Carrying a quarter of Canarian blood on the part of my paternal grandfather made me especially receptive to everything lived on this land. But under the spectacular sky of the Canary Islands, for centuries inhabited by aboriginal populations, the grandmothers appeared to me as the true watchtowers of the tribe, guardians and protectors.
As O’Connell explained, in tribes like the Hadza, grandmothers help their daughters in two main ways. On the one hand, they contribute to their provisioning, sharing with them the food that they themselves actively collect. And, on the other hand, the grandmothers also participate in raising their grandchildren, taking care of them, feeding them and educating them, teaching them a wide repertoire of useful activities for their survival —among them, the harvest itself—. This care of the grandchildren allows weaning, in these groups, to occur relatively quickly. By ceasing breastfeeding, the children are less dependent on the mothers and they would be released to carry out other productive tasks or to conceive a new child. But we don’t need to go visit the Hadza tribes. Let’s think about our mothers. Let’s think about our grandmothers. In our great-grandmothers. Is the power of his bastion familiar to all of us? This fundamental role of grandmothers has been maintained over time and in all economic and social contexts, from rural environments to more urban lifestyles, changing the format but not the content of their key role as support for the group.
In the light of biology, it appears that a prolonged “senior age” is the result of a successful strategy favored by natural selection to pull the chestnuts out of the fire for a species with threats of high infant mortality and prolonged juvenile dependency. In other words, there is an added value in the contribution that our elders make to the success of our species and that value is of such magnitude that evolution has favored longevity in those groups in which individuals are highly dependent. Biology supports with data what we might think was only written in our feelings. This fundamental contribution of grandmothers, today also extended to grandparents, is one of the identity marks of Homo sapiens within the hominin lineage.
Maria Martinon-Torres She is the director of the National Center for Research on Human Evolution (CENIEH)
HOMO IMPERFECTUS
Publisher: Destination Editions
Theme: Human evolution
Author: María Martinón-Torres
Number of pages: 272
Price: €18.90 (paper) / €9.99 (ebook)
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