A study at Stanford University of California – Santa Barbara found that a potential reason for increasing human longevity is because older men tend to chase younger women – which according to the research increase the longevity survival of the human species.
According to the evolutionary theory, people decease at old age after their reproductive cycle have been completed. According to Cedric Puleston, a doctoral in biological Science at Stanford says that for human, this is usually at the age of 55. However, fatherhood tends to end later for men, and they are faced with shortening mutations, which lead to the end of their reproduction cycle.
According to Puleston, giving birth to babies when you are in your 50s or 60s will not have any personal survival benefit. Yet, it does have an effect to the overall population. If such trend continues the longer reproduction capacity of men will be transmitted to women through their genes, which will have some positive effect on children of either males and females.
In an earlier study published, by Puleston & Micheal Gurven, who are assistant professors of anthropology at UCSB and Shripad Tuljapurkar, who is the Morrison Professor of Population Studies at Stanford, a research on “Why Men Matter: Mating Patterns Drive Evolution of Human Lifespan,” The aim was to clarify that human’s longevity doesn’t stop as soon as the female reproduction cycle ends.
There is something that has been connoted as the “wall of Death” – which literally means living longer than the reproductive years. In the recent 50 years, the question of why species continue to live despite that their reproducing abilities are gone, has been at the epic of scientific controversy.
The evolutionary theory suggested that as reproduction ends, the species will be affected by dangerous mutations that will reduce survival. Nevertheless, in the early hunter-gatherer society, which is a typical representation of early human mating patterns – 30 percent of the population lived beyond the age of 55 – or female lived-on even after their reproductive lifespan had ended.
In today’s society life expectancy is between 75 to 85 years in the developed world, with longevity increasing slowly – even for female following their menopause.
The Grandmother Effect – Grandmother hypothesis
According to Puleston, since 1966, the “grandmother hypothesis” has become the contemporary explanation why humans live beyond the age of 55. The grandmother effect was described by a British Evolutionary Biologist, William Hamilton, with a mathematical formula for the “wall of death.” The underlining idea of the hypothesis is that female improves the potential survival rate of children & grandchildren by simply living long enough to help them – and thereby enhancing their genes. Yet the representation of Hamilton’s mathematical calculation on genetic (grandmother effect) is a very complex description to demonstrate why people live to old age.
New Study – Including Males – Contrary to the Grandmother effect –
A new more complete study was recently released. The demarcation of this new study from previous research on human reproduction is that for the first-time information on males has been included into the picture. This inclusion allowed scientists to finally provide a response to the “wall of death” with an aligned answer to human mortality patterns. Previous study emphasised primarily on women (female survival and fertility rates) which led to incomplete conclusions.
Often, people dislike doing models, including both male and female, due to the high level of complexity. Yet, women and men’s fertility are contingent – the only difficulty is to comprehend the relation between the two. – for us to understand fully patterns of human evolution – reproduction is at the core – it holds answers to how we have evolved and also how we will evolve.
Longevity & Men
The study on both sexes showed that the longer male can reproduce the better is it for survival. Males older than the age of 50 years do commonly mate with younger females. This was usual in early human civilisation as well. According to Puleston, males being fertile at old age help to counteract on damaging cell mutation, for women beyond the menopause stage – this does thereby remove the so called “wall of death.”
This new research demonstrates how old-age male fertility eliminates the ‘wall of death’ that Hamilton stated. It is not necessary to rely on the grandmother effect or equal economic status – mortality will slowly increase as a function of longer male fertility.
In the research, the scientists gathered fertility and longevity information from two hunter-gatherer groups:
1. Dobe – Kung of Kalahari
2. Ache of Paraguay (amidst the world’s most segregated population)
Date of Forager-farmer was also included in the study:
1. Yanomamo of Venezuela and Brazil
2. Tsimani – Bolivia (indigenous group)
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Puleston said that these four groups had a lifestyle similar to that of our ancestors. Their living habits are also very similar. In addition, some village farm in Gambia was included as well as a modern group of Canadians to give universal understanding of contemporary to modern society.
In the traditional societies, the gender age gap between partners was 5 to 15 years. Currently, In Europe and the United States the average age gap between partners is two years. It is a general pattern that men are older than women during marriage. The age gap differs from one group to another, but remained true across the populations.
For all the groups involved in the study men were sexually reproductive at a more advanced age in comparison to females. They also had a slower decrease in fertility contrary to female fertility, which ends by menopause.
In all the groups involved, they stopped having more children in their 50s. However, in special cases, high-status males tended to reproduce to the age 70. Mostly, men who lived in societies that favoured polygamy were reproducing in their 70s.
However, old male fertility is equally present in societies that encourage serial monogamy. Men are more likely to remarry, and are fertile at this advanced age. Fertility of elder men has been present in human history as to reflect mortality patterns and increase in longevity.
The “grandmother hypothesis” might be accurate – but to have a more precise explanation male fertility needs to be elucidated. According to Puleston, males reproduce at a later age with younger females have a positive effect on population longevity.