Theorizing on the Origins of Human Longevity and Gender Differences in Life Span
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It is a given that women tend to live longer than men, a difference that becomes ever more apparent as demographic cohorts approach extreme old age. Four of every five centenarians are women, for example. This disparity has long been noted, and yet, as is the case for many apparently simple questions about the biochemistry of aging, there are as yet no definitive, final answers as to why this situation exists, how it came about though natural selection, and what exactly determines gender differences in longevity at the level of metabolic processes and epigenetic patterns of gene expression. There are a great many theories, however. If you want to see scientific undergrowth at its thickest, tangled and disputed, look no further than this topic.

Any proposed comprehensive theory on gender longevity differences in humans should probably also touch on two oddities in our species: firstly that women live long past menopause, and secondly that we are very long-lived in comparison to other mammals of a similar size, including our closest relatives among the primates. The grandmother hypothesis is often put forward in connection with these points, but is by no means universally accepted: it is the idea that our intelligence, tool-use, and culture allow older post-reproductive individuals to increase the reproductive success of their direct descendants in ways that simply don't happen in other species.

Here is a very readable open access paper that walks through one researcher's assembly of a theory of human aging and its unusual aspects, touching on a range of research from recent years, some of which you might recall being mentioned here. As for many similar topics this has very little bearing on the pressing issue of building effective treatments for degenerative aging, but it is a part of the very interesting backdrop to that work: there is great complexity to aging, and the genius of the repair based approach of SENS and similar initiatives is the recognition that most of that complexity can be bypassed and ignored. We simply don't need to fully comprehend degenerative aging in order to remove it from the human condition.

Evolution of sexually dimorphic longevity in humans

The pattern of human aging exhibits a number of salient features that have long engaged evolutionary biologists. For one, among the higher primates, human being are unusually long lived. The maximum lifespans of orang-utans and gorillas are 58.7 and 54 years, respectively, and those of our closest relatives, bonobos and chimpanzees are 50 and 53.4 years, respectively. By contrast, maximum human lifespan varies from 85 in foraging groups such as the Aché in Paraguay and Kung bushmen, to 122 in the developed world. This implies that an evolutionary spurt of increased longevity must have occurred since the last common ancestor of humans and chimpanzees/bonobos walked the earth some 5-7 million years ago.

Another striking feature of human aging is its sexual inequality. As life expectancy has increased with improvements in living conditions during the last century, there has consistently emerged a survival difference between the sexes, with women living longer. For example, in the UK estimated life expectancies for women and men from birth (2012) are 82.4 and 78.0 years, respectively, a difference of 4.4 years. Some other examples of gender gaps are the USA 5.0 years (81.0F - 76.0M), France 6.4 years (84.7F - 78.3M) and Russia 13.0 years (73.1F - 60.1M). The gender gap reflects a greater susceptibility of men to a wide range of aging related pathologies, including cardiovascular disease, type II diabetes, infection and sarcopenia (aging-related loss of muscle mass). The basis of male frailty remains unclear, either in terms of its evolutionary origins or somatic causes.

The other gender gap in aging affects reproductive lifespan. Women's capacity reproduce is lost in their late 40s, as they undergo the menopause, while men can remain fertile at least into their early 80s. The significance of the early cessation of reproduction in women is a topic of much discussion, in particular, whether or not it is an adaptation and contributes to evolutionary fitness.

Why do humans live longer than other higher primates? Why do women live longer than men? What is the significance of the menopause? Answers to these questions may be sought by reference to the mechanisms by which human aging might have evolved. Here, an evolutionary hypothesis is presented that could answer all three questions.

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