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. Homo Ancestors
... Trachillos bipedal hominids
... Homo erectus
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... Sea Gypsies/ the Moken
... Homo sapiens - water afinity
... Coastal Migration
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... Semi-Aquatic Animals

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Menopause

Animals almost always continue to reproduce until they die. There are just five exceptions that we know of: short-finned pilot whales, killer whales (orca), beluga whales, narwals and humans. [1] In all these species, females lose the ability to have children at an average middle-age, but continue living for decades after. Human females, with an average maximum lifespan of 100 years, tend to go through menopause when they are around 50. Female killer whales go through menopause in their 30s or 40s but can live to about 90. But what sense does it make in nature for a species to live maybe half their lives without being able to pass on their genes?

"One of the most popular explanations, first proposed in 1966, involves helpful grandmothers. Even if older women are infertile, they can still ensure that their genes cascade through future generations by caring for their children, and helping to raise their grandchildren.* There’s evidence to support this “grandmother hypothesis” in humans: It seems that mothers can indeed boost their number of grandchildren by stepping out of the reproductive rat-race as soon as their daughters join it, becoming helpers rather than competitors." [2]

One theory about why killer whales experience menopause [3] is that it provides the older generation of matriarchs a better chance to look after their existing children and grandchildren. Orcas, like humans, invest a lot of time and energy into rearing single offspring that take many years to fully mature.

"It’s clear that [orca] mothers who had been through menopause were just as useful to have around, and probably more so, than those who are still fertile. On average, a 30+ male is 8 times more likely to die in the next year if his mother passes away, but his odds actually go up by 14 times if mum had gone through menopause. This confirms that mothers are helping their sons well into adulthood, since older orcas actually benefit from mum’s presence more than young ones. Perhaps she helps them to hunt, or maybe she watches their backs during fights with rivals." [4]

Similar survival benefits have been observed in human societies where grandmothers can help look after their children's children:

"Economic subsidies that older women are observed to supply to their daughters and grandchildren point to the distinctive selection pressures likely to have favored post-menopausal survival in ancestral populations." [5]

But why does menopause affect humans alone of all primates? [6] Female chimpanzees tend to remain reproductively active most of their lives with fertility dwindling towards the end of their natural lifespans, and no obvious disadvantage to the survival of their grandchildren has been observed from this. Yet human menopause must have at some stage conferred a large enough evolutionary advantage to a woman's own descendents that she would forgo reproducing herself for almost half her life. Her non-reproductive years must be useful in some evoutionary regard. Evidence has shown that a woman's mortality increases once her own children pass reproductive age themselves, in turn becoming grandmothers or entering menopause. [7]

Most other animal mothers manage to protect and provide for their offspring well enough by keeping them close at their sides, on their backs, in their pouches, or leaving them in their lairs, dens or nests. The male individual /biological father rarely contributes to his offspring's survival after mating. (There are obvious exceptions to this, notably in birds, who equally share parenting duties). A semi-aquatic human female would not have been able to leave her babies or small children alone on the beach, or in the shallow waters while she dived for clams. The presence of another, closely related female, who did not have small children or babies of her own to care for, would be better able to invest her time and energy in looking after her children's children, clearly offering a distinct survival advantage and ensuring the survival of her own genes.

Elaine Morgan wrote about menopause in her 1990 book: The Scars of Evolution. In it she said:

In apes, the non-stop cycle of pregnancy and feeding continues till death; in human females it ends with the menopause. No one is quite sure why women are granted this dispensation, which is very rare in nature – though it has recently been claimed that we share it with whales and dolphins.

One suggestion is that the menopause tends to prolong the active life of grandmothers by lessening the physical strain on them. The young of humans take a very long time to mature, and the success of our species depends to a unique extent on the passing on of acquired knowledge and skills to the next generation. So it would have been advantageous for a breeding population of hominids if there were some experienced non-breeding females present to help in the task of child rearing.

It seems a little odd that just around the time of the menopause, when there are not going to be any more children to gestate and suckle, there is a general tendency in women of all races for their weight to go up instead of down, and for an increased percentage of their body tissues to consist of fat.

Changes in fat and weight distribution in perimenopausal and postmenopausal women

Another factor of menopause that seems to indicate an aquatic connection in human evolution is that women tend to gain weight as they enter the menopause and this fat tends to be distributed more around the waist and upper body.

Several studies have shown that perimenopause, independent of age, is associated with increased fat in the abdomen as well as decreased lean body mass. This suggests that menopause plays a role in many midlife women’s transition from a pear-shaped body (wide hips and thighs, with more weight below the waist) to an apple-shaped body (wide waist and belly, with more weight above the waist). [8]

We have already seen how subcutaneous fat plays an important role in survival for aquatic mammals, offering both buoyancy and better thermoregulation while in water. By putting on weight when her menses cease, especially more weight around the upper body, a woman in the water is better able to support the additional weight of her grandchildren, while her own offspring dive for food.

Reduced bone density

It is well known that oestrogen levels are in decline in a woman's body as she enters menopause. Oestragen protects bones and it is known that up to 50% of caucasian women aged 50 and over have low bone mass, with approximately 20% developing osteoporosis.

"The most important risk factor for bone loss in midlife women is the menopause. Women lose about 50% of their trabecular bone and 30% of their cortical bone during the course of their lifetime, about half of which is lost during the first 10 yr after the menopause." [9]

As previously noted, increased bone density (pachyostosis) may have played a role in our human ancestors' semi-aquatic evolution. Homo erectus had dense, heavy bones, a feature normally only seen in shallow diving mammals such as manatees and dugongs. Acting as ballast, heavy bones enable the individual to remain underwater for longer periods at a time. Conversely, low density bones, as seen increasingly in human females after menopause, would help the individual remain at the surface and better support her offspring and her offspring's offspring.

Other observations:

- Human hair strengthens and grows longer during pregnancy and breastfeeding. It has been suggested that swimming babies and infants would be able to grab hold of the long floating hair to stay close to their mothers. Unlike males, human females do not normally experience hair loss and balding in later years.

- Several species of animals belong to matriarchal groups, including orca. The offspring tend to remain with their mother's / grandmother's family/pod, while adult males leave the group to mate with females from other pods. This is also true of elephants, another species with a semi-aquatic history. It is therefore possible to suppose that early, pre-civilisation human societies may have formed around matriarchal groups. The earliest figurines (variably attributed to Homo erectus, Neanderthals and/or Archaic homo, clearly depict fat, rounded female figures. [10] Perhaps the mother/grandmother figure was once revered in these early social groups?

References:

Killer whales experience menopause just like humans
http://www.theguardian.com/environment/2013/oct/17/killer-whales-menopause

Why do killer whales go through menopause?
https://www.nationalgeographic.com/science/phenomena/2012/09/13/why-do-killer-whales-go-through-menopause/

https://www.sciencemag.org/news/2017/01/study-suggests-surprising-reason-killer-whales-go-through-menopause

https://ocean.si.edu/ocean-life/marine-mammals/menopausal-moms-mammal-mystery

Beluga whales and narwhals go through menopause
https://www.sciencedaily.com/releases/2018/08/180827080847.htm

Female Menopause
http://carta.anthropogeny.org/moca/topics/female-menopause

Evolution of the human menopause
https://onlinelibrary.wiley.com/doi/epdf/10.1002/1521-1878%28200103%2923%3A3%3C282%3A%3AAID-BIES1038%3E3.0.CO%3B2-9

Grandmothering, menopause, and the evolution of life histories
https://www.pnas.org/content/95/3/1336

Maternal grandmothers improve nutritional status and survival of children in rural Gambia
https://royalsocietypublishing.org/doi/10.1098/rspb.2000.1190

Analyses of ovarian activity reveal repeated evolution of post-reproductive lifespans in toothed whales
https://www.nature.com/articles/s41598-018-31047-8

Fitness benefits of prolongued post-reproductive lifespan in women
http://www.nature.com/nature/journal/v428/n6979/abs/nature02367.html

Hazda Women's Time Allocation, Offspring Provisioning, and the Evolution of Long Postmenopausal Life Spans
https://www.journals.uchicago.edu/doi/10.1086/204646

Changes in weight and fat distribution
https://www.menopause.org/for-women/sexual-health-menopause-online/changes-at-midlife/changes-in-weight-and-fat-distribution

Bone Mineral Density Changes during the Menopause Transition
http://ncbi.nlm.nih.gov/pmc/articles/PMC2266953/

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