What evolutionary pressures could turn a group of divergent hominids into matriarchal giants?

Question

QUESTION

Other questions have covered what the physical structure of a humanoid giant might look like. Assuming that anatomically modern humans (or something close) were dispersed into several distinct population groups over an adequate time span, what evolutionary path might cause one of these groups to become giants?

CRITERIA

Giants in this case meaning a mostly human-ish creature about 10 feet tall, perhaps with a general build resembling gigantopithecus. One big problem is, from what I understand, size in mammals is usually driven up by competition between males over females. But in this case I'd like the female giants to be noticeably (i.e. several feet) taller then the males, and form the dominant and more sedentary core of their communities. I'd like to do this is the simplest and most plausible way available, keeping them otherwise as close to the human baseline as I realistically can.

Ideally, these giants might evolve on a rainy tropical archipelago, in which the largest landmass is a bit less than half the size of Australia. This would be convenient for me, however it is not required. I can pretty easily tweak the world or their origin point to fit a good evolutionary path.

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EDIT: Just to clarify for posters, "matriarchal" is title question shorthand for "sexually dimorphic with larger females", which I tried to specify in the text below. I certainly appreciate the deep importance of culture, but this question is about justifying a physiological difference, not about ways to produce a matriarchal culture for giants. I plan for the size difference to have a wide variety of social effects, beyond just power structures. Thanks!

Answer 1

• Humans have selective advantage for big heads.
• Big headed babies require big pelvised mothers.
• Big ladies have big pelvises.

Why do humans have such big heads? It is a matter of conjecture but we do. Apparently though head size stopped growing about 5000 years ago.

https://news.nationalgeographic.com/news/2012/06/120606-americans-heads-getting-bigger-science-health-skulls-evolution/

Beginning with the dawn of the first Homo species, human skulls evolved to be increasingly bigger until about 30,000 years ago, when head size plateaued.

And about 5,000 or 6,000 years ago, when agriculture took off in earnest, skulls began shrinking. The cause of the shrinkage is a mystery, but scientists have tentatively fingered more efficient brain wiring and easier access to food and safety—the idea being that people no longer had to be especially smart to survive (aka the Idiocracy theory).

I wonder if this is true for Africa and Australia? In Europe and the east I think that the reason might have been a degree of vitamin D deficiency - rickets - in regions where there was a lot of agriculture. Low vitamin D means your pelvis does not develop correctly. Then when you are ready to have a baby, it lodges in the mothers pelvis: end of the line for both. That probably drove the evolution of white skin in under 8000 years.

But what if the factors selecting for big headedness were not countered by obstetric difficulties? Or could bigger pelvises evolve to accommodate big head babies? They do!

Covariation between human pelvis shape, stature, and head size alleviates the obstetric dilemma

Because of the tight fit of the large human neonate through the narrow maternal birth canal, childbirth is remarkably difficult. In this study we show that the dimensions of head, stature, and pelvis in a human body are linked in a complex way that was not recognized before and that contributes to ameliorate this tight fit. We show that females with a large head possess a birth canal that can better accommodate large-headed neonates. Because mothers with large heads usually give birth to neonates with large heads, the detected pattern of covariation contributes to ease childbirth and has likely evolved in response to strong selection.

So: your people have big heads. Huge heads! Maybe they are really smart - or whatever other fitness advantage bigheadedness confers. To get those big babies out you need big pelvises. And a good way to have big pelvises is to be big in general. Just as pelvis size and shape differs from male to female in humans, your humans take this difference further - in the women not just the pelvis but the whole rest of the body is big.

And you might be man the hunter: a hairy badass full of testosterone - but you will mind your manners around a lady who is 9 feet tall and triple your weight. Best to do as she says.

Answer 2

Abundant resources, low/weak predation, female dominant breed selection.

You look at peaceful megafauna and they tended to get there by simply having enough to grow larger, and predators that couldn't easily take them...

There are multitudes of variations on how this could happen. Think of an evolutionary path where the "smaller" ones would get killed or die, leaving the larger ones to continue to breed. As long as the environment can support the increase in dietary/shelter/space needs relatively easily, they can grow larger.

This isn't too far from what you see in disparate races here on earth, different environments have different requirements of our bodies skin tone, muscle mass, and size until they become "traits" of our heritage... your world just takes it to another order of magnitude.

Ensuring that the females are fighting over the males, or at least choosing the males in a method other than who can punch who the hardest, will remove the genetic benefit of males growing larger.

=== Edit for the clarification about the "matriarchal" requirement ===
You get larger females the same way you get larger males... have them fight each other.

It benefits a male to have a breeding pool, multiple females he can keep pregnant and safe...

A matriarchal society tends not to fight over males... cause why? There is no genetic advantage to them keeping a harem. There is no "downtime" for the male where he will need a strong female to provide for him or protect him.

You would have to find a motivation, a genetic one, not a social one, to enable this. Extremely long (months) refractory periods where the males are not only unable to breed, but maybe too lethargic to hunt or protect themselves? Even then, at that point the female, biologically speaking already has what she needs, there is no reason to "protect" the male... she can just find a new one later.

An argument could be made that the females have to be bigger and sturdier to be capable in a predatory environment during pregnancy... but that only makes sense if the males are somehow not able to provide them with safety. This doesn't add up... whatever time it took for the females to get stronger would make the males stronger as well.

The "easiest" way would be socially... but there would have to be cultural motivations in place for tens of thousands of years before this would be significant... which is extremely unlikely.

Answer 3

There doesnt necisarily need to be an evolutionary reason for a societey to be matriarchal, many matriarchal societies exist already. There are 3 main kinds of descent that a culture can place emphasis on. In egalitarian and industrialized cultures that place an emphasis on equality and legality you mostly find bilinial descent is valued. Bilinial descent is where either male or female can inherit and lineage is equally valued from either parent.

Patrilineal is where wealth and power are inherited by males, matrilineal is the same but for females. Whether a less developed more traditional societey is concerned patriarchy or matriarchy seems to simply be based off of which lineage the culture values for sociological or cultural reasons. It really just depends upon which gender a culture chooses to inherit the wealth and positions of power.

As for gigantism in organisms, a common set of environmental factors seems to be found in most cases that it has been observed. An abundance of resources and space, and few large predators. If you really need there to be size based sexual dimorphism you could simply have it be due to cultural effects on biology. Males value bigger females because they survive childbirth more easily, or its simply just a commonly held belief that bigger is more attractive kinda like how we currently seem to think tanned skin equates to beauty.