What evolutionary pressures would lead to giants?

Question

Somewhat similar questions have been posed before, but what I'm really interested in is what selection pressures would be necessary to produce a subspecies of giant humans, about three meters in height. In asking this question I am making several assumptions:

• Humans can climb trees, and therefore would not benefit from growing in size in order to better harvest food from trees as giraffes did, for instance.
• Humans are relatively clever and highly social pack hunters, and so can hold their own against the average predator. As a result, they are already safe from most predators, including predators stronger than them, and would not necessarily benefit from growing in size.
• Humans are mobile, and therefore will simply move away if they live in an environment wherein averagely-sized individuals are dying at high rates.
• The environment which this theoretical branch of humanity lives in is capable of supporting the dietary needs of these giant humans.

I think it's worth noting that gigantopithecus managed to reach about three meters despite there being no obvious benefit to this increase in size, so while I'm not really sure if all the above assumptions hold true in the case of gigantopithecus, it does illustrate that gigantism can occur viably in homonids, so I'm assuming it's theoretically possible with humans as well.

What kind of pressures would result in giant humans under these terms?

Answer 1

Is There Any Region We Could Look At With an Unusually High Amount of Very Tall People?

Very tall people occur in many different climates (Africa, India, Northern Europe). It does not seem like there is a specific region that can be focused on for examples.

What benefits does being very large confer?

Superior visibility ###

Standing humans enjoy a better viewing angle of the terrain than animals walking on all fours. Combined with our ability to adjust our height by crouching, the superior viewing angles allow human kind to defeat cover and camouflage by predators and prey. A three meter tall humans would enjoy a less significant, but still substantial, improvement in being able to see over and around a crowded or complicated environment.

Greater body weight vs. surface area

Assuming the giant remains proportional, gains in body volume exceed gains in body surface area. As a result, the giant can hold on to body heat longer because the giant has less of their total body mass directly exposed to the elements.

Stronger

Because the giant weighs more, a giant of roughly the same "fitness" (or activity) level as a normal human will be capable of carrying more weight or lifting heavier loads.

Better reach

A larger human will be able to reach across larger crevasses without jumping, or swing across wider gaps without ever needing to let go of their existing hand hold. He'll be able to throw a spear further and pull a larger non-composite longbow (meaning heavier arrows, or faster arrows)

What weaknesses does being very large confer?

Heavier, slower, lower stamina under "equal" loading

The maximum strength of vascular walls doesn't increase with height, and muscles carrying more weight tend to fatigue more quickly.

Cardiovascular health

A giant's heart has to work harder to pump blood up to the head, a task it must do constantly. With age, cardiovascular health becomes a problem.

What evolutionary pressures might select for these benefits?

• Better visibility: complicated landscapes where predators and prey may take advantage of low terrain or possess good camouflage
• Better heat retention: hard winters, flooding, winter showers
• Better reach: frequent flash flooding, mudslides, unreliable terrain
• Stronger: big animals, hauling heavy loads

What environmental conditions would minimize the selection pressure of weaknesses?

• Slow, low stamina: Slow, big prey / farming
• Cardiovascular health: easy access to water (swimmimg) where the heart does not have to pump up, but just around.

Answer 2

When Gigantopithecus blacki remains are dug up the sizes of the bones fall in to a bimodal distribution which suggests that the two sexes had very different sizes. Males are estimated to have been roughly twice the size of females. This is known as sexual dimorphism and is common in the great apes. The general explanation for this in primates is that the males tend to be larger than the females due to sexual selection. To be more specific, in many apes reproductive success for males is determined to a large extent by male-male competition. Larger, stronger males are better able to compete for reproductive access and so size is positively selected for in these species. The more intense the male-male competition, the stronger the importance of size, and the larger the sexual dimorphism gets. Humans have relatively small differences in sizes between sexes which many anthropologists suggests means male-male competition was only a minor factor in our evolutionary past and that our ancestors skewed more towards monogamy. On the opposite side of the spectrum if we look at the gorilla we see a large degree of sexual dimorphism with males being up to twice as large as females. This large difference is explained by their social structure. Gorillas live in troops of one male and many females. The alpha male prevents any other males from procreating with the females in his troop. For this reason reproductive success among male gorillas depends heavily on their size and strength and the ability to take a troop and defend it from other males. Presumably this is what caused male gorillas to become so large over evolutionary time.

Now, what is the limiting factor to this arms race? As males get bigger and bigger they become more and more reproductively successful so when does it end? Barring other size-specific selection pressures male size will be capped primarily by their ability to feed themselves. This means that if resources are abundant male size in primates could conceivably become quite large. So, the recipe for your giant hominids is two simple things.

1. A social structure with large amounts of male-male competition for females. The stronger this competition the stronger the selection for larger male size.
2. Abundant food which the question already provided.

Of course, this only gets you large males. Female size may increase somewhat if only to keep up, but I’m not sure on that. Either way, this is likely one of the main reasons why Gigantopithecus grew so large.

Answer 3

Herbivory.

Humans do not have rumens like cows. The rumen is an evolved adaptation that allows cows and related ruminants to digest plant matter in the foregut. There is a limit on the size ruminants can obtain before the giant rumen full of fomenting plant matter starts posing problems with microbial overgrowth.

Hindgut fermenters however can get big and probably the bigger the better. Elephants ferment in the handout (read: colon) and it is thought that the huge extinct herbivores like giant rhinos and sauropods used this form of digestion.

http://www.zora.uzh.ch/id/eprint/2393/2/Oecologia_body_size_2003V.pdf

An oft-cited nutritional advantage of large body size is that larger animals have lower relative energy requirements and that, due to their increased gastrointestinal tract (GIT) capacity, they achieve longer ingesta passage rates, which allows them to use forage of lower quality... We propose that, among the extant herbivores, elephants, with their comparatively fast passage rate and low digestibility coefficients, are indicators of a trend that allowed even larger hindgut fermenting mammals to exist.

Our relatives the gorillas are herbivores. They are big and part of that bigness is to accommodate a big colon where they digest their food.

https://ucdintegrativemedicine.com/2016/03/youre-not-cow-gorilla-dont-eat-like-one-either/#gs.b0KpPP4

The Big Belly Factor. For example, a big-bellied gorilla is considered healthy while a big belly on a human signals obesity — with all its associated health problems. For gorillas, that big belly is necessary because the size of their colon is both longer and larger than humans so they can ferment the plant fibers they eat and accommodate all the bacteria needed for the fermentation process.

Human (whole food) plant-based eaters rarely have a big belly. Because humans rely on starches and intact sugars that are broken down through enzymatic digestion, a plant-based eating human tends to be thin, muscular and healthy.

Now consider a future scenario where there is lots of low-value plant food. Humans evolve to be gorilla-like, and then elephant like to maximize body size and colon size. Our teeth are not well suited to chew lots of low value plant food but fortunately we have hands and tools to help with that.

Thus: the giant humans will be herbivores subsisting on fast growing low caloric value plant matter. They will have huge bellies and will poop immense amounts, like elephants.

They will not be opposed to a bite of meat when they can get it.

Answer 4

You might consider your three-meters tall humans as a combination of island gigantism combined with the founder effect and sexual dimorphism (the latter point has been covered in another answer).

Some hundred thousands years ago, a restricted group of Homo sapiens individuals made it to a previously uninhabited island. For whatever reason, these men were already all taller than the mean. They found themselves before a land full of preys bigger than the ones they'd seen in the mainland and dwarf predators that didn't pose any serious threat to them - except for children, of course. Freed from these ecological constraints, these primates began growing larger than their mainland counterpart. Their social structure, however, predisposed them to battle violently over the control of females. Body size increased even more: being able to feed almost limitlessly, the males grew to the size of giants.

One might think that this arms race affected only the size of males. Well, not at all: the competition among males became so violent that infanticide against male children became the norm. This favoured male children with a fast development: male babies were born bigger and bigger, and this tendency selected for females big enough to not die from childbirth.

There we are: probably, females will still be smaller than males, but not so much.

Answer 5

Well, if it was a humanoid, it would have to have evolved in another planet where the gravity is weaker and there is a lot of air pressure. The air pressure will help the giant heart to keep beating, and half the gravity will let more big stuff work.

The heart will have to beat a lot to make all of that blood up, but if its in a small gravity place then normally will work

So, if you want something smaller a great example is the gigantopithecus which will grow more if it had survived the food extinction (what is presumably what killed the other species of humans) and things grow larger with evolution (except if they do not need to) so the gigantopithecus will have grow a little bit larger enough to sustain gravity and the air pressure.