What would be the tallest possible height for humanlike creatures in earthlike conditions?

Question

This is about "giants" inhabiting a fictional earth-like planet somewhere in the Universe (like those in the movie Prometheus, for example).

Assuming that all the conditions (temperature, gravity, atmospheric pressure and composition, etc) on the planet are about the same as on earth, what would be the tallest possible height that a bipedal humanoid organism could reasonably attain?

The assumptions here would be that:

• these humanoid organisms are built similarly to humans, with a brain, heart, lungs, an oxygen carrying circulation of blood, a human-like skeleton and so on
• there should not be any hydrogen or other mechanisms that make these creatures lighter
• the organs and the skeleton could be allowed to be stronger but their operation would have to follow standard laws of physics
• the civilization of these humanoids may have developed some way to obtain large amounts of energy to support their physical energy needs, but they are still dependent on consuming the nutrients in the form of solid or liquid food

Answer 1

Scaling laws are an important aspect of biology. When you take a particular object (such as a human being), and make it twice as tall (while keeping the proportions the same) its weight will not increase 2-fold. It will actually increase by a factor of $2^3$ (8-fold)! A 6-foot person weighting 160 pounds, if doubled in height, will therefore be 12 feet tall and weigh 1,280 pounds if you kept the proportions the same.

There is a problem with this. Although weight increases $2^3$-fold, the strength of the bones would only increase $2^2$-fold. It means that the strength-to-weight ratio of the bones is half that of a normal person. A giant with these proportions would stress their skeletons more easily and be at greater risk for injury if they fell down. In order to fix this problem, you would need to make the bones wider in proportion to their length so that the weight of the person produces less pressure on the bone.

A well-proportioned giant would therefore be a rather wide, burly-looking person with thick arms and legs.

Potentially your bones could generate more compact bone, and less marrow, and would compensate reasonably well. But, one's muscles, tendons, and etc would also have to compensate. The knees, ankles, and hips would also take a beating.

Would a person that big have to eat about 20,000 calories per day? Possibly. Think about large theropod dinosaurs like Tyrannosaurus rex. Those creatures demonstrate that bipedalism is possible for very large animals.

In order to reach that size (5-7 tons), a lot might have to be changed about human physiology. A better cooling system might be needed (surface area does not increase as quickly as volume does, so the heat-generating tissue of a large mammal has less surface area to release that extra heat from). Might such giant humans require elephant-sized ears for temperature regulation? More sweat glands? Or perhaps a lower average body temperature? Larger or more efficient lungs would be needed too (the diffusion of oxygen and carbon dioxide gases is limited by the surface area of the alveoli in the lungs).

I calculated my basal metabolic rate to be around 1,730 calories a day, so if you were to scale my mass up to that of a T-rex, my metabolic rate would would increased by a similar amount (~112,000-156,000 calories per day). If you went for the "lower the body temperature" solution to the heating problem you'd get less calorie burn than that, though. In fact, warm-blooded creatures expend around 90% of their caloric intake just warming their own bodies up. A cold-blooded giant might not be so bad!

The lung volume should be ok due to the fractal space-filling nature of the lungs, unless your creature panted like a dog as part of its temperature regulation.

Humans vary in height from around 4 feet to about 7 feet with relatively "normal" physiology, although perhaps head size varies less than other parts of the body.

Wadlow's greatest recorded weight was 222.71 kg (35 st 1 lb) on his 21st birthday and he weighed 199 kg (31 st 5 lb) at the time of his death. His shoe size was 37AA (47 cm, 18½ in long) and his hands measured 32.4 cm (12¾ in) from the wrist to the tip of the middle finger. He wore a size 25 ring. His arm span was 2.88 m (9 ft 5¾ in) and his peak daily food consumption was 8000 calories.

The cause of death is very telling... Wadlow died at 1:30 a.m. on July 15, 1940, in a hotel in Manistee, Michigan, as a result of a septic blister on his right ankle caused by a brace

The current record holder, Sultan Kösen at 8'3" is often photographed with crutches. However, since there are many basketball players over 7', perhaps the leg problems are not always found with tall individuals.

Answer compiled from here

Answer 2

Shamelessly based on this answer. Because of the square-cube law mentioned already, a 50 foot human would weigh over 50 tonnes. I do not see a possible way for a biped to support, how big these people can get depends on how humanlike you want them to look.

In the real world, very tall people are caused by excess growth hormone rather than genetics, which is why they often have many problems. Giant Ground Sloths stood around 5 metres tall and weighed around 5 tonnes, however they were tripedal, having a load-bearing tail. If an alternate evolutionary path is acceptible for you then your giants could have similar tails, but then we lose the humanlike segment.

For Their great size, these giant humans would need a series of size adaptations:

• Bones. Proportionally thicker bones are needed to cope with the excess weight (compare a horse to an an elephant). This is especially true for the legs bones, while theoretically we can say that the bones are made of a stronger material on this world, like calcium carbonate, this will allow them to look humanlike, but only on the outside.
• Muscles would be larger/stronger for the same reason as above. You could also increase the efficiency of the muscles - Apes have shorter muscle fibres than humans which makes them a lot stronger pound for pound at a trade-off of losing your fine motor skills. The problem here is that the muscles at a large size would break the biones they move, a solution here is to have fat deposits act as coushins, this would also help caloric intake, see below.
• Proportions. As you scale up, relative leg length scales down in order to preserve balance, this is especially true in bipeds, which have a poorer sense of balance. The bigger they are, the harder they fall, which is why longer arms would be needed for balance, stability and for softening impacts, also the possible reason of collecting more food. I would expect the distribution of mass moves lower and lower to keep a low-center of mass, so the tallest humanoid may have thinner upper bodies or less broad shoulders.
• Internal organs would have to compensate for extra body mass, shocker there I know. Because volume scales faster than height, the heart and lungs become less effective and would need to be scaled up at a higher rate. These beasts would likely have a deep ribcage and massive heart relative to their size.
• Facial features. Larger eyes only become effective up to a certain point. The eyes of the tallest possible beast would be much smaller in proportion to his head than a regular human. The surface area of the nasal area would increase faster than the length of the nose, giving them a much keener sense of smell. The larger nasal cavity and larynx would also give your giants deeper voices.
• Brain. Our brains use vast amounts of energy so the brain would probably scale up a lot slower than the rest of the body. Extra mass and the accompanying buffs to your senses would be more taxing on their brain so the brain could scale to compensate, how much it scales depends on how intelligent you need them to be.
• Blood pressure. Keep blood pressure high or the giant will die. The giant would need to keep blood pressure high in order to circulate blood and oxygen around the body. Elephants have very tight skin on their feet to increase the blood pressure in their limbs as the blood needs to travel against gravity for quite a height. Elephants also have large fatty pads on their heels to cushion the impact of walking. Tall human feet may not resemble a regular humans.
• Diet. As the intestines scale up, digestion becomes easier. More body mass means higher body temperatures which means more nutrients can be extracted from food. The problem here is that animals with large intestines lean towards vegetarianism, which in turn supports lazier animals as size increases. The solution is for the vegetarian diet of the tall people to consists of planets high in fat and caloric value, nuts for example.

I have also read that the size of a certain reproductive organ scales up more with giant animals but I chose not to read up on that one, I do have principles believe it or not.

I would say that while keeping them humanlike the tallest we can make humans is 4 metres as the largest prehistoric mammals never peaked above 5 meters (even quadrupeds) and the largest bipedal animals of all time - the therepods - didn't really exceed 3 or 4 metres at the hip and it's specualted the atmosphere contained a lot more oxygen in prehistoric times. Even a slimmer 3.5m giant would weigh over half a tonne.

Answer 3

About 10 feet. That's the realistic limit to hominid creatures we have excavated so far (read Gigantopithecus article for details).

Generally, people taller than 7 feet develop arthritis, circulatory disorders and, in rare cases, autoimmune problems (read this list for details).

The mentioned 10 feet height is based on the tallest hominid discovered yet, which only went extinct due to (assumed) shortage of food some 100,000 years ago.

Answer 4

I see no physical reason why a bipedal couldn't grow to the size of dinosaurs or at least giraffes. The bones are not a problem, mass goes up with $height^3$, but the second moment of area (to which the "rigidity" is is proportional) of a tube is proportional to $r^4$, so the bones won't break. The tubing scales well, too, dinosaurs got along with one heart, as do giraffes (which have a few extra biological tricks because of their long neck).

It's a matter of the size of the area (dwarfism of species on islands is well known) available for living during evolution, which is unlikely to be so small.

Main problem is evolution itself: Large species are longlived, so respond less well to changing circumstances, and large species have smaller populations and larger individual food demand, so there is a larger risk for them to not go through a bottleneck and just die out.

Or put it this way: If two species want to evolve into intelligence, and one becomes fertile at 10, the other at 25, it's rather clear who will win.

One other snag with height: Once you're rather clever, there is hardly evolutionary pressure to become higher. So height will have to come first, or en route. Perhaps.

Answer 5

Basing things solely on human existence that has been documented or discovered so far and adding in all sorts of mathematics and genetics on based on known earth values one could propose a relative value. It has been documented (not acceptably proved) that Easter Island had a humanoid race of inhabitants that exceeded 12 feet in height.
Based on unearthed bones it has been proposed that dinosaurs could have evolved (if not for extinction) into humanoid like creatures one can only ponder if a something like a tyrannosaurus were to evolve into a humanoid type creature. Or maybe a brontosaurus with compact body and a long neck but still humanoid.

And in todays age of genetic engineering it is only a mater of time (even if banned) that some government or person fiddles with making larger humans with greater strength and power for one purpose or another. This possibility is only countered with over population which drives global warming and would have greater impact on larger size beings.

But I could envision one other possibility that fits the earth like planet that could allow larger body style even with the earths physical loading. This would be a humanoid that is much greater percentage toward amphibian. A water planet with more shallow salt water areas could force a humanoid to become taller to stand above waters. More sleeker in body style to swim faster. With a large portion of the planet cover in swampy growth of tall trees bearing fruits but waters mostly exceeding depths at 8-10ft or more could force evolution to follow two paths grow tall or at least long necks and arms or learn to swing in the trees.