Is humans' height ideal?

Question

More or less we humans are around 1.70 meters tall.

My question is, would it be possible to scale us? that is, is it in principle possible having humans 1 cm tall, or 10 meters tall or is there some reason that makes our height somehow ideal?

Answer 1

Humans can, to some extent, be scaled: While most humans are two meters tall; the human body essentially works throughout the entire range from around a meter to around two and a half meters, although you tend to run into problems at the extremes.

However, outside of that range you run into the tyranny of the square-cube law:

As you scale a creature (or, in fact, anything), it's mass increases proportionally to the cube of the scale, while its surface area increases proportionally to the square of the scale.

This means that as you scale humans down, their mass, and hence their ability to generate heat would drop faster than their skin area, and hence their ability to dissipate heat; meaning that if unaltered, smaller humans would rapidly run into problems of freezing to death.

(Small animals typically solve this through a number of methods: Higher metabolic rates, rounder bodies, and more fur. Small humans, i.e. babies also suffer under this, and solve it by having what is typically referred to as "baby fat", a blubber that, among other things, aids thermoregulation.)

Similarly, as the human body is scaled up, the opposite problem arises: The body becomes unable to dissipate heat quickly enough. Larger animals solve this problems with giant organs specifically suited to radiate heat, such as elephant ears; or through submersion in water, as seen is cetaceans and hippopotamuses; or simply through living in cold climates, such as the polar bear. (All of this is typically augmented by lower metabolic rates.)

Heat economy isn't the only problem with size, however:

Especially when scaling up, the strength of various structures in the body become problematic. Unusually (yet not even freakishly) tall humans generally have some manner of bone problems, and also often have problems with too small hearts, leading to low blood pressure, or too big ones, leading to heart failure.

If the change was gradual enough, with sufficient adaptation, this wouldn't necessarily be a problem. After all, giraffes and elephants get on perfectly well; but a pump-valve heart such as we have cannot be scaled up indefinitely.

With a continuous-flow heart, this limit could probably be stretched, but how that would even begin to work biologically I have no idea.

Further problems with very large humans include getting enough air flow through the lungs, the enormous chore it would be to stand up or sit down, and of course, just getting enough food to drive the enormous body.

The long and short of it is: Anything less than about half a meter would probably leave us freezing to death; and anything more than two and a half meters is quite frankly a stretch (if you'll pardon the pun).

(There are people alive now that are more than two and a half meters tall, but they are not generally speaking in good health, although some do get on.)

Of course, all of this is predicated on the specific gravitational and atmospheric conditions on Earth.

On another planet with a greater or lesser gravity, a different proportion of oxygen in the air, or just a denser atmosphere; many of these limits and limitations would be quite different indeed. (Which is why whales, living in such an alien world as the sea, can be an order of magnitude bigger than any animal that lives on land.)