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What genetic traits would be needed to have a human that is "normal" in all aspects, but roughly 50 to 80 feet tall?
This doesn't seem completely out of the realm of possibility considering some whales can be 10x larger than other species of whale. What would prevent a line of humans from giving birth to gradually larger and larger children?
This question is part of the Anatomically Correct Series.
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 such a size, especially not one of human descent. How big your giants can get depends on how human 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 I believe they were tripedal, having a load-bearing tail. If an alternate evolutionary path is acceptible for you then your giants could have similar tails.
For giants humans to exist, they would need a series of adaptations:
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 would reccomend keeping your giants under 4 or 5 metres as the largest prehistoric mammals never peaked above that (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.
Welcome to the tyranny of the Cube-Square law. It goes something like this
Mass increases proportional to the cube of an objects size
Strength increases proportional to the square of an objects size
It comes down, fundamentally, to muscles. Our muscles are bundles of fiber that can expand and contract when ordered. The more fibers you have, the stronger you are. A man with strong, bulging muscles has lots of muscle fiber running the length of a muscle, where as a skinny man has only a few. To pack in more fiber, I need a larger cross-section (A larger planar area, thus the square) to fit the fibers in. If I increase the cross section of a somehow square muscle from 1x1 to 2x2, it has 4 times the strength.
Total mass is dependent on volume. If I Increase the dimensions of a 1x1x1 cube to 2x2x2, its volume is now 2x2x2 (8! 8 Whole times!) times larger, hence the cube in cube-square law. Mass can be treated as volume * density, so an object twice the total size weights 8 times as much.
Now, ignoring all the issues of circulation or bones snapping under their own weight, a giant that's a modest 3 times the size of an average human would have 9 times the strength, but 27 times the weight. It's doubtful he would even be able to stand.
Place them on a low-gravity world.
It's definitely possible to create anotomical structures on the size scale of a giant. Large therapods had legs of the right scale for such a beast and some, like therazinosaurus, even had arms approaching those that a proportional giant would have. However, it's probably not possible to create an anatomically correct giant of that size that lives on Earth. The biggest problem with having a 50 to 80 foot tall humanoid is blood circulation. Unfortunately, there probably isn't much you can do about blood circulation while remaining 'humanoid'.
Looking at the lower end of your range, around 50 feet, we encounter sized at which your giants are about as tall as a dinosaur like Spinosaurus is long. However, Spinosaurus and other gigantic carnosaurs had mostly horizontal bodies, with their heads about level with their torsos and hearts. Sauropod dinosaurs also follow this general body plan, though some probably raised their heads to some degree, at least 20 feet or so above their hearts.
The easiest way to get around the problem of blood pressure from being tall is to weaken gravity. On a planet with only .25 g, for example, pressure differentials for a 50-foot high giant would be similar to those faced by a 12-foot high humanoid, which is well within the realm of possibility.
Beyond that, you should expect your giants to have proportionately shorter limbs, since long limbs would be effectively more fragile for a huge creature, gigantic barrel chests to contain proportionately larger hearts (blood pressure is still a problem, even with our lower gravity, just not as much of one), and proportionately thicker arms and legs.
No, it is not possible. Not even remotely. At least not with Earth mammalian biology being what it is. Bones can only get so big. But hey, we had some pretty tall dinosaurs walking around, so why not a giant human?
It comes down to joints, blood circulation, and a bunch of other fun stuff like that. Basically, once you reach a certain size our basic biology just can't keep up anymore. An 80 foot giant would swing an arm, and snap its bones in half, as the muscle power required to move its limbs would be greater than the strength of its bones.
If we were devising a different creature, with multiple hearts, a different circulatory system, etc. it might work out, however. Some of the earlier, massive dinosaurs are believed to have had 2 hearts anyway, so it can happen.
If you look at the history of our world in fossils you will notice that even the dinosaurs got progressively smaller as we get closer to modern times. Same with the megafauna that dominated our planet a few hundred thousand years ago. The more complex our biology became, the smaller we evolved to be. This is not a coincidence. Even blue whales, whose weight is suspended by the oceans they live in suffer from diseases related to their size by the time they die.
At best you are likely to reach 15 feet, but what we end up with, does not look human.
