I've created a genetically modified race of humans with an extreme height. I've taken into account the cube square law, and added some bones and muscles to help cope with that. I'm planning on making said humans 15-18 feet tall. The creation of this species is more or less handwavium. Is there anything I'm not accounting for, or is my logic not sound? Should I have made other adjustments to account for the cube square law? Is it purely impossible?
Get ready to wave those hands
How active do these giants need to be? I ask because Wikipedia's list of tallest people notes a lot of health problems. The 25 tallest people who are no longer living had an average lifespan of only 39 years. Below is a photograph of John Rogan, the second tallest man in history. He's sitting because he was unable to walk after the age of 14.
Joints, bones, and muscle take a real pounding in extremely tall people, doubly so if your story involves extensive physical activity. It's possible to evolve a structure to support that kind of weight (lots of mammals do it) but it would take some re-engineering, either through many generations of evolution or an extra dose of handwavium. You'd also have to have a heart and lungs powerful enough to supply the farthest reaches of a 15-foot body with oxygenated blood. That requires more handwavium. Shorter people also have lower rates of afib (source) and, as listed by a 2013 study, other heart problems:
When shorter people are compared to taller people, a number of biological mechanisms evolve favoring shorter people, including reduced telomere shortening, lower atrial fibrillation, higher heart pumping efficiency, lower DNA damage, lower risk of blood clots, lower left ventricular hypertrophy and superior blood parameters.
The conclusion is that you could theoretically have super tall people, but they might be riddled with health problems and have trouble being very active.
If you lower the gravity, it is possible.
Otherwise the wear&tear will cause a short lifespan (ligaments, joint cartilages, heart). Aside larger height, the bipedal position comes anatomic disadvantages:
- stagnating blood in leg veins
- higher pressure on back spine and leg joints - with the whole body weight distributed on half the number of legs of a quadrupedal
- equilibrium maintenance is sensible to aging and a range of diseases (electrolyte imbalances, ear infection)
- making reproduction more dangerous for women (narrow the birth canal)