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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?

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    $\begingroup$ Give those giants elephant legs like in game of thrones and bones as thick as whale ones and it's fine $\endgroup$ – user75689 May 19 at 8:59
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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.

Photo of John Rogan

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.

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  • $\begingroup$ Maybe he was unable walk because he was all skin and bone $\endgroup$ – user75689 May 19 at 9:02
  • $\begingroup$ @Kyu Tall people are usually skin and bones because their Basic metabolic rate is very high. A 240 cm man need to have much more muscle weight than 190 cm one. And need to eat accordingly. So not a 3k kcals meals but rather in the 8k kcals range (or even more). I doubt that in "ye olde day" eating that much of calories a day would be an easy task. Both time and financial consuming. $\endgroup$ – SZCZERZO KŁY May 19 at 13:14
  • $\begingroup$ @SZCZERZOKŁY this is a limiting factor only in poor countries, first world people have no trouble consuming diets of up to 10k or even 30k kilocalories a day, basically up to 15 times the average dude. Obese people, Strongmen athletes and Sumo athletes are a good example. Appetite is genetic but can be built up with time and adapts to weight, people starving to death tend to never be hungry. $\endgroup$ – user75689 May 19 at 13:24
  • $\begingroup$ @Kyu ummm "In 2017, 40 million people struggled with hunger in the United States". Yes, the strongman are good example. Halfthorn need to spend time and money to have that 10k kcals a day. And those calories need to be properly proportioned. Strongman ans sumo are good examples. They eat to perform and suffer a lot of problems that goes with it. IT's also very small portion of people. If you scale that up (to all people having 250 cm) you discover that 70 millions of obese (in USA) are eating enough but there is much more people who don't. And are skin and bones. $\endgroup$ – SZCZERZO KŁY May 19 at 13:34
  • $\begingroup$ @SZCZERZOKŁY which means that those people in skin and bones would just die out and the examples I made would replace the gene pool... Evolution works just like that, it doesn't care about poverty or people dying. $\endgroup$ – user75689 May 19 at 16:08
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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)

Example: in the 18' (=6m) you have the giraffe, with an average life span in wilderness of 25 years even if they can reach up to 38 years.

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