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Can we go from this:

enter image description here

to this:

enter image description here

Suppose over millennia (or as long as it takes) humans on Earth are artificially selected by aliens to be big and healthy.

Is there any theoretical reason that they couldn't grow to be as tall as large dinosaurs?

If the proportions (thickness of bones etc.) had to be changed, what would those changes have to be?

Assumptions

EDIT for clarity (I hope)

*These are intended to be giant humans. If someone found their skeleton, they would be able to identify each individual bone that occurs in a human skeleton. Imagine an archaeologist saying, "Wow! These are humans!". They should walk in an upright position rather than being able to use their knuckles like a typical ape. Relative bone lengths should be consistent within normal human variation but bone thickness or size of joint can vary in order to carry the required weight. *

A target height would be about 26 ft (8 m) but the taller the better.

They must be able to support their own weight and walk easily and bipedally.

The giant-sized humans should still be recognisably human even if their proportions are somewhat different - for example they might be stockier or spindlier.

The aliens will provide the humans with all the food and comfortable living conditions necessary. Only the healthiest and largest humans are allowed to breed. There is no manipulation of DNA, just selective breeding for size.

This occurs on Earth and on land so everything happens in 1G.


Note

The tallest man to ever live was Robert Wadlow at 8' 11.1" (2.72 m)

enter image description here

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  • $\begingroup$ Related, but possibly not direct duplicates: (a) worldbuilding.stackexchange.com/questions/109716, (b) worldbuilding.stackexchange.com/questions/8569. $\endgroup$ – JBH Jan 26 at 19:48
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    $\begingroup$ Everyone here is arguing about semantics. Stop. I know, scientifically it's a poorly posed question, but not to the point that it's unanswerable. The question stands, "Supposing humans are specifically bred for height, how high can the desendents of humans get before ceasing to be humanoid in body shape?" $\endgroup$ – tox123 Jan 26 at 23:25
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    $\begingroup$ Wow, people want this question closed. This is the third time I've voted to keep it open. Folks, if you insist on voting to close, please leave a clear explanation as to why. Chasly, as people post those explanations, listen to them, there's obviously a disconnect between you and a persistent group of people. Everybody: don't argue in comments. Post your concern, edit the question to clarify, and move on. Cheers. $\endgroup$ – JBH Jan 27 at 18:18
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    $\begingroup$ I can't define exactly what a human looks like. Search Google images for the following: smallest person in the world, tallest person in the world, heaviest person in the world, skinniest person in the world. Their body proportions are vastly different but they are unmistakably human. I can't even say things like 'no body hair' because look up hairiest people in the world. Look at the difference between say a world champion weight-lifter and a female gymnast. Again they look vastly different but they are human. What more do you want from me? Look up human anatomy for a detailed description $\endgroup$ – chasly from UK Jan 27 at 18:29
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    $\begingroup$ @Karl - I stated in the question, " ...over millennia (or as long as it takes) ...", so literally that. If it takes millions of years, so be it. Of course the aliens may have to protect their giants from apocalyptic occurrences but they will do that. $\endgroup$ – chasly from UK Jan 27 at 20:31
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Possible, maybe, but with some caveats.

  1. They will only look vaguely human. Their proportions will be drastically different the square cube law is unforgiving. Their legs will have to be massive to hold up the weight, luckily humans are plantigrade. They have to compensate for scaling difference but they also need to account for the much larger heart and lungs. Expect legs 2-3 feet in cross section at a minimum, and they will need some serious shock absorption in the feet. The internal anatomy of the legs will have to be massively reworked to keep fluid from pooling in the legs. Giraffes have a thick inelastic structure around each leg to prevent expansion of the tissue.

Their lungs and heart will need to be much larger proportionally, the body is basically a fluid column, and the heart needs to overcome that massive pressure differential, that's why humans with gigantism have huge hearts and still die of heart failure. Their entire circulatory system is going to need tweeking, because that massive pressure going to the legs is also going to the brain, they will need some kind of adaptation to keep this pressure down in the head or they will bleed into hteir brains.

Their heads will be smaller proportionally, brains scale less than one, so bigger bodies need smaller brains proportionally to get the same functionality.

I mentioned the lungs, mammalian lungs are crap (dead end sacks). One of the reason dinosaurs got so large is they have a very different breathing mechanism, their lungs are highly efficient stiff one way structures fed by air. Your creatures lungs will have to be huge, and not just because of the amount of oxygen they need to extract, they also need to move all the air out of a much larger esophagus before they get any fresh air. This is why giraffes have such disproportionally massive lungs.

The single biggest problem however is the spine, This may be what stops them from getting to the size you want. The mammalian spine has been seriously compromised by the mammalian breathing system. Having to have a portion of spine unsupported by ribs makes for a very weak structure. dinosaurs did not have this problem. The spines for your giants will be proportionally gigantic, and even then their is going to need to be some serious changes, something closer to a ball and socket joint between vertebrae, or perhaps a bunch of interlocking joints like a hero shrew, will necessary to prevent dislocation. That will take a long time to evolve, probably more than anything else. The musculature supporting it will be equally massive, all together they may not even be able to bend their spine much.

  1. It will have some big impacts on their lifestyle. If your humans stand up from a sitting position they will likely pass out, giraffes have a similar problem because it is a drastic change in pressure. Their spine is going to be very stiff just to work, they may not be able to bend over. Don't expect them to move fast either, swinging their arms in a pitch will tear it out of the socket and rupture every blood vessel in it. Of course they will also spend a lot more time growing, expecting adulthood to take 40 years to reach would not be unreasonable. Food is not an issue, animals get more metabolically efficient as they get larger.

  2. It is going to take a long time to create them, tens of millions of years at a minimum. These are some massive changes and they will not evolve quickly. It took sauropods ~30 million years to reach massive sizes, even with selective breeding half as long (15my) is not unreasonable.

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    $\begingroup$ "They will only look vaguely human" : So, they might look like a hominid, but won't "look human" [+1]. $\endgroup$ – Pelinore Jan 27 at 19:45
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    $\begingroup$ even with selective breeding half as long (15my) is not unreasonable - I think you're greatly underestimating the power of selective breeding vs natural evolution. We turned wolves into chihuahuas in a few thousand years and we weren't even doing it scientifically. Natural evolution is really slow because it requires a significant advantage to produce a noticeable change and a lot is up to chance; by making the "advantage" distinct and controlling the chance aspect it can be accelerated WAY more than merely twice as fast. $\endgroup$ – IndigoFenix Jan 30 at 6:49
  • $\begingroup$ @IndigoFenix breeding smaller is far far easier than breeding larger. Your not just working with existing genes you need some very specialized adaptations to evolve. this isn't wolves into chihuahua this is wolves into t-rexs. Wolves also reproduce MUCH faster than humans, wolves can breed 2 years after they are born, humans take almost ten times as long, and will take even longer as you make them larger. You are also starting with humans who almost no genetic diversity to start with, that's your biggest problem, inbreeding will be much more of a problem than in normal artificial selection. $\endgroup$ – John Jan 30 at 13:57
  • $\begingroup$ @IndigoFenix Also 14,000 years plus is hardly a "few". $\endgroup$ – John Jan 30 at 13:59
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Can we go from this: to this:

Assuming we are talking about on earth & the aliens haven't relocated their breeding stock to a planet with some combination of lighter gravity & denser atmosphere.

It very much depends on what you mean by "still look human" but for 8 meters tall I'd have to say.

No

A human bipedal bodyplan is extremely maladapted for a size more than a little over seven feet tall.

Robert Wadlow had a whole host of health problems associated with his size, he couldn't walk unaided for more than very short distances, the pressure caused by all that weight on his bones & joints did a lot of damage to them & he had a serious heart condition as a result of his size as well.

And he was only eight feet eleven inches, an 8 meter human is entirely out of the question.

To go much over seven feet & hope to be reasonably functional the human body needs substantial re-engineering of its organs, the skeleton & limb structure (especially the legs) to a degree where the end result won't resemble a normal human anymore even if it was still broadly a bipedal humanoid.

In short you can't just scale up the human body by selective breeding & suppress good adaptions (for the increased size) to maintain a "normal" human appearance, while allowing those good adaptions is going to result in a very significant change in the species appearance.

They must be able to support their own weight and walk easily and bipedally.

Just scaling up the human form to 8 meters they simply wouldn't be able to walk without snapping their legs like dry twigs, not (as you specified) in earths gravity.

and still look human?

If you allow sufficient modifications to occur during the selective breeding so that they can walk easily and bipedally then there's no way on Earth you would ever be able to show me a picture of one & for me to mistake it for a human*, even if you remove all indications of scale.

*Which it wouldn't be, by then it would be a new hominid species.

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    $\begingroup$ This seems a bit of a chicken-and-the-egg reasoning. Was Robert Wadlow unhealthy because of his size or because of the ineviteable growth problems that occur for a human to reach this size? A healthy, slow genetic evolution could very well reach Robert's height with much less (if any) problems. The question is, what height would this stop? What height would mean radical changes to the human plan and thus be the limit? 8m will likely be impossible without metamaterials added, but going off the largest human with growth issue's doesnt seem to be the right idea. $\endgroup$ – Demigan Jan 27 at 8:05
  • $\begingroup$ @Demigan - Exactly. That's why I specified that they are also selecting for health, not just picking those suffering from individual gigantism. I don't want any reinforcement by metamaterials that have to be artificially added. Everything must be biological. $\endgroup$ – chasly from UK Jan 27 at 9:27
  • $\begingroup$ @Demigan : No "chicken-and-the-egg" involved he was unhealthy because of his size pure & simple, while he did die from an infection most human giants will die young from heart complications brought about simply because the human heart isn't designed to pump all that blood at all that pressure, there are also significant bone & joint problems from all that weight that are nothing to do with "growth problems" it's simply a case of the design being inadequate to support a body that size if it's just scaled up. $\endgroup$ – Pelinore Jan 27 at 16:43
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    $\begingroup$ That is the point. Gigantism isnt a full body increase that increases the body proportionally with the intent of sustaining that size, this means that the heart doesnt grow as fast and is designed more for a "small" human body, or that the vessles designed to prevent blood to flow back present in the legs to extend to more parts of the body. $\endgroup$ – Demigan Jan 27 at 17:11
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    $\begingroup$ We have humans that are big, small, irish White, african Black, evolved to handle low-oxygen at height and even humans that have much more natural bodyfat without the associated problems with the heart and the like. If the question had been "how fat could people get through selective breeding before problems arise" then going off someone with diabetes and heart problems would be wrong as we do have an essentially subspecies that can handle it. Using giantism as a role model for what if a human is larger is simply a bad choice. $\endgroup$ – Demigan Jan 27 at 20:55
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It depends on what you mean by "still look human".

Picture the difference between a deer and an elephant: the elephant has a squatter shape, thick, pillar-like legs, wide cushioned feet, and numerous other adaptations that allow it to work at such massive scales.

You could potentially get something that looks like a human to work on big scales, but its proportions will be distorted. It will be squat, muscular, and have proportionally shorter, thicker legs than a human. Although this might not be such a bad thing, after all a lot of artwork of fantasy giants and ogres have these very same distortions.

Another interesting fact is that larger animals tend to retain heat better. These giant humans will probably fare better in cold climates. If you want them to live in more equatorial regions, some other adaptations typical of large African animals may be helpful, like large ears or a tendency to wallow in mud or water.

As for how big they could get, now the speculation becomes harder to determine. But I'm going to go out on a limb and say that they can grow as tall (though not as massive) as any terrestrial animal that has ever existed, provided that their legs are around as thick as that animal's own legs. After all, a sauropod's front legs are strong enough to hold up the "slice" of dinosaur that is above its legs, so if you took a chunk of meat roughly the same size and shaped it into a human-ish form, it should still be able to support its weight at least...though balancing on two legs and walking may be a different story.

EDIT: One caveat I didn't consider - you're going to need a proportionally bigger heart to move blood to the brain. Elephants can get away with a normal-sized heart relative to their body size because their head is on roughly the same level as their heart, but this wouldn't work for a species that holds its head above the heart. Giraffes have very large hearts with thick walls for this reason. This may cause some issues if you want to stuff it into a human-sized torso. Not saying it would be impossible, but it may place additional limitations on how big an upright biped can grow.

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  • $\begingroup$ "It depends on what you mean by "still look human"." +1, & that's a matter of opinion of course, personally I think going up to the OP's desired 8 meters the end result would look less like a human than a Gorilla does, or than a deer can be said to look like an elephant. $\endgroup$ – Pelinore Jan 27 at 16:52
  • $\begingroup$ "you're going to need a proportionally bigger heart to move blood to the brain. Elephants can get away with a normal-sized heart relative to their body size because their head is on roughly the same level as their heart" more & stronger one way valves on the up-flow portions of the bodies blood supply system might help as well. $\endgroup$ – Pelinore Jan 27 at 17:10
  • $\begingroup$ Also note that the pressure in the jugular vein will go from 0 to 140 in 1.5 seconds whenever the "human" decides to tie their shoes. Not fun if you're the wall of that vein. $\endgroup$ – John Dvorak Jan 27 at 19:24
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    $\begingroup$ @JohnDvorak : The Giraffe is probably our friend there, it's already solved that issue after all so we can look to it for an answer to that one. $\endgroup$ – Pelinore Jan 27 at 19:39
  • $\begingroup$ It's not just the blood pressure in the head; these ultra-tall humans are going to have a problem with blood and other fluids pooling in their legs. Giraffes have, essentially, a built-in g-suit to compress their legs. $\endgroup$ – Keith Morrison Jan 28 at 4:59

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