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I totally get why humans who have colonised a low gravity environment (Mars or an asteroid, for instance) are skinny. Muscles doing less work, square cube law, force output of a muscle proportional to its cross-sectional area, etc, etc.

But I don't get where the trope of them being tall comes from? Why would biology bother to go to the effort of making longer bones, when you could save all that calcium and phosphate for something else? Like when person X is growing their bones to 7 foot tall, person B instead uses the resources to build their first baby's skeleton. So to me, a low gravity person with normal sized (but thinner) bones and the expected skinny muscles seems a far more sensible solution for natural (or artificial) selection to have produced.

What have I missed? Any clues gratefully received!

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  • $\begingroup$ I would take into account what people would wear. In a lower gravity environment people could literally wear rocks as clothes in an armor fashion. So with heavy clothing having longer limbs could make it easier to use material that doesn't bend so well. think of a slinky. limited bending related to its length but when u stretch it out u have near limitless flexibility the longer you go. My personal opinion of low G height related instances would depend on atmosphere. If people lived in a dome because there was no livable atmosphere on the planet that would restrict space on the planet and peop $\endgroup$ – Sarfaraaz Jun 6 '16 at 7:31

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Considering that all those lower-gravity environments have artificial life support, which (presumably) is kept at a nice, even temperature, then if we presuppose that the temperature is slightly warmer than what a human on earth in the natural environment experience (in other words, nobody ever freezes or feels chilly), then perhaps some observed phenomena come into play:

  • Bergmann's rule, which states that humans (and other animals) are heavier in colder climates,
  • Allen's rule, which states that in warmer climates body surface increases for the same mass - which fits the tall, long-limbed, slender, low-G-er well.

Then, considering that food in an artificial life support environment may be a resource that is under pressure, a more efficient metabolism for a given mass may be a desirable trait:

  • Kleiber's law seems to state that larger individuals have a slightly more efficient metabolic rate than smaller ones. (The graph in the article linked also has surface area in it, as per Allen's rule.)

At this point I ran out of imaginative ratios to google. There might be others. E.g., in chemistry, a couple of centuries ago (<-- this is irony - was long, long ago at any rate), I was taught that Alpine people were shorter and stockier due to a lower oxygen level at high altitudes - but for all that you know that might simply be genetics.

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Update on oxygen levels in the artificial habitat (thanks, Drunken Code Monkey, for the comments): seems that a slightly increased oxygen level might also lead to better endurance, bigger humans, and an increased fire risk, perhaps toxicity at a certain threshold. It seems that the partial pressure of the atmosphere is also important. See e.g.:

I could still not find anything related to the height or body size of populations living in high-altitude/low-oxygen environments, although other adaptations are well known:

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    $\begingroup$ Kleiber's law is suspicious. An individual slightly larger than another might be "slightly more efficient" at utilizing all his energy intake, but A) he needs more energy intake and B) is LESS efficient because he consumes more resources for the same amount of work as the slightly smaller individual. Also, the size of life on earth has also been historically directly proportional to the oxygen ratio of the atmosphere. More oxygen means bigger creatures. $\endgroup$ – Drunken Code Monkey Jun 6 '16 at 1:20
  • $\begingroup$ I actually agree with you @Drunken Code Monkey. This would probably require that any extra mass would be muscle, and that it (in conjunction with the low gee!) makes the person considerably more efficient in performing work. Also interesting your statement: More oxygen means bigger creatures (any reference?). $\endgroup$ – fr13d Jun 6 '16 at 5:32
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    $\begingroup$ Check here: sciencedaily.com/releases/2010/12/101217145647.htm . $\endgroup$ – Drunken Code Monkey Jun 6 '16 at 12:21
  • $\begingroup$ Longer limbs = more leverage = greater efficiency runnersworld.com/sweat-science/… $\endgroup$ – called2voyage Jun 6 '16 at 21:29
  • $\begingroup$ I would actually think longer limbs may lead to worse leverage in some ways. Think about it this way: it's easier to swing a two foot sword than a ten foot sword of the same mass. $\endgroup$ – Dan Jun 6 '16 at 22:59
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I think this may be a combination of some natural effects of low gravity and an opposite of the "Heavy Worlder" trope.

For the first; astronauts actually do "grow" when spending time in Zero G. True, this is just a matter of the spine expanding a bit (just 2" or so total) in the absence of gravity, but it would be easy to extrapolate "low gravity" to "taller people".

For the second; heavy gravity lends itself to a thick and short build to withstand the gravity and to make falls shorter. This would also be easy to extrapolate "low gravity" to "taller people".

These beliefs both lead to tall thin light-worlders whether or not it is actually realistic.

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  • $\begingroup$ yeah I'm pretty sure some science fiction author back in the day misinterpreted 'astronauts grow about 2" in space' and ever since then lunar colonists have been like seven feet tall. $\endgroup$ – Tacroy Jun 6 '16 at 15:59
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    $\begingroup$ My friend says that Out of the Silent Planet by CS Lewis, published in 1938 has tall low-g people, so that pre-dates real space travel by quite a bit. Though he does point out those people are aliens rather than native to Earth. $\endgroup$ – DrBob Jun 7 '16 at 7:38
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The usual logic for such tall low-gravity creatures is that we tend to assume taller is better, and low gravity removes some very real shackles which make tallness unstable.

I think one of the most fundamental advantages of such tall creatures is the ability to put sensor apparatus such as eyes and ears and noses far from the center of gravity of the creature. It takes a lot of energy to move the center of gravity of a creature, and that movement is often a commitment that predators can identify and act on. The ability to move a small sensor ball a distance, observe what the world looks like from that vantage point, and then decide to move is quite valuable.

A related trope would be that low-gravity worlders would move slowly and gracefully. This is highly related to the extended size of the creatures. Neuron transmissions are slow. They top out around 120m/s. The bigger one is, the slower the reactions must be unless one distributes those reactions (similar to how our patellar reflex, aka the "knee jerk," is handled in the spinal column, rather than the brain). However, this trope may have some truth to it. In lower gravities, sudden movements can be more dangerous. You can shift your momentum in ways that are hard to arrest. Thus, slow graceful movements may be valid.

(This forms a tail-chasing loop in movies. One easy way to make creatures feel slow and graceful is to make them very tall, because it is very hard to walk in a non-graceful way if you're tall in 1g, and most filming is done in 1g).

However, if we consider fast movement for a bit, there may be an advantage to long appendiges. The further your mass is from your center, the higher your moment of inertia is (a measure of how much you oppose changes in rotation). Control over moments of inertia is key to maintaing stability in high speed low gravity environments. A spine, in particularly, is very good at controlling moments to cause remarkable movements. Nowhere is this more obvious than a cat turning itself right-side-up when it falls. Cats do some really clever tricks with their spine to allow them to control their rotation to a remarkable degree. A creature with long appendages, such as arms and necks could do the same. In low-gravity combat, this could be a make or break skill.

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  • $\begingroup$ "it is very hard to walk in a non-graceful way if you're tall in 1g" youtube.com/watch?v=IqhlQfXUk7w $\endgroup$ – user2390246 Jun 6 '16 at 9:37
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    $\begingroup$ Why would you have to move slow and graceful in low gravity? Only walking/running in very low gravity with low friction might be dangerous because you can’t stop/swerve fast (or e.g. jerking your arm up fast because it might send you flying). $\endgroup$ – Michael Jun 6 '16 at 10:15
  • $\begingroup$ @Michael Walking/running makes up quite a lot of our motion as an upright bipedal. It's also not just the obvious can't stop/swerve fast. It also affects our ability to do corrections. The decreased friction means we have less lateral control, so must be more precise. This precision also typically means you need great integrated control over many muscles moving together at once, such as integrating the spinal movements with the legs. For an excellent example of this, find a dancer and an ice rink. Both of you try to walk on the ice in sneakers, and watch each other's motions. $\endgroup$ – Cort Ammon Jun 6 '16 at 14:32
  • $\begingroup$ Predators?!! How the *** are you ever going to get people in a low gravity environment threatened by predators. Your not going to pack some bears or lions when going to mars are you. And even humans with stone age tech are enough of a threat that bears and lions will avoid them. Predators go for creatures unable to defend themselves. $\endgroup$ – Donald Hobson Jun 6 '16 at 17:52
  • $\begingroup$ @DonaldHobson you should read Larry Niven's The Integral Trees. $\endgroup$ – Tin Man Jun 6 '16 at 19:02
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Being taller gives you a whole set of advantages:

  • You can easily reach to fruit produced above ground (fruits, leave)

  • You get a way better field of view.

  • Assuming your members are proportionated, you can move faster.

Of course, these are offset by two disavantages:

  • Weight increase, which means your body constitution must be harder than that os regular humans (and again, another probable weight increase due to the sturdier constitution). And, with weight increase, a probable need of more food.

  • The need to pump blood to your brain. Many human who are abnormally tall die young because the heart just cannot maintain the increased stress of pumping blood to the head. Again, a solution to it (with improved heart) will need a heavier heart, a reinforced circulatory system (to support the increased pressure) with additional weight, plus the additional energetic cost of moving the more powerful heart.

The combination of the pros and cons for every animal form and ecological niche will give the aproximated optimal height of the species.

Now, if you weaken gravity, the cons suddenly become way less restrictive, allowing the optimal combination to result in a taller species.

But in low gravity systems, those two issues are lessened, so it allows

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  • $\begingroup$ The 'reach' benefit of being tall is also less important in low gravity, since climbing becomes much easier. If you measure gravity using mm instead of metres it will seem much lower, and indeed ants can climb up a wall almost as easily as they can walk along a floor. $\endgroup$ – bdsl Jun 5 '16 at 19:06
  • $\begingroup$ As an old soccer player, I'd add torque to your list of disadvantages. Taller people in my experience are much more likely to have joint (knee and ankle) problems, particularly as they age and gain weight. If the weight was less of an issue, you could support a much taller person with the same joint designs. $\endgroup$ – T.E.D. Jun 6 '16 at 15:59
  • $\begingroup$ @bdsl That is a good point $\endgroup$ – SJuan76 Jun 6 '16 at 20:35
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So to me, a low gravity person with normal sized (but thinner) bones and the expected skinny muscles seems a far more sensible solution for natural (or artificial) selection to have produced.

As a general rule, increased height of humans who colonize low-gravity is produced not by natural or artificial selection but by reduced gravity. The hypothesis is that increased gravity makes people's bones grow shorter; reduced gravity allows them to grow longer.

Of course, we've never tried it with an actual human being. We don't send people into space until they're already full grown. We haven't even tried it with animals. We still send them from the ground and then take them back. We don't try raising them on the space station to see what happens.

It would take a long time for any slight advantages from reduced dietary resources to show up via natural selection. And you also might consider the possibility of American diets. If space is colonized by people who overeat, there is no benefit from taking fewer calories during childhood. Otherwise, everyone would be four feet tall rather than being mostly in the five to six range.

Evolution isn't smart. It doesn't do careful consideration of all options. It tries solutions randomly. Any that work can prosper. That's why there can be long-necked giraffes and shorter-necked zebras in the same area. It doesn't rely on sensible solutions, merely successful ones.

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    $\begingroup$ Exactly! All the answers about why being tall is better in low gravity are missing the point. Humans didn't evolve in low gravity, and if the story isn't playing many thousands of years after the colonisation, possible benefits just don't play a role. $\endgroup$ – BlindKungFuMaster Jun 6 '16 at 8:16
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First, I think writers like Niven and Smith before him found it obvious without question that a heavy-gravity person would be short and squat: Smith wrote of the Family D'Alembert starting in 1963, who were adapted to a 3G world. Later Niven (perhaps an homage) wrote of people from a 3G world a decade layer. Not only are they similarly described physically but their great strength is used a a plot point. Since the specific "3G" is also the same, I would suppose that Smith influenced Niven.

Niven, in the same Universe of stories and then in related but distinct works, wrote of people either adapted to or born in low G. Literarily, it made sence to make them the opposite: people adapted to 3G are short and squat, so people adapted to 3/5 G are very tall.

Literarily, it makes sense to have people from different planets look different as well as have different cultures, just as we are used to today with diverse populations from across the globe. Other traits are chosen sometimes arbitrarily, and we suppose they may be due to founder effect: in Niven's example, the Crashlanders are prone to be exceptionally tall albinos. The latter is not a developmental trait caused by the environment!

But, the traits that do take over the population might be permitted due to environment. Founders who had the albino gene did not find it a disadvantage because they lived underground without natural sunlight, anyway. Familial tallness was in the founding population, and was less of a disadvantage on the low-G world, so it flourished moreso than it would have on Earth.

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To expand on Michael Richardson's answer, bear in mind that evolution only operates over huge numbers of generations. Modern humans are taller than we were a hundred and fifty years ago, and the difference is pretty much all down to better food as children http://www.bbc.com/future/story/20150513-will-humans-keep-getting-taller.

In a lot of science fiction, the colonists have only been where they are for a few thousand years. That's still not many generations on an evolutionary timescale. However, it's plenty of time for spines to expand. It's also possible that it's easier to grow in low gravity, so people who are there from infancy may grow more than people who arrive after they have finished growing.

Balanced against this, exercise is pretty important to skeletal growth. If "low gravity" means "floating off the walls", then these people will need some serious exercise regimes.

[Bonus section: the effects of height on probability of reproducing are probably pretty tenuous anyway, and thus the relevant author can make it a positive or negative thing as they feel like. Not many people are likely to die of starvation, or of falling over too many times, before they reproduce. If they spend more on food, they might have less money for other things, or work more. They might tend to do manual labour more. But that has a really ambiguous effect on reproduction, likely affected by society's structure in that particular setting.]

Edit: the article I linked also mentions that colonising a new place is going to be stressful and that will make people shorter. This will wear off, though, once the colony gets nice and settled.

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Consider that low gravity is synonymous to "nearer horizon", for a given planetoid BMI. Or more familiarly, low gravity generally comes from smaller planets with nearer horizons. Thus giving an advantage to tall creatures with eyes as above described, not just further away from their center of gravity, but also further away from the planetoid surface enough to expand their horizon. The advantage of course may be minimized as the horizon is extends further outside the range of "clear far vision". And so the reverse consideration is that lower gravity planetoids might discourage (and reversely higher gravity planetoids might encourage) evolving to enhance "far vision", due to lesser need/benefit, for those who can already see clearly to the "nearer" horizon (or reversely can't already see clearly).

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Maybe the trope about people raised in low gravity being tall is not about being tall for a particular purpose, but about growing taller because that is just how growth under low gravity works given the genetic makeup their ancestors brought from originally evolving in an higher-gravity environment

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    $\begingroup$ so your example is the assumption that our genetic makeup is built for defying gravity therefore having lower gravity ends in greater height growth? $\endgroup$ – Sarfaraaz Jun 6 '16 at 10:26
  • $\begingroup$ Much of the low gravity tallness I've read doesn't wait long enough for evolution to come into play. The first generation born on Mars is taller. The idea seems to be bones can become longer when they don't have gravity pulling them back. Much as a helium balloon goes higher when tied to a longer string. Whether that makes any biological sense or not. $\endgroup$ – Jontia Apr 11 '18 at 12:41
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Disclaimer: out-of-universe answer:

I think it's mere extrapolation of Earthen observation that humans are getting taller generation after generation. Basically we don't understand why taller is better, but our experience shows us it is.

An intuitive understanding is that in low-gravity environment the costs of being tall are smaller, so the above trend will be exaggerated.

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I kind of assumed it was a side effect of growth in low gravity... That is, normal growth represents a balance among biological and external forces including gravity, and lower gravity could allow the bones to overgrow, becoming long but perhaps fragile. But I may be crediting Niven with scientific prescience. :-)

(On the other hand, the 3G build was explicitly the result of genetic engineering.)

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