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What would be the effects of high or low gravity on human body development? How would the human body develop under a different gravity? Many science fiction stories explore the idea that people from low gravity environments would be taller and slimmer, whereas people from higher gravity environments would be shorter and stockier. How realistic is this? How malleable is the human body? I figure that high gravity would lead to a lot of muscle and low gravity to a lot less, but Would the human skeleton really develop to be shorter in a child which weighed more since its birth, or taller in a child who weighed less? I'm interested in the range of 2 to 0 G's.

I'm asking for morphological changes on the human body that are the result of being born and raised in a different gravity. I'm not interested in evolutionary changes as a result of selection over time in that environment.

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    $\begingroup$ Related question. But only a little. $\endgroup$ – HDE 226868 Nov 7 '14 at 14:43
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    $\begingroup$ I think it is basically a dup of this question worldbuilding.stackexchange.com/questions/3181/… $\endgroup$ – bowlturner Nov 7 '14 at 14:50
  • $\begingroup$ @bowlturner The answers to that comment mostly discuss what life would be like having evolved on a planet with high gravity. I am interested in the effects on human body development of being born in different gravities, such as on the Moon, Mars, or some other planet. $\endgroup$ – Mike Nichols Nov 7 '14 at 14:54
  • $\begingroup$ it's also similar to this one worldbuilding.stackexchange.com/questions/480/… But I agree this does seem to focus a little different than all three $\endgroup$ – bowlturner Nov 7 '14 at 14:57
  • $\begingroup$ I voted to close this question as a duplicate of the question that the first close voter suggested (worldbuilding.stackexchange.com/q/3181/2072), but I think it can be saved if this one only focuses on low-grav, cuz that one focuses (I think) only on higher gravity. $\endgroup$ – Shokhet Nov 7 '14 at 15:33
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We actually have very little information on this question. I'm assuming from your "not interested in evolution" phrasing that you're interested in a 1 generation case, where a baby is born in low gravity and stays there. NASA is very interested in questions like this, because it helps them deal with the physiological effects of space on astronauts.

The answer is really complicated because the human body grows in response to stimuli. However, not all stimuli are gravity related. Some of our spinal growth after birth is due to gravity. However, bed-ridden children still grow to reasonably normal heights, so there are clearly many other factors wedged against each other to support spinal growth. (This would change if you opened the question to evolutionary effects, but in 1 generation, its a bit simpler).

The real issue for human growth in space is the few cases where we need gravity to develop. It is known that our hearts atrophy in space because they don't need to pump as much blood. It is not yet known if, in that atrophied state, it is strong enough to raise a healthy child. For all we know their immune system could be stunted because it didn't get enough oxygen! These unknowns are why we don't allow anyone but fully fit, grown adults into space. Our society believes it is not worth the risk.

If you want humans to be taller due to low gravity, I would look at what could cause a human to want to be taller. Look at gymnasts and contortionists. I guarantee nothing they do was ever "planned" for by genetics. They simply want to be more flexible, so they put their body into situations where it can grow into a more flexible shape. If there were strong advantages of being tall (such as reaching fruit of local fauna which grows tall and slender due to gravity), I think you would find remarkable effects, especially in the 1-3 year old region of a child's life. That would go doubly so with a few generations of social evolution, to build a family structure that raises children to want to be tall from a young age.

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Sheesh, he's not interested in a multigenerational evolutionary hypothetical here.

enter image description here

Doesn't anyone read?

Likely humans born in high gravity would develop stronger hearts, lungs, and musculature. Bones would likely be thicker as well, if not caused by developing in high G then from healing from constant breakage. (Falling would be very dangerous on a high G planet) Children would learn to walk slower and may be significantly shorter, no one would ride bicycles, and planes and off planet trips would be very expensive fuel wise.

Low G humans would likely lose the ability to survive in a normal G environment due to atrophy or, in children born on the planet, lack of development but it's doubtful that they would be rendered unable to survive in their own environment. Children would all ride unicycles but would be very jealous of the off-worlders who could fly pedalcopters. Depending on the lack of gravity low cost compressed air jetpacks may be available and people would not need to worry about falling from (again depending on the gravity) almost any height. Expect to see people safely disembarking from air transportation without needing it to land.

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    $\begingroup$ Second-to-last paragraph should be "Likely humans born in high gravity ..."? $\endgroup$ – user253751 Jan 18 '16 at 6:57
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(this is my own theory related to a story line I'm working on, very unsure on it's validity, but it involves multiple generations during a prolonged space flight in a zero g environment...was actually going to post it to this forum eventually for input). I'm also working on the assumption that critical functions such as respiratory, circulatory, and digestion can continue to function in different gravities.

Assumptions...not only is it zero-g, but we are lacking sun exposure as well. Air pressure is earth like in the zero-g environment.

The human body will lengthen significantly if raised entirely in a zero g environment. Limbs will become longer, including the neck. Bone density will be significantly less and it's quite likely this being will weigh significantly less than a Human raised on Earth. Bone structure (although elongated) won't change in a single generation...however multiple generations in zero-g may start to see their bones loose rigidity, become flexible structures instead. Several generations later may result in a elongated semi-eel like being that can quickly bend around curves and corners of the ship/structure it resides in.

Hands could have an interesting change. Where a human on Earth needs to grip and pick-up things against the weight of gravity, and zero-g human would not...rather they would be spending most of their time on 'button' like interfaces. Gives me this image of elongated fingers exceedingly adept at a 'key press' up and down motion instead of a bulky hand designed to lift against the weight of gravity. Precision in a zero-g human would become more required than strength (precision often being sacrificed in favor of raw strength) and the hand will reflect that.

Muscle structure would also develop significantly differently. Where our strongest muscles (Glutes) tend to be focused on keeping us upright, a space child wouldn't have that same need to develop the ability to stand. Nor would walking muscles (Quads) have the pressure on them to grow to the strength a human on Earth's would. Pushing off would be the preferred locomotion of a zero-g human, perhaps giving them larger calf muscles in relation to their other muscles (yet still not as developed as one that needs to fight gravity on a constant basis.

Skin colour would also be significantly different as the pigments that protect us from the sun wouldn't be required. Don't ask why, but I have the image of eventually greying skin, especially over multiple generations.

We may develop a new 'sense' or a variation on one that we already have but don't much consider. Orienting yourself on Earth is a 2-d exercise for the most case...up and down are in relation to gravity and is easier to enforce. As divers and deep sea swimmers know, up and down isn't quite as simple to detect without this obvious gravitational pull. How exactly a zero-g human would learn to orient itself and perceive it's 3-d location could be considered a 'new sense' to some degree.

All of this is purely speculation as we really have no idea what a zero-g human would look like.

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Consider a 200 pound man, here on Earth. If he were to set foot on a planet X with 2X the gravity, he would weigh 400 pounds. Consider the endurance, momentum and other physical factors on this man, having to lug around 400 pounds! Conversely, if he were to step on a planet Y with 1/2 the gravity, he would weigh 100 pounds. For life to form on either of those two planets, further evolving into some sort of intelligent life form, it is likely the reverse of many of the comments would need to be true. Additionally, we need to stop thinking of those alien life forms as humans or on human terms. On planet X, this individual may very well need to be smaller and lighter than humans, yet with a greater ratio to muscle mass. On planet Y, far less muscle mass would be needed and this individual would likely be much taller, as gravitational forces would not place as much strain on his or her skeletal structure. This may answer some of the basic physical characteristics, with regards to overall size, but a plethora of other factors would need to be considered.

What is the atmosphere like on the planet? This will affect how they breathe or even derive energy.

What chemicals make up that atmosphere? Heavy oxygen? Heavy CO2? Limited nitrogen? All these can contribute to what types of early life forms are given the extra advantage to develop further.

How bright is the midday sky? This will affect their vision and the shape and size of their eyes.

How long are the days? This can have a great affect on their routines and adaptations to such cyclical internal mechanisms. Consider humans an melatonin, the enzyme responsible for our sleep cycles, as just one example.

How long is the year? This may or may not be as relevant, based on the inclination of the planet. But if seasons are pertinent to agriculture, this could have a positive or negative effect, as we know it. As a result, the greater the variance, the greater the adaptivity, in the evolution of the alien species.

How hot or cold are the average temperatures? This would result in more or less average body fats, amount of hair or some sort of protective dermal layers. It would also have an affect on overall size and mass of extremities, as well.

What is the ratio of land versus water? Again, we are talking adaptation to environment. Low water resources would equate to a decreased need for water. Low land resources could equate to a greater adaption for amphibious or aquatic living.

What was the progenitor of the species? In humans, the great apes and humans shared a common ancestor. It is very possible, their life forms may have evolved from some form of plant-life.

In order to give a fairly detailed description of some form of alien life, all of these questions - and many more - would need to be answered. The most difficult of all these question would definitely be the last question. Sure, we, here on Earth, may be able to determine: average temperatures, day/night cycles, length of a year, inclination of the planet, degree of brightness, atmospheric properties, mass of the planet, gravitational force, percentage of land to water, etc... But... until we can answer, "What was the progenitor of the species?" the best we can do is take a wild guess!

{Ph.D. Astrobiology, Cambridge}

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Evolutionary changes would be the most significant here. Look at deep sea fish, they live in an environment that would crush shallow water fish, yet, they proportionally look the same as shallow water fish. Adaptability is paramount here.

If the present human form is the best suited for our gravity, I would suspect that a similar form would evolve to take advantage of a low or high gravity environment. Just too many neck problems otherwise for the low grav person, and too much of the waddle factor for the high grav guy.

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I think humans would get stronger. Their bones would be thicker, and so will their muscles. So let me say someone from a planet with 100,000,000g came to earth they would be a lot stronger and could lift weights ranging to the megatons with the same amount of effort he/she lifts 100 pounds on their planet.

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I've been writing a fantasy novel and trying to justify the existence of giants, I'm gonna go with 'an inherited magical power to make things much lighter'.

Seriously though.. a low gravity environment, IMO would raise much larger mammals, a size that, when eventually exposed to higher gravity, they'd adapt to life and just be much bigger an stronger.

2nd and 3rd generation in low gravity would be larger still as the embryo size would increase with its mother.

Our stance on this really depends on whether you believe the animal kingdom is in any way evolved to deal with different gravitational environments.

That belief hinges on how we think gravity works on earth and in it.

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