I'm creating a fantasy planet with an interior that has a god phasing in and out of our dimension, adding to and taking away mass in mysterious fashion while keeping everything but the influence of gravity stable, including air pressure. The world will fluctuate from +50% to -50% earth gravity smoothly over the course of 200 years, with humans retaining earth-like lifespans for the most part.

In order to adapt, I would like to know if the following is feasible: a gland within the female detects the current level of gravity and secretes a hormone that influences the development of the fetus. Perhaps fetuses naturally develop for .5gs early, since those bodies require less mass. If the hormone fires, the fetus continues developing until it resembles 1g humans (us), and if it achieves a certain higher concentration the fetus continues developing until it's adapted for a 1.5g environment, possessing thicker bones and muscles, etc. Gestation period would, therefore, vary directly with increasing gravitational force.

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    $\begingroup$ 200 year cycle, 9 month pregnancy, I don't think anyone would even notice. That is, evolution would have to handle the extremes across the entire span of a human life, the 9 months spent in the womb are irrelevant: 0.0075 delta-G. $\endgroup$ Jul 11, 2017 at 20:25
  • $\begingroup$ The idea behind adapting during gestation is so that the infant is born with maximum fitness. If you're at 1.5g and come out of the womb with increased bone density and musculature, you're way ahead of an infant not adapted for the gravity. The fact that the individual will have suboptimal fitness late in life is irrelevant, since evolution doesn't care what happens to you in your post reproductive years, by and large. $\endgroup$ Jul 12, 2017 at 2:10
  • $\begingroup$ Unfortunately "adaption" in this sense doesn't happen "during gestation." As for how Life would handle the bonkers situation you've thrown at it, all that we can do is make wild speculation. $\endgroup$ Jul 12, 2017 at 13:53
  • $\begingroup$ I'm all right with wild speculation :). The problem with speculating about fluctuating gravity is that everything has evolved with the assumption that gravity is a constant, so it's difficult to imagine what type of adaptations would have developed if gravity were not. Certain explanations are more plausible than others, however. Since it would make sense that infants optimized for their environment would be selected for, it seems phenotypic differentiation respecting gravity would happen in the womb. $\endgroup$ Jul 12, 2017 at 15:00

2 Answers 2


I would look at everything that changes with that much gravity change.

If you break it down by year that is a .5% gravity change every year up or down and some unlucky people at the end of a cycle feel it increase and decrease over their lifetime. It doesn't seem like a lot but people would go from 50 lbs to 200 lbs in the same timeframe if they weighed 100lbs. If people live 50 years they can expect to lose or gain 25lbs, for a 100lb person. Not a big deal. I don't think most people will require too much change for this to be handled but the longer they live the more uncomfortable they will be if it is getting heavier or more lively if it is decreasing.

Air pressure follows the same change. It will go from 45PSI to 15PSI. That is the equivalent of going from 7,000 feet underwater to 18,000 feet up. So air pressure is a real problem. More so for plant life than for animals that can just go higher into the hills if needed. In a 50-year life span that is a change of 0.15PSI a year or 7.5PSI in total. Again it is something to worry about in plant life.

There is a chance that trees will grow like crazy in low gravity years then the upper branches die off as the gravity increases. Maybe it segments the body of the tree so that it has rings like bamboo with each level mostly independent of the lower levels for when the gravity stops water from going up to the higher branches. Or maybe trees begin to droop or grow out so it is easier to get water to the branches instead of up.

So I think life on this world would have an extra sense organ like you mentioned, that has a pressure chamber that acts as a barometer. The organ can tell when the pressure is getting higher or lower and can influence live to move to higher altitudes or lower ones. Maybe a slow migration of animals over the years.

Bone density and muscle density may also be changed as you mentioned but even on Earth people born in higher altitudes have different lung capacity and increased red blood cells. So we can assume that not much change will be needed to deal with the air unless they live really long.

So I think your idea works fine. The gestation period would be the same just the result at the end would be different.

  • $\begingroup$ Thank you chaiboy, air pressure is something I neglected. To give you more details about the planet: the closer you get to the core, the more magic works, especially when the gravity is high (since there's more core, the source of magic). Also, those conceived closer to the core live longer, so you can end up living over 100 years if you were conceived low enough. So those who live in the lowlands will tend to live into their 70s and 80s, or a little more than a third of a cycle, which can mean you were born at .5g but live to 1.2 or so. If you were born adapted to low g you might have trouble $\endgroup$ Jul 12, 2017 at 2:05
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    $\begingroup$ @AdamHalatek "Closer to the core" in the scale we're talking about is virtually negligible on a sphere of planetary size. You know that thing about Earth being smoother than a billiard ball? That. Living on the tallest mountain in the world would be 0.048% farther away from the core than someone living in the deepest valley. Air pressure remains a huge problem though. When the planet has half Earth's mass, its atmosphere is going to leech into space (like Mars's atmosphere did millions of years ago) and it won't get it back. $\endgroup$ Jul 12, 2017 at 14:02
  • $\begingroup$ True, your point about being closer to the core is salient. It would be more accurate to say that the god either grows out from the core or that its concentration or saturation increases throughout the planet's layers, perhaps just all of the layers beneath the crust. The atmosphere escaping is a major issue, though. I may need a fix for that. $\endgroup$ Jul 12, 2017 at 14:36
  • $\begingroup$ What if the god "inhales" as it phases in and gravity increases and "exhales" as it phases out and gravity decreases? That way it could keep air pressure somewhat steady regardless of gravitational fluctuation. A touch inelegant, perhaps, but it would solve a few of my problems. Additionally, I now feel obliged to flesh out this god: what else is it draining from the planet? How does the planet react to its presence, if at all? $\endgroup$ Jul 12, 2017 at 15:47

No special gland required, Humans do this already

Humans already have the adaptability to handle this environment. We see it most visibily​ in weight lifters as they develop increased muscle mass and bone density but all humans do this. There isn't one gland or hormone that regulates bone density/muscle mass though for the sake of a story, your could make such a thing.

The gravity changes happen over such a long period that it is well within a human's ability to adapt. What this changing gravity will do is ensure that the overall human gene pool is capable of adapting to +0.5 and -0.5. Gene lines that can't handle the +/- 0.5 won't survive long.

If this show variation in gravity has been going on for million then the entire ecosystem on the planet has already adapted to "The Heavy Times" and "The Light Time".


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