Are there gravity-dependant human/animal physiological processes?

Evolution adapts our bodies to survive in the natural environment. Our eyes perceive the “visual spectrum” because that’s where the peak output of the Sun lies. Animals and plants have learned to use the wind for various purposes - survival, propagation. Similarly water - large or small, standing or flowing.

Given that life has learned to exploit every available natural resource, are there any biological processes that directly depend on Earth gravity to function? If yes, which are those? If no, why not?

(AFAIK, humans in space do not suffer disruption of essential bodily processes in microgravity. I am not talking about discomfort and long-term effects such as osteoporosis due to underuse of muscle groups. None of the major organs - the brain, heart, lungs - will cease to function in a zero-gravity environment.)

So what, are we destined for space travel, with dogs, cats and plants in tow?

The vestibular system that controls balance, especially the otolithic organ.

Astronauts lose bone density and muscle tone after spending significant time in microgravity, so I guess the processes involved in normal maintenance of those are gravity dependent.

I would at be at all surprised if it turned out that fetuses and babies do not develop normally in microgravity.

Recent thread: Weird question: How do seeds know what direction to grow?

Although it isn’t a physiological process internal to the human (or animal body), one of the very significant things gravity does for us is ensures that the physical vectors of disease (referred to in hygiene literature as “fomites” or “fomes”) are limited in their travel and persistence in the environment. In a terrestrial environment with normal gravity, carriers like mucus, pus, blood, excrement, and other expectorants and bodily fluids which provide protection, nutrition, and transport for viral and bacterial vectors will settle on horizontal surfaces which can be readily sanitized. This substantially limits the spread of such diseases to a manageable level. In freefall, however, fomites will remain suspended in the air or will stick to surfaces in every direction, increasing their distribution and persistence. And even debris that does not carry disease vectors is still problematic, as John Young demonstrated with his “contraband” corned beef sandwich on Gemini III with concerns about the crumbs from the bread causing electrical shorts or blocking the HVAC systems. In current habitat systems, this is dealt with by having constant air flow and filtration to remove contaminants, but it doesn’t prevent vectors from attaching to surfaces, especially where air flow tends to stagnate. The infamous black mold on the Soviet/Russian Mir station, which threatened to make the station uninhabitable prior to its destruction is the most notable example, but the general problem will remain an issue in any freefall habitat.

njtt notes the long term physiological problems with living in freefall conditions. Our longest continuous experience with people in freefall conditions is about 14 months (Valeri Polyakov) so we don’t have a comprehensive model of human tolerance and variability to long term low or zero effective gravity conditions. (Yes, I am aware that freefall in orbit is not technically “zero gravity” but as far as the subject is concerned there is no acceleration field in their orbiting frame of reference.) However, the more we learn about the response of human physiology to freefall conditions and the radiation environment in unprotected space, the more apparent it is that in order to support long term human habitation we will have to simulate terrestrial conditions to a substantial degree of fidelity, e.g. radiation protection, Earth-like atmosphere and conditions, a significant simulated gravity field. So while there are no essential physiological functions that are immediately dependent up on gravity, in the long term we are not “built” for space travel any more than we are adapted to swim in the ocean continuously.


I actually studied similar topics for a couple years in grad school, while I was tooling around for a thesis topic.

Turns out cellular development is greatly affected by microgravity. There’s a sort of “skeleton” structure of microtubules in cells that depends on gravity to develop properly, among other protein reactions that are affected.

This was somewhat controversial when it was first demonstrated, since we didn’t expect these chemical reactions to be affected by gravity.

It’s been suggested, though not proved, that this abnormality in the regular development of cells may have quite a lot to do with muscle and bone weakness after prolonged exposure to microgravity.

As noted, nobody has been in space for long enough to determine the long-term health effects of such abnormal cellular development, but the safe guess is that it’s probably not good. The more we learn, the more it turns out we are built specifically for Earth in ways we could not have conceived even 10 or 20 years ago.

No human has been in space long enough, but we’ve also done experiments with faster-growing animals in zero-G. Have any of those yielded any insight?

Incidentally, I think that is a considerable oversimplification. Of course we would not have evolved eyes if there were not a good deal of visible light around, but there is plenty of other EM radiation about that we cannot sense. I think the issue is more that it would be difficult to create detectors out of biological materials that are of a reasonable size and that have much practical use (in particular, that have much resolving power or directionality) for EM wavelengths very far outside the visible spectrum.

Not really; the Sun, a G2V star, produces light which perfectly matches the sensitivity of our eyes.

If we went to a planet in another solar system with a slightly hotter or cooler star, and that planet was heated to the same temperature as the Earth by the incident sunlight, in both cases the sunlight would look slightly dimmer than the light we see on Earth - because a larger fraction would be invisible to our eyes. To our eyes, G stars are the brightest of all possible stars, when see from an Earthlike world. That is not a coincidence.

Under a K2V star we would have almost certainly evolved to see more infrared; under an F2V we would have been able to see more ultraviolet.

I believe microorganisms growing in space increase in virulence and grow faster than their terrestrial counterparts. In fact, this article suggests some families of bacteria may actually grow better in zero-G, even with less nutrients to feed on.

Bees, birds and even plants can see or sense ultraviolet and infrared radiation for their own purposes. The Mantis Shrimp has the most complex visual system in the animal kingdom, with the ability to perceive everything from ultraviolet into longer wavelengths, including plane and circularly polarized light. Apparently nature did manage to evolve “eyes” of such complexity with organic materials, and also conferred clear evolutionary advantage to the animal.

I can’t remember where I read this, but one of the reasons animals largely don’t have infrared vision is because of interference from their own body heat. Exertion even in cold-blooded animals raises body temperature, degrading resolution significantly. Pit vipers use temperature differences, not actual photon detection, for thermal rangefinding; they don’t “see” heat, they just compute distance and direction.

I’ve read that many birds need gravity to swallow because they lack muscles to push food down their throats to their stomachs.

One thing often mentioned by astronauts is that their sinuses don’t drain in microgravity, so they have constantly blocked noses.

No not really. Peak visibility for us is not just the output of the Sun; it’s also about what makes it through the atmosphere, which screens higher and lower wavelengths than “visible” light.

I believe you are highlighting Science to a Creationist.

The Sun came first, our eyes adapted second.

i was just going to mention that. food looses its taste(smell) for them then.

Perhaps if they had simulated gravity, as in the space station in 2001: A Space Odyssey, their tastes buds might fall into line. :wink:

There have been experiments with spiders that you can see on line. Here’s one: Spiders … in … Spaaace…
If you google “spider webs in zero gravity” you’ll find other accounts.