I’ve always wondered why we are able to survive in no/low gravity environments. With gravity such a present, constant force, why have we evolved systems that aren’t dependent on this force? I know there are health problems for long term exposure in micro gravity, but I’m wondering about being able to function long enough for those effects to even set in.
Probably because like most animals our bodies have to cope with our bodies being oriented in a variety of different positions within Earth’s gravitational field. If there was any biological system that required gravity to function it would stop working when we fell down. That seems like an invitation for disaster.
Our vestibular system requires gravity in order to determine which direction we have our heads oriented. However, we can compensate for a dysfunctional vestibular system visually. So provided you aren’t trying to do work blindfolded in low gravity…
Plus, many of our internal organs and biological process evolved while we were still in the water. Gravity isn’t much of a factor there-- except that it’s helpful to evolve so that it’s no factor at all, and you can truly operate in a 3-Dimensional world.
Plus, gravity isn’t really a particularly strong force in comparison to capillary forces, adhesion, peristaltic pumping, et cetera, and in fact most dynamic systems in the body have to work against gravity at some point. This isn’t to say that there aren’t adverse reactions to free-fall–aside from the long term health issues there is also the nausea, vertigo, bloating, histamine misregulation, et cetera–but they don’t happen to be ones that have a fatal impact on humans, and in fact I can’t think of any animal offhand that would be seriously threatened by short-term exposure to a microgravity environment.
Evolution, or rather, the theory of Natural Selection tells us that environmental inputs cause more ultimately reproductive characteristics to be preferred over others, but it doesn’t negatively select against characteristics that “aren’t dependent” on environmental conditions unless those characteristics are otherwise significantly detrimental to reproductive success. There is no teleology, no goal, no intentionally filtering out of unneeded capabilities in the selection process.
Stranger
As far as most internal organs are concerned they might as well still be suspended in water, being as they are completely supported within the peritoneal greater (mesenteral) and lesser (omental) sacs. Being submerged in water helps support limbs and excess adipose tissue, but these don’t have any function that would conceivably be regulated by gravity.
I’m actually kind of curious, now, what “evolved systems” he thinks could plausibly be dependent upon gravity to function. The only one I could think of would possibly be the digestive system (assuming you can straighten out the curves) but anyone who has suffered a bout of constipation would definitely agree that one would not care to leave such an operation to gravity alone.
Stranger
Just because we evolved to survive under certain conditions, doesn’t rule out our survival under some different conditions as well.
Who says we don’t have any bodily functions relying on gravity? You try drinking a gallon of milk while hanging upside down from your ankles and get back to me, bud.
Notice: Do not try this. Trust me.
Cite (preferably video)
Si
It’s not a gallon of milk, but I think it demonstrates the point.
Doesn’t our sense of balance depend on gravity? Not the visual system we can use, but the part of our auditory system.
Not a gallon of milk, but as a child I tried drinking a glass of water while hanging upside down. It can in fact be done, though with a little spillage.
1.)
You obviously haven’t done the experiment where you hang upside down and eat a banana to get you to understand Peristalsis (or your winky smiley is just saying you do know and are playfully ignoring it)
2.) Some tiny creatures that live in the water are so little affected by gravity that they use the variation in local electromagnetic fields rather than gravity to determine “up” and “down”. Stephen Jay Gould wrote one of his Natural History columns about it.
3.) Some body functions certainly use gravity – excretion and birth both use gravity to help (If we didn’t assume the poop would fall away from us, we’d be built to forcibly eject it with some velocity, an evolutionary possibility I’d rather not dwell on. We’d have to redesign airplane toilets.)
4.) There are plenty of examples of evolution working to take the effect of gravity into account. Goiraffes require high pressure to get blood up that lomng neck to the brain, but they have to still be able to bend that head down to drink without causing serious pressure problems when they do so. Hence the giraffe has the “miracle net” of blood vessels that expand to take up the excess when the head is lowered.
You are talking about the vestibular system. While one of the purposes of the vestibular system is to help determine the orientation of the local gravitational field (and hence allow you to stand upright) it doesn’t do this alone but rather in conjunction with proprioception (a sense of the body’s orientation from internal nervous feedback about the muscular system) and visual cues. As experienced SCUBA divers know, it is very easy to become disoriented in midwater without visual cues due to the lack of tactile feedback to provide a basis for gravity normal. (Note that, while you are suspended in midwater, you and your utricle and saccule are still fully in the grip of gravity, and yet are not sensitive enough to determine gravity normal on their lonesome). The primary operation of the various organs of the vestibular system are actually to determine differential movement, i.e. changes in acceleration rather than a constant acceleration, and it’s ability to determine differential rotation isn’t really dependent upon gravity at all. So, in microgravity, they still function in terms of telling you when you start and stop rotating, and after an initial period of adjustment, people demonstrate equilibrioception function just fine in a microgravity environment.
So I’d say that it is clearly evolved to use gravity (or other applied accelerations) as part of its function, but is not dependent upon gravity to work.
Stranger
Although it is difficult to eat and drink with gravity working directly against you, you can certainly eat and drink in reclined position (if you’re careful) and can of course perform these actions in a microgravity environment.
Stranger
IANAXenobiologist, but I humbly disagree that our hypothetical zero-gee aliens would have projectile poops. After all, our old friend, peristalsis, works just as well in microgravity as full Earth gravity; at least, I never hear of any astronauts being unable to do a number one in orbit. Unless I’m wrong, in which case, well, I’m wrong.
Try drinking a gallon of milk while comfortably sitting at a table. Don’t you ever watch Jackass?
You misunderstand – peristalsis is fine for getting it out of the system, but under ordinary circumstances, it doesn’t impart velocity, and the waste just drops. Getting it away from the body is as important as getting it out, and our bodies don’t normally do that. And excrement is an irritant – something that’s been verified in the lab. You do want it away from your skin.
Under extraordinary circumstances, of course, people can experience “projectile pooping”. (Look up Sir Richard Francis Burton’s account of the "Human Cannon’ sometime), but that’s not our normal experience.
Also, but less scatalogically, I’m always hearing how gravity assists in “natural” childbirth, and it seems we rely upon that as well. Certainly, as gravity is in our environments, we take advantage of it. And, just as certainly, when it’s removed, we can have problems. Astronauts in space in zero G for extended periods of time experience bone loss and bloating.
I did misunderstand. My apologies.