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I have two "new" planet earths -- we'll call them Light Earth (LE) and Heavy Earth (HE). These planets are generally the same as our Earth in terms of land masses, oceans, the moon and sun and other terrestrial objects, weather, and so forth. The difference between them and our Earth is in the force of gravity. On LE, gravity is less than that of our Earth, and on HE, gravity is greater than ours.

How does gravity affect the evolution of flora and fauna? Would trees be taller on LE and bushes more prevalent on HE? Would there be no birds on HE? (And if there are no flying birds, are there more dinosaur-like things still present? or more emu-like birds?) Would there be more fish at greater depths in LE?

This question is only about the evolution of life on the HE and LE planets. I am aware that a difference gravity would change how the planet itself would evolve (plate tectonics, atmosphere, etc etc). We're going to ignore that for now.


I'm looking for the general overall effects of a different gravity on evolution -- something like "there will be fewer flying mammals in higher gravity, and they will compensate by XYZ." I've intentionally not given numerical values for "higher" and "lower" gravity, but let's consider a range of [10%,190%] of our Earth's gravity, if such a thing matters.

Please note that I'm intentionally not changing the gravity of our Earth. These are brand new almost-duplicate Earths (minus the gravity) whose wildlife and plant life evolved in that environment. The majority of the gravity related questions here appear to be about taking an existing organism from our Earth and placing it in a new gravity environment; this question is intentionally not about that.


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  • $\begingroup$ I tried to explain as much as I could what I thought (I like the idea a lot), hope this will suffice ! $\endgroup$ – Yassine Badache Sep 28 '16 at 13:41
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    $\begingroup$ A sufficiently light Earth wouldn't be able to hold on to a nitrogen-oxygen atmosphere over millions of years, which would have quite radical effects on evolution. But I assume that's something you want to ignore. $\endgroup$ – Michael Seifert Sep 28 '16 at 15:09
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I think you would end with two really differents planets. I will probably forget some factors, but here is what might happen.

The Heavy Earth

Gravity there is way stronger that normal. Animals, fauna and ourselves would need a stronger body in order to be able to fight it, or evolve to be able to keep living without much effort. Plants would need great growing strenght blossoming and keeping their leaves up to the sky.

(Thanks Burki for the details) However, it would not be total grounding. Stronger gravity would mean more oxygen per cubic meter, which means your animals and plants will be able to compensate some of their constraint with higher muscle irrigation, thus efficiency. And this is an excellent news because you can imagine one or two species which may take advantage of this point (improved breathing system e.g) to live rather normally, with an edge on other creatures !

De facto (I love this term), you would find yourselves with two cases of animals / plants, if we take out the oxygen barons:

  • Bulky animals, who are normal in height compared to our animals but way more muscular to sustain the pressure of the gravity. Your trees would be hella shaped as weel, with huge trunks.
  • Small animals, very slow, but very strong. Close to the ground in order to reduce the effort, maybe crawling animals like snakes would be more common. Insects would be little affected, I think: most of them have an outstanding power by weight ratio. Plants, too, will be either close to the ground or growing to a little extend before falling to the ground. Small cute little arch tress, maybe.

I don't really know about fishes, but pressure there would be times and times superior. You would find deep creatures earlier in the depth map, and further ... You go boy, you can really create whatever you want.

Basically, I think our Earth would be flat as hell, with animals close to the ground, little birds only which would fly not that much. The flora itself would, too, stay close to the ground. Trees will be smaller, bushes more common, and most '*high-ground plants' would mutate to be able to keep growing on a flat surface, or at least near the ground. Lucky strawberries lovers out there.

The Light Earth

On light Earth, this is where things are getting fun. Your animals can deploy some power on something else that fighting against gravity. They can grow taller, fitter, with more speed, more power to deploy while hunting and fleeing. They, too, would be a bit muscular, because if you deploy such power into running, your body still needs to sustain the charge. A lot of birds might grow, too, bigger and faster than anything.

Plants, on the other side, are free to grow a loooooot taller, too. Small or big trunk, they can extend to meters and meter ahead of our current trees. Bushes would blossom, and even ground plants could grow up in size.

BEHOLD HOWEVER !

This does not mean you multiply anything by two or three. If you grow, you still need energy to feed youself. It just mean that your structure (animal or plant) can sustain higher without collapsing. Keep that in mind while designing your new species.

TL;DR

Heavy Earth will keep species close to the ground. They would evolve in a slow planet, in my opinion.

Light Earth will let them evolve in some interesting manner, letting them going as far as possible.

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  • $\begingroup$ Very nice! a question, though: would not part of the effects be cancelled by oxygen pressure levels? Muscles are only helpful if provided with energy which requires oxygen to be converted into work (yes, shortened...) and on High-g-earth you get more oxygen per cubic meter than on low-g-earth. $\endgroup$ – Burki Sep 28 '16 at 14:02
  • $\begingroup$ I don't know if OP meant to go to such extend, but this is an excellent point to take in consideration. I think, still, that they would not have enough oxygen to compensate for the gravity itself ? It would be cancelled a bit, but not to complete nullification ? $\endgroup$ – Yassine Badache Sep 28 '16 at 14:18
  • $\begingroup$ Air pressure doesn't have to be connected to gravity. Venus has a slightly lesser gravity than Earth and has an atmosphere of mostly CO2 versus Earth's N2/O2. The relative masses of those gases would suggest that Venus should have an atmosphere around two to three times denser. Venus' atmosphere, however, is actually about 90 times denser, which is far beyond anything I can explain even to myself. Are we free to just speculate that however many air molecules we desire are in the air, and that air pressure/density can thus be whatever we want it to be? $\endgroup$ – Palarran Sep 28 '16 at 14:46
  • $\begingroup$ Ask OP, my friend. I love this kind of very technical discussions, to be honest. $\endgroup$ – Yassine Badache Sep 28 '16 at 14:47
  • $\begingroup$ Water pressure has a linear relationship with gravitational acceleration, so doubling or tripling the gravity wouldn't compress the pressure gradient in the ocean all that much. Considering the immense diversity of aquatic animals that regularly migrate several hundred meters vertically, I don't think this would be a huge factor in differentiating the biospheres. $\endgroup$ – abestrange Oct 17 at 22:54
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Trees

Light Earth(LE): Stresses on branches will be reduced. Tress will be able to spread enormous canopies from a single slender trunk. The weight of a water column will be reduced, so trees will be able to grow to greater heights. Flowers will still have an evolutionary advantage, but perhaps not as much on our earth, as broadcast pollination will be more effective on this world. So maybe fewer and more primitive flowers.

Heavy Earth (HE): Height will still be a tremendous evolutionary advantage for plants, so there will be significant pressure to evolve systems to grow tall even against the greater gravity. One adaptation may be buttressing; trees that grow in clusters and lean into a central tree, grafting to the central trunk at various heights and creating a flying buttress for the central tree would be able to attain significant height against the gravity of the planet. This same support strategy would allow a spreading canopy to be supported by multiple limbless buttresses. Without such a support system, other trees on this planet would be very limited in the length of limbs they could radiate out from the trunk.

Land Animals

LE: Leaping and hopping locomotion will be a much more prevalent evolutionary choice on this world as it will prove to be more efficient than standard quadrupedal gate. Gliding adaptations may prove to be more prevalent, as less significant body mutations on this world will result in glide capabilities that can be selected for. On this world, some species may take the the air and never come down for their whole lives. Large bodied animals will evolve more readily than on our world.

HE: Semi Aquatic habits may be prevalent, using the buoyancy of water to offset the cost of larger bodies on this world. Multi limb arthropods may have an advantage dispersing weight over a larger footprint and using a exoskeleton strategy vs an internal skeleton may be a much more efficient solution, ultimately requiring less skeletal mass to support similar mass of soft tissue. The highly segmented arthropod body plan may also allow for smaller, repeated respiratory/circulatory systems in larger animals to compensate for fluid pressure issues that will be present on this heavy world. HE might be a bug world.

Flight

LE: Flight is a common adaptation on LE, with some species staying airborne most of their lives. Many and varied gliding adaptations will evolve.

HE: Flyers will need larger (relative to body size/mass) or more efficient wings to generate sufficient lift to fly on HE. Most animals larger than a sparrow will not be able to take off from a standing position on the ground. Needing instead to drop from elevation (or jump of a cliff) or run at speed along the ground. The idea here being that ground effects and the general unweildlyness of their large wings relative to their bodies will make it impossible to generate sufficient lift from a standing start near the surface.

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  • $\begingroup$ Jim, review my edit for subsequent reference. There is a header formatting button on the editing toolbar and multiple levels are supported. $\endgroup$ – JDługosz Sep 29 '16 at 6:58
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While not an expert, I can think of a few things right off the bat:

Gravity is going to affect skeletal structure, height, whether animals develop which are bipedal vs quadrupeds, or even hexapeds (the higher the gravity the more likely for animals to evolve to be lower to the ground).

Animals which climb, or fly might be quite small, and trees might also be quite short in a high gravity environment.

Gravity will also have a big impact on the cardio-vascular system. Higher gravity world dwelling creatures might evolve to be more territorial, and hunt by ambush, rather than roam the land, while creatures in a lower gravity might evolve with no such restrictions, and generally be bigger in size.

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Two sci-fi books that deal with this directly: Dragon's Egg by Robert L. Forward https://www.amazon.com/Dragons-Egg-Del-Rey-Impact/dp/034543529X

Heavy Planet by Hal Clement https://www.amazon.com/gp/aw/d/076530368X/ref=mp_s_a_1_1?ie=UTF8&qid=1475076330&sr=8-1&pi=SY200_QL40&keywords=heavy+planet&dpPl=1&dpID=51mNsaXkjoL&ref=plSrch

Both of these look at life evolution under very high gravity and may guide your own development.

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Good technical reading for the less-obvious implications of microgravity (and a little on high gravity) is Fundamentals of Space Life Sciences Vol. 1 (edited by S.E. Churchill, numerous authors, Krieger Publishing, 1997). Low-gravity environments aren't that well known so you'll have to interpolate from the known effects of microgravity and centrifuge experiments.

I'll summarise a few points:

  • Gravity redistributes blood pressure in the body. Humans blood pressure in orbit goes down in the limbs and up in the aorta; the head gets so much more blood that everyone has a puffy face. (See chap. 4.5 & 4.6.) Your high-gravity species need better ways to pump blood against gravity, low-gravity species need it less. Expect the heart size to differ correspondingly; the major muscles (in Earth animals) also help pump blood but these should already be scaled for gravity anyway.
  • Humans have fewer red blood cells in orbit. This is believed (*in one likely hypothesis) to be an adaptation to less strenuous exercise. The body also reduces its total blood volume in space (our faces get less puffy after some time). (See chap. 4.7 & 4.10.) Your high-gravity and low-gravity species are likely to have correspondingly more or less blood.
  • Human bones are not simply there for structural support. Calcium is a vital element for various functions. Some authors even go so far (!) as to describe the structural function as secondary to the calcium storage function. If animals on an alien world have a similar calcium metabolism, there could be important implications if their bone mass is scaled up or down. The last trimester of pregnancy and lactation place severe loads on the human calcium store. (See chapter 6.2.) Aliens with a lighter frame on a low-gravity world may have to have fewer young, or smaller young, or more independent young, or be more prone to osteoporosis. (Of course the young would then require less calcium for their bones, but not for other functions.)
  • Our immune systems are somewhat more efficient in a centrifuge and less efficient in microgravity. The reasons behind this are complex but it seems that cells are better able to read chemical messages in high gravity (which is relevant to aliens) and that some types of suppressor cells may not work as well there and the total number of immune cells is also increased (which probably isn't). (See chapter 8.3-8.6.)
  • Bacteria reproduce much more aggressively in microgravity, which is believed to be because they spend less energy moving and maintaining their position against gravity/buoyancy. Simple eukaryotic cells show similar effects. (Most of chapter 3 is relevant.) Overall, aliens in low gravity would probably need to spend a bit more energy on their immune systems and those in high gravity a bit less. With no immune system, plants should have more or less aggressive defences (bacteriocidal sap, etc.) than on Earth.

Also:

  • Animals require more food in higher gravity and less in lower gravity, for locomotion. Their oxygen requirement for burning that food scales similarly. An animal in a low-gravity environment should thus have smaller lungs but nonetheless greater endurance. Heart scaling for blood pressure has already been mentioned; an even bigger effect on the size of the heart should be due to the change in oxygen requirements.
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We could even consider the effects of Earth's gravity on the evolution of human morphology that in turn determines how we move (bilateral symmetry, e.g. opposing hands) that in turns determines how we sort objects, that in turn determines the form of math we have created. Gravity influence our metaphysics, that is, the bias we project into reading our world. The variation in the strength of gravity, while interesting, can also include gravity per se or gravity (humans) versus "no" gravity (fish).

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    $\begingroup$ Not bad for a first post, but you should be aware that this is an old question which by modern standards would be considered too broad to be answerable.When you have a moment , please take the tour and check-out the help center to give you a heads-up about how we work. Enjoy the site. $\endgroup$ – We are Monica. Oct 17 at 21:07

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