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This question already has an answer here:

Is it possible for such creatures/plants and planet to exist without magic? Assuming the creatures are sapient genetically engineered 30-40 meter tall species of various form: humanoids, snakes, insects...

What kind of planet would they require? Weaker gravity, perhaps?

EDIT: I think at least some of those who answered the question missed the point. I'm not asking about scaled-up animals. I'm talking about artificially designed creatures, which, although looking similar to common animals, may have completely different inner/outer biology.

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marked as duplicate by Vincent, Josh King, Hohmannfan, John Dallman, Xandar The Zenon Aug 6 '16 at 22:53

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Welcome to the site Mary. We have had a few questions on scaling up creatures. I would recommend a search on the square cubed law for a good sampling of questions that may help answer your questions. $\endgroup$ – James Jul 29 '16 at 20:23
  • $\begingroup$ For more info on the site and guides on how to ask good questions/answers check out the help center once you hit 20 rep feel free to join us in Worldbuilding Chat as well. $\endgroup$ – James Jul 29 '16 at 20:23
  • $\begingroup$ Search on the square cubed law was helpful, thanks. Although my question is about a world for large creatures, not really the creatures. Although, if it's not possible for these creatures to exist at all, it would be useless to think about the world for them. $\endgroup$ – MaryCole Jul 30 '16 at 1:58
  • $\begingroup$ If you're not talking about scaled up animals, then this is either too broad or unclear what you're asking. You don't include a lot of specifics or requirements or additional information in general, which makes this too broad/unclear. $\endgroup$ – Xandar The Zenon Aug 6 '16 at 22:52
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It's very possible, but you need some specific conditions. For animals and invertebrates, your ideal environment for gigantism is low gravity (perhaps half-Earth or lower), high oxygen content, and if you really want to be on the safe side, an evolutionary history that resulted in either very large, squat animals or towering giants with bones and tissue made of some sort of naturally-occurring supermaterial, like graphene or TMDCs (these don't occur naturally on Earth but nothing strictly prohibits it from happening). This is because the square cube law makes it both more difficult for bones to support the animal's mass and harder for muscles to exert the force needed to pump blood throughout the body as its weight increases exponentially in proportion to its linear increase in size.

For trees, they can already get pretty gigantic on our planet but if you want them really big, just apply the conditions I already mentioned and then give the trees something plants on our planet don't have: a circulatory system, to help push nutrients up towards the top of the tree against the pull of gravity.

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Size appears to be less genetically determined and more determined by the environment. For example, Oxygen is a primary driver of size in species...a low oxygen environment simply cannot support too massive of creatures. In reverse, high oxygen environments actually cause oxygen toxicity in small creatures, killing off smaller life and only leaving the large behind.

Genetically engineered or naturally evolved will make little difference in this discussion...where the creature came from is irrelevant...it's how well suited it is to it's current environment that determines if it's survives. So with that in mind:

Oxygen - this appears to be the single greatest size driver from what I can see. High oxygen is better for larger creatures and toxic for small ones. There appears to be a pretty direct collation between the creatures on earth size and the oxygen content of our atmosphere

Atmospheric density / pressure - most of the reactions that keeps our bodies going depend on the pressure around us (look up the 'bends' and divers issues if you want to see impacts). Higher pressure would appear to benefit larger creatures

Hot/cold - Warmer weather seems to be good for larger creatures, however you have to be careful as eventually their sheer size will begin generating more heat than it dissipates. Much of dino evolution appears to be around dissipating extra heat.

Gravity. Fits into the squared cube law...eventually a creature when enlarged will hit the point where the body collapses under it's own weight. Lower gravity will of course make this collapse point higher.

Environment. As silly as it sounds...if you are enlarged and slow, but all the other creatures around you (trees/life in general) are also enlarged in slow, then you're all going the same speed and it kinda works. If there is a particular limitation that size imparts, if all other creatures also possess that limitation, is it really a limitation? Of course, the balance here might be easily upset if a creature without those limitations is introduced.

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  • $\begingroup$ Your answer is helpful. Thanks. However, I think your're going about it backwards. My intent isn't to design creatures for some world. I designed giant creatures (if it's possible) and then I try to find/terraform a world for them to live. That's why I'm saying they're artificially designed. They don't have to compete with other creatures and they don't have to evolve. $\endgroup$ – MaryCole Jul 30 '16 at 1:50
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Highly recommend the essay On Being the Right Size by J. B. S. Haldane.

In short, no. In addition to things like the square cubed law for musculature, there's the simple matter that different shapes are more adaptive to different sizes and environments. While weaker gravity would probably allow creatures of the same shape with a larger size, simply increasing in size would not be the most adaptive form. That is, a 40m tall human, on a planetoid of weak enough gravity that it wouldn't collapse under its own weight, would still be incredibly slow, without natural weapons/defenses, and have extreme difficulty breathing (either the air would be too heavy for its lungs to pump at that size, or, more likely since weaker gravity means less atmosphere, too thin to provide adequate oxygen for its mass).

This can be seen on Earth in reverse with pygmy animals. They are not the right size. Imagine a teacup pig trying to out compete a wild boar in the wild; it's not going to happen.

Anyway, the more interesting question is not, "What environment could giant animals live in?" but, "I have this environment, what shape of animals would live in it?"

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  • $\begingroup$ It's worth noting here that atmospheric density is more complex than just weaker gravity = thinner atmosphere. Case in point Venus being slightly less massive than Earth has 90x the atmospheric pressure of Earth. It really has more to do with the availability of gases that don't easily reach escape velocity given the temperature and gravity of the world the problem would be that mostly that would mean lots of things like Carbon dioxide, Xenon and other heavy gasses, retaining Oxygen is hard on a low gravity world unless it's cold (molecules less easily hit escape velocity when cold). $\endgroup$ – MttJocy Jul 29 '16 at 21:37
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According to Fergus Simpson's big alien theory, it is statistically likely that aliens would be larger and less numerous than humans. Others have stated that it would be hard but with very specific planetary conditions (such as low gravity, extremely high oxygen content, etc), it may be possible. Of course it is not likely that creature as large as 30-40 meters will spring up, but statistically speaking it is not unlikely that some decently large extraterrestrials would exist. If you really want species that large they may have to have been created by an advanced race as an experiment, and they may have to live on an unnatural world (low gravity but decent atmosphere, etc) that is kept alive by alien technologies. You might have some trouble keeping that part entirely scientific, however.

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