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I am working on a planet were life (trees, plants, food, animals, humans) would be bigger/taller than Earth. I have learned that oxygen level or gravity, for example, could help change the size of life forms.

So here are the characteristics of the planet :

Nitrogen : 72,4%, Oxygen : 26%, Argon : 1,05%, CO₂ : 0,5%

I have settled the air density at 1.9 kg/m3, so approximately 148 892 Pa and 1,4 ATM (but I have doubts on whether it's good or bad for creating taller life forms). And I still don't know how much I would have to lower gravity (0,9g or maybe 0,8g ?)

There is a big continent near the equator, so there is a lot of huge forests (one of the reasons the level of oxygen is at 26%). A big part of the forests are near mountain range and/or coasts, so there are a lot of rivers, and so very fertile soil. These forests have very rich and abundant biodiversity (with different species of plants and animals than Earth, that I still have to create). And the trees would have similar behaviors than Redwoods (https://www.latimes.com/archives/la-xpm-2002-sep-01-adme-redwoods1-story.html), having water sources from both the ground and the canopy.

So considering all those characteristics, how tall trees, animals and "humans" could grow ? (bigger it is, the better)

Thanks for all the answers you will be able to give me !

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The reduced gravity would help a bit to produce bigger trees but I wouldn't expect more than a 10-20% increase. What would make life very difficult would be the high level of oxygen in the atmosphere especially at the higher pressure. Wild fires would be an enormous risk as would storms so the giant redwood trees might not get a chance to grow to maximum height.

Whilst trees greatly benefit from being taller the same is not true of humans and animals. They might well remain at roughly the same size. The extra availability of oxygen and reduced gravity might allow them to be smaller as less muscle mass would be needed and smaller lungs. or the same sized muscle and lungs and a greater endurance and performance. Very hard to say as it would depend on the evolutionary pressures over millions of years.

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  • $\begingroup$ Thanks for your answer ! So instead of increasing the pressure, I should decrease it ? Like less than 1 ATM ? $\endgroup$
    – Emie
    Commented Nov 5, 2019 at 18:12
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    $\begingroup$ I think what he is saying is that you should not worry so much about the planet and worry more about the evolutionary pressures. There will always be niches for both big and small lifeforms. Your planet may allow for some really really big life forms, but when you look at Earth, elephants are far from the biggest animals that could be, just the biggest they make since to be right now. The most important feature your planet needs for bigger life is a very long and stable period mild weather. $\endgroup$
    – Nosajimiki
    Commented Nov 5, 2019 at 19:48
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    $\begingroup$ @Emie Concerning evolutionary pressures, it’s not clear how they would act. The situation would be very complex as there would be many factors to consider. Longer stronger legs would allow faster running, but with a bigger more expensive to build body. Shorter weaker legs would alllow slower running but a smaller less expensive body to build. Which wins out is hard to say it depends on the fine details of the circumstances over millions of years of evolution. Where trees are concerned light is king so taller is better. Where oxygen is concerned above 21% O2 combustion is an increasing problem $\endgroup$
    – Slarty
    Commented Nov 5, 2019 at 23:51

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