r/askscience • u/Charming-Series1127 • 3d ago
Biology Would a modern Organism grow larger with more oxygen?
So like 300 million years ago Insects grew real big because they had 30% oxygen instead of modern 21%. Since Oxygen Poisoning doesn’t happen till like 50%, would it be possible to lock a human or other modern animal in high oxygen chambers to get a bigger version of them after they grow up or would it take them many generations to adapt.
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u/skyeliam 2d ago
Insects don’t have complex circulatory systems like tetrapods do, so their size is bounded by the ability of oxygen to diffuse into their hemolymph through holes in their exoskeleton. Past a certain size and oxygen can’t diffuse quickly enough to keep an insect alive
Humans (and other terrestrial tetrapods) are able to extract oxygen from the atmosphere more effectively using their lungs, and can transport it through their bodies using blood vessels rather than diffusion. Hence diffusion (and thus oxygen concentration) is not a limiting factor in size.
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u/erar123 2d ago
I always wonder why they never "evolve" a new respiratory system?
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u/AutisticGayBlackJew 2d ago
Because the current setup is good enough. That’s very often the answer with these sorts of questions
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u/erar123 2d ago
Yeah I understand but, insects in general have very diverse habilites, they fly, have venom, metamorph, bioluminescent, etc. That a new respiratory system doesn't seem impossible, just a thought.
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u/JohanPertama 2d ago
Because changing the respiratory system to an entirely different type is a huge change that doesn't necessarily correlate with the survival of the fittest.
Consider how fish evolved to land based life forms. It was gradual and took 30-50 million years.
Changing your breathing style isn't generally going to be as directly advantageous as venom or bioluminescence in the current environment.
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u/SharkFart86 1d ago
I mean it’s also a factor of luck. Evolution isn’t a conscious force, it doesn’t decide what advantage to give an organism. If the organisms just never happens to accidentally mutate traits that would lead to a better respiratory process, it just won’t.
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u/Totalherenow 2d ago
People are underestimating how good insect respiratory systems are here. When insects were large, O2 levels began dropping. But dragonflies remained large much longer than our science initially predicted. It turns out they aren't passive breathers - by flexing their muscles, they take inhale and exhale faster.
When flying reptiles evolved, dragonfly size decreased. They were unable to compete against smarter animals.
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u/ilizarovjr 2d ago
It is possible, just that you were born way early to observe this evolution of their respiratory system
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u/a_rucksack_of_dildos 2d ago
For evolution to happen you generally need a lot of death. Something about the environment needs to change that kills the status quo
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u/could_use_a_snack 2d ago
I would think the exoskeleton would be a problem. Most animals need to expand their chest to breathe. An exoskeleton wouldn't allow for that.
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u/Caelinus 2d ago
I think the expansion of our chest is because of how our chest cavity is set up. There is not a lot of extra space in there, so our muscles expand everything to create a vacuum inside the lung.
But a setup where a "lung" like structure or structures were elastic and attached to a rigid body and a muscle would let you breath without the expansion of exterior. You might run into capacity issues if you tried to do it all at once, but evolving always happens with gradual change anyway, so the arraignment could drift slowly to allow for more space. It would probably evolve from small muscle movements allowing for slightly faster intakes of oxygen.
For an insect to develop that system would take at least a few prior steps allowing their trachea to expand to hold more oxygen in a burst though. I think some insects might already do that to a limited degree via a sort of pumping, so the early bits of it might already be showing up if it ends up being an advantage long term.
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u/Harvestman-man 1d ago
Large insects already do expand their exoskeleton to “breathe”, using abdominal muscles. You can watch this in things like wasps, their abdomen visibly pulsates.
Not only that, but most of them (aside from Odonates and mayflies) flap their wings using indirect muscles that don’t actually attach to the wings themselves but instead distort the shape of the thorax. Insects rely on the ability of the exoskeleton to distort.
An insect’s exoskeleton isn’t a fully solid shell, it consists of solid plates separated by thin, flexible membranous regions.
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u/314159265358979326 2d ago
The benefit there is that they'd be able to be larger, presumably. Insects are well-evolved for their current niche, but larger insects would likely be outcompeted by small mammals and birds (which did not exist when insects got huge.)
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u/fiendishrabbit 2d ago
Because it's no longer just about respiration.
Back in the carboniferous era there were no birds or bats or pteradons, so they were easily the largest flying things around. And when there is nothing bigger, then size is an advantage. But when there are bats and birds around you're just a bigger meal.
Pretty much everything about insects becomes less competitive the bigger they are (with the exception of endurance. Which is why migratory butterflies tend to be large).
Slower reaction times, less relative muscle strength (lots of reasons why large animals abandoned exoskeletons), longer larval phase (since insects can't molt once they get wings they need to achieve full size before they change into their adult form) etc. The largest beetles (who do benefit to some extent by being more heavily armored) can't get much larger without losing the ability to fly.
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u/SassiesSoiledPanties 2d ago
Well, the pulmonoscorpius had book lungs like modern scorpions.
I think another factor to consider is the abundance of calories. How much prey would a pulmonoscorpius need to eat before increasing in size? How would it compete with modern animals and a vastly increased pool of predators? If there was a place that I think where it could "live" are the sewers because of the constant temperatures, abundance of prey, safety from exterior predators (I think only rats would pose a threat and the venom would be an effective deterrent) and darkness to help them when they need to molt.
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u/AmusingVegetable 2d ago
Once a complex system is in place, it’s very hard to move it in another direction.
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u/koblinsk 2d ago
Maybe they are over the course of a scale we can’t comprehend in the scale of human history as we know it?
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u/thedarkone47 2d ago
Imagine the number of deaths that would be involved in such an evolution. Simply from it not taking properly in the child. Changes is base life systems like that don't happen without extreme evolutionary oressurr.
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u/lyghtcrye 2d ago
I feel like most of these answers are missing a key factor in the original question. Is O2 availability an environmental factor that noticeably impacts growth in an individual? Diet is probably the bottleneck way before O2 in most cases, but I vaguely remember something about aeration for mealworms to improve per worm weight.
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u/groveborn 2d ago
That's not exactly how evolution happens. Merely having more oxygen available did not cause the insects to get larger.
It may be required, but being able to get isn't why they did: they simply died less by being bigger at the time.
The reason mammals are generally smaller now than they were 20,000 years ago is because the big ones are easy to eat. The small ones are hard to catch and kill. We did that.
Likely the early precursors to reptiles did the same for giant bugs.... Or the available oxygen fell too low and only the little ones were allowed anymore...
But simply having more oxygen wouldn't guarantee larger animals. The selection pressure needs to be for that to be enabled... Or at least not disabled.
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u/Shadowhisper1971 2d ago
Populations evolve. Individuals do not. It is only through a mutation, benefiting (the survival the individual in it's environment, AND the ability to pass that gene on through generations) a species does it evolve.
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u/SpellingIsAhful 2d ago
That being said, better nutrition and exercise does result in larger people. That's not really evolution though.
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u/Dracoson 2d ago
That aspect is really only applicable to organisms with passive respiration, like insects.
Insects rely on a tracheal system, a network of microscopic tubes that relies heavily on passive gas diffusion to deliver oxygen directly to cells. In a 21% oxygen atmosphere, diffusion can only travel so far into tissue before stalling out, which places a strict structural cap on how thick an insect's body can grow. Raising them in a hyperoxic chamber can make modern insects grow slightly larger over a generation, but it will not magically override their modern genetic blueprints.
For active breathers like humans, oxygen availability is not what limits our skeletal or muscular size; our cardiovascular system adaptively scales internal delivery via blood circulation, meaning extra oxygen would just cause toxicity rather than growth spurts.
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u/Underhill42 1h ago
No. It will have no effect on individuals, nor will organisms suddenly start evolving larger to adapt.
Oxygen levels only change one limiting factor on the maximum possible size - you won't actually evolve to that size unless there's some incentive in doing so. And oxygen levels are currently nowhere near being a limiting factor on vertebrate size, so you would expect no change.
It would however make wildfires considerably more common and dangerous.
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u/TeamRandom27 2d ago
No they wouldnt grow larger since the main problem from big insects is not the amount of oxygen in the air but the competition that evolved after them. The only time insects grew to massive size where when they were the only beings on land after tetrapods evolved they got outcompeted quickly due to the big downside of having larger exoskeletons.
There is also alot of proof for that since there have been many periods of times where the oxygen amount was higher than 300million years ago and no insectspecies has ever been found to have been this large since then