Straight-faced_solo: Nitrogen is pretty inert when it comes to chemical reactions. It will naturally form a triple bond with itself and its pretty hard to get it out of that state. Even if you have a lot of nitrogen in the air it pretty much just binds to itself and refuses to react with anything.
dragons_scorn: That is how our atmosphere is today, as life developed it was quite different. When life first developed there was a LOT of CO2 in the atmosphere and nearly no oxygen. Life back then didn’t even use oxygen, at least not how we do today. In fact, the first organisms to photosynthesis are believed to have caused a mass extinction due to how drastically they changed the atmosphere as atmospheric oxygen was toxic to many organisms at the time. If anything, the Atmosphere today owes much of it’s current status to life rather than the other way around.
Jdazzle217: Nitrogen gas (N2) has a covalent triple bound. This is really really really ridiculously strong and makes doing any chemistry with nitrogen gas pretty really hard (in fact when chemist need to stuff in non-reactive atmospheres they frequently use nitrogen). Splitting that triple bound is so hard that basically the only things that split nitrogen gas on the entire planet naturally are lightning and nitrogen fixing bacteria (such as those that hangout in the roots of legumes and make them great sources of protein).
Getting that N2 triple bond to break and converted to a bioavailable form requires really really complicated enzymatic chemistry that is difficult to evolve and does not work efficiently in the presence of oxygen. Basically things that can split N2 can’t survive well in an aerobic atmosphere which is a pretty major limitation (the bacteria do survive nicely in the root nodules of legumes that evolved a nice system of incorporating these bacteria in a largely oxygen free environment).
Bioavailable nitrogen (NH3/NH4+, NO2 and NO3) is one of the most significant factors limiting the primary productivity of an ecosystem. You need nitrogen to make protein and every living thing needs protein to build enzymes and structures. Things like legumes that can fix nitrogen from the air have massive advantages over other organisms in nitrogen limited environments and introducing nitrogen fixing plants to places that didn’t have them to begin with has wrecked ecosystems across he world.
The Haber-Bosch process and the development of nitrogen fertilizer basically allowed the world to get this many people because without nitrogen fertilizer our agriculture yields wouldn’t be able sustain 7 billion people.
Oxygen on the other hand is super easy to do chemistry with, in fact it’s a little too easy and oxygen sometimes reacts a little to indiscriminately (see rust). CO2 is harder to use than oxygen but still easier than nitrogen.
Photosynthesis does some very complicated chemistry to convert CO2 in useable carbon and O2 but even photosynthesis sometimes gets messed up by oxygen’s tendency to react with everything. Sometimes the main enzyme in photosynthesis accidentally grabs an O2 instead of a CO2 which sucks for the plant as this doesn’t net any energy. Some plants evolved a cool new type of photosynthesis that solves this problem by only doing photosynthesis deep inside the leaf with little oxygen around to make it more efficient (that second type of photosynthesis is basically way more efficient in every single way we’ve tested yet most plants still do the first type for some reason which is a major mystery in plant biology).
TL;DR: nitrogen gas is very inert and the chemistry to make it bioavailable is really really hard and the only chemistry life has evolved to make nitrogen bioavailable doesn’t play nice oxygen which is a problem in atmosphere that contains a lot of oxygen. Oxygen is super reactive and easy to use to do lots of chemistry (it’s a little too easy and actually causes a lot of problems). CO2 is somewhere in the middle (much closer to the N2 side of things) but photosynthesis is pretty neat and works well.
TL;DR(TL;DR): It all comes down to thermodynamics in the end.
Edit: random spelling and grammar things
HarryPFlashman: Chemistry. Oxygen is what allows animals to function because it reacts with everything. It is essentially a highly volatile poison that we have adapted the use. Nitrogen is inert which means it can’t be easily used for energy to make animals move.
Jumpman9h: Lot’s of good answers, I’ll add that carbon is the stuff of life because it’s a universal middleman. It’s a perfect scaffold for complex molecules, nothing else on the periodic table works that way. The next best thing would be silicon, which doesn’t work.
doppelwurzel: I’ll start with oxygen and finish with CO2.
Organisms obtain useful energy through chemical reactions that move electrons from high energy states to lower energy ones. Oxygen is abundant and a really great electron acceptor so transferring electrons from, say, a sugar molecule to oxygen atoms is easy and has a high energetic payoff. The most well known process to do this is called [aerobic respiration](https://en.wikipedia.org/wiki/Cellular_respiration) and in this system the oxygen atoms used as electron acceptors are converted to CO2 and H2O. Organisms able to use this oxygen-dependent process generally outcompete organisms using less ideal electron acceptors and as a result they spread over evolutionary time and came to dominate earth.
CO2 is trapped (fixed) by photosynthesis in essentially the reserve process and this requires an energy input. Free electrons removed from water molecules by the sun’s energy are bound up along with the carbon atoms from CO2 to produce large carbon-based molecules like sugar. This represents an energy store that the plant can later degrade (through aerobic respiration) to free up energy in a useful place and time.
There are a few living organisms that do use nitrogenous molecules, and various other atoms, in processes equivalent to those I mentioned above. They’re only “better” in very specific circumstances so that’s why they’re fairly rare. For example, there are bacteria that use [iron atoms instead of oxygen](https://en.wikipedia.org/wiki/Dissimilatory_metal-reducing_microorganisms) and others that use [sulfur](https://en.wikipedia.org/wiki/Sulfur-reducing_bacteria).
BlueKnightBrownHorse: Oxygen is a really great chemical for causing chemical reactions. Hydrocarbons (what food is made out of) have a lot of energy in their bonds, and can be used to power things (like your car). Carbon Dioxide, on the other hand, is an incredibly boring, stable, and low energy molecule.
Your body uses oxygen to force a reaction which turns hydrocarbons into boring carbon dioxide. The difference in energy is something you get to keep, and you use it to power your body. This process is called *cellular respiration*, and it’s the same reaction as setting fire to a pile of logs, it just happens at lower temperature inside your mitochondria.
The reason oxygen is so great is because it has one of the highest electronegativities of all the elements (it’s close to the top right on the periodic table). This means it is powerful at coming into a party alone and stealing all of your girlfriends (You’re hydrogen, and your girl is carbon: there’s probably a better metaphor). It produces a lot of energy this way, and your girlfriends like him so much that they’re never coming back. Fluorine might be even better than oxygen, but there isn’t a lot of that floating around in our atmosphere, so we haven’t evolved to breathe fluorine (there may also be more practical reasons why we don’t breathe fluorine).
This is also part of the reason CO2 buildup in the atmosphere is a problem. It takes incredible amounts of energy to get our girlfriends back from oxygen, the Chad of the periodic table.
Edit: I forgot to answer part of your question. Nitrogen is the emo guy standing in the corner. Noone is leaving with him.
greatchocolatecake: It’s sort of like saying “There is vastly more water on earth than gasoline. Why is it that we’ve developed cars that run on gasoline and not water?”
Water has little potential energy that’s usable for chemical reactions compared with gasoline. The same is true for nitrogen relative to oxygen.
ShitInMyCunt-2dollar: Carbon lends itself to far more varied compounds than nitrogen. Carbon is tetravalent, whereas nitrogen is generally trivalent. That means carbon likes to have four bonds per atom and nitrogen likes three (usually – nitrogen can also be pentavalent).
As such, carbon is the basis for all life that we know of. Its tetravalent nature makes it perfect to form large and stable molecules. Molecules that just don’t form with other elements.
maximumplague: During the Carboniferous period (~300 million years ago) the atmosphere was so oxygen rich that insects got bigger to decrease the risks of oxygen toxicity. That was around 30% atmospheric oxygen levels.
capn_ed: You build a metabolism with the chemical reactions you have, not the chemical reactions you’d like.
Thomas9002: The same reason you don’t put water in your gas tank: you can’t take any energy out of it
secretwarrior1: There’s a guy here that posted something about organisms consuming fluorine, and other elements.
If I find his post, I’ll link it here.
im_thatoneguy: Oxygen is like harnessing an elephant: It’s a bit risky since it might rampage through your village but if tamed it’s an incredibly powerful tool since it has so much energy and utility. Nitrogen is like a sloth, it’s not dangerous but it also has very few practical uses since it has very little energy potential.
jumpinbeaan1: Maybe its some sick failsafe nature instilled in an effort to avoid over population of any oxygen dependent species, not that “nature” is by any means sentient but ya know natural order checks & balances and all that…🤔
rhomboidus: Nitrogen (N2) is a noble gas. It doesn’t readily interact with anything. Atomic nitrogen really, *really* wants to be N2. It will take any and all opportunities to rapidly disassemble itself from other molecules to go play with another nitrogen and make an N2 molecule. This is why nitrogen is an essential component of all modern explosives, and why nitrogen chemists are often deaf, short a few fingers, or both.
So as the vast majority of available nitrogen is N2 and doesn’t want to be anything else, it isn’t an easy molecule for life to work with.