Your body’s energy to live comes from oxidizing carbon. That’s why you eat carbon rich foods (carbs and fats) and breath in oxygen and breathe out carbon dioxide. It’s not too different from to very slow, controlled burning of the food. Just like a fire without oxygen the fire stops, without oxygen all the processes that represent your life stop, too.
Fire is rapid oxidation. Rust is slow oxidation. Oxygen is the molecule that breaks down various compounds. As such, oxygen is necessary for our body to regulate (accelerate or slow down) our biochemical reactions (like breaking down food, extracting and converting proteins, etc). These processes are constantly running and require oxygen. Now, some animals evolved to hold their breath for extended periods of time, but that usually comes at slower metabolism. Humans, as land-dwelling animals, never evolved the mechanism of manual regulation of metabolism, therefore we need constant supply of oxygen to stay alive, as even just a couple minutes of oxygen starvation is enough for cells in our brains to start dying off. Animals never evolve organs or abilities without a real need, as those things require energy and resources to maintain. That’s why humans didn’t evolve the ability to hold our breath for extended period of time, fish didn’t evolve extra limbs for walking (just in case they need to), and cave animals are losing eyes over time.
Oxygen is all around us in quantities that can be used for cellular fuel, but not high enough that it can hurt us.
And so as our multi-cellular ancestors evolved in that environment, they naturally harnessed the plentiful not harmful thing around them to grow more complex life. And now most complex life relies on that. Plants did a similar thing with sun. You could say “why do plants not grow in the shade, seems like a design flaw?” but the reason is that there were elements in the environment that made it advantageous for plants to grow in the sun, and so that’s how they evolved.
Because it is readily available so our evolution was never pressured to become efficient with oxygen unlike sea mammals that have limited access so there was pressure to become very efficient with oxygen use.
That doesn’t mean we can’t adapt to low oxygen environments. There are groups of humans that can function perfectly fine in thin air environments like the Sherpa in the mountains. Their bodies have adapted to running on less oxygen.
There are also tribes located on water fronts that free dive for minutes at a time with bodies that have become very efficient at storing extra oxygenated blood cells.
We can also train our bodies to become better at storing oxygen or running on less as is frequently done with many free divers and some military groups that are specialized to operate in the mountains.
But to gain natural affinity to needing less breathing, that takes millions of years of evolution under reduced oxygen environments to start seeing.
> What is it actually doing to me that is so necessary I need it constantly?
We don’t. What we *do* need to do is constantly get rid of carbon dioxide. If our bodies made no CO2 from metabolic processes we could survive on a single breath of air every 3 minutes.
The problem is that we don’t have any way to directly pump out this carbon dioxide. We instead need to displace it with something else (oxygen) which is why we breathe continuously.
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