It cracks me up when you tell people that when bacteria first started filling the atmosphere with oxygen it was a massive crisis for all living things because oxygen is a highly reactive, toxic, corrosive, dangerous gas that reacts with almost everything it comes in contact with... and they try to tell you that you're making that up and that oxygen is just nice stuff that we breathe into our lungs all the time. Yeah... where it is IMMEDIATELY sequestered by hemoglobin molecules designed to transport it without letting it damage anything, and then handled by special molecules inside the cells to keep it from destroying stuff. Mitochondria are ancient bacteria with a metabolic pathway that can utilize oxygen's highly reactive nature... at a price. Escaped oxygen from mitochondrial metabolic pathways results in "free radicals", which by definition are formed by oxygen reacting with other molecules in a way that makes them unstable and causes them to become damaging to other molecules.

"That's stupid. If oxygen is dangerous and reactive, why do we need it to live?"

Because we evolved in an environment where cyanobacteria were massively populating the oceans and producing massive amounts of oxygen as a toxic byproduct. Everything either had to learn to hide from it (anaerobic bacteria) or live with it (pretty much everything else. The evolution of multicellular organisms hinged on the formation of the eukaryotic cell... a cell that kept captive bacteria inside it as little factories capable of using the reactive potential of this toxic gas to drive their machinery. The price for this energy is the production of free radicals that may, according to some evidence, be part of the inevitable process of aging. It is known that in cells where some step in the mitochondrial production breaks down, the release of free radicals in unacceptable amounts is a signal to the cell that something is very wrong and will trigger the cell's suicide mechanism (there are actually lots of things that will trigger a cell's suicide mechanism, including uncontrolled cell division. Mutations that knock out genes responsible for cell suicide are a key cause of cancer).

Mitochondria store a lot of their genes in the nucleus of their home cell, meaning that they rely on the cell to produce the necessary proteins for them. When it comes to a few essential proteins that control the transport of highly reactive molecules back and forth across the mitochondrial membrane, these former bacteria have retained control of those genes. The process is too delicate to be regulated by a feedback loop involving communication with the nucleus... adjustments have to be able to be made onsite.

The vast majority of oxygen on Earth is locked up in the earth itself in compounds with basically every other element that makes up Earth's crust (particularly silicon, but it isn't picky). A bunch more of it is fairly happy being water, where the unstable oxygen molecule can ease its desperate demand for electrons by partnering with a pair of hydrogen molecules, which are agreeable to sharing. Even though water is nicely stable under most conditions, it is a polar molecule, with the oxygen dominating the negative charge by hoarding the shared electrons, which means that many other polar molecules or molecules that don't mind becoming positively or negatively charged ions will happily go into solution in water, which is why it's such a critical component of living organisms. The most basic reactions carried out by living cells of any kind involve transporting charged ions across a cell membrane, and there wouldn't be charged ions to transport if they weren't dissolved in water.

ANYWAY.

I am rambling off the top of my head because this is what my brain was up to, so if I've messed up my facts feel free to remind me.


.
.
.