Thread by @ck_eternity_

Most people see a meteor as the ultimate symbol of mass extinction, in reality though we should be looking at oxygen the same way

The first major extinction event on earth was known as the great oxidation event and occurred as a result of increased O2 in the atmosphere

(thread)

At this point the earth was still covered in single-celled organisms

Opportunistic cyanobacteria developed photosynthesis as a way to leverage the abundant CO2 in the atmosphere for energy production, however the fundamental metabolism of other cells wasn't configured for this

Molecular oxygen is highly reactive with certain forms of minerals, particularly the water-soluble ferrous form of iron

This reactivity leads to the creation of a cascade of free radicals in response to the formation of iron oxide, I've discussed this often in health/disease

At the time the organism the existed were used to a high CO2 atmosphere which actually creates a "reducing" environment fundamentally opposed to oxidation

As a result they had less antioxidant production and their metabolism was more permissive of the use of ferrous iron

When oxygen levels increased, however, this was disastrous

This resulted in the extinction of roughly 90% of organisms on earth, mostly composed of the non-photosynthetic organisms

The echoes of this extinction can still be seen in cells and mitochondria today

Mitochondria are particularly interesting here in that they were previously prokaryotic bacteria themselves, and their ancestors survived this oxidation event

We can still see the adaptations to this high-oxygen environment in their use of copper as an "electron switch" for iron

In modern organisms copper acts as either an electron donor or acceptor to convert iron from the ferrous form to the insoluble ferric form and vice versa

Iron is transported in the ferric form to limit its reactivity, then it is reconverted to the ferrous form in hemoglobin to allow it to bind to oxygen to transport it

In mitochondria, we see a microcosm of this in the enzyme cytochrome C oxidase which uses copper and iron cores

Mitochondria also display all the enzymes necessary to synthesize melatonin, one of the most potent antioxidants found in nature

We know melatonin mostly as a hormone, however its role as an antioxidant is billions of years old and still occurs today in both plants and animals

Melatonin is amphiphilic, meaning it is soluble in both lipids (cell membranes) and water (cytosol), so it is able to protect all parts of the cell

It also can be used as an antioxidant multiple times in a row before it is used up

I would go so far as to argue that other core signaling molecules like serotonin may have even developed out of their presence as precursors to melatonin

Originally serotonin signaling probably consisted of it binding to and modifying proteins (called serotonylation)

All in all mitochondria are exquisitely tuned to their electron environment

They leverage it both as an abundant and efficient energy source, and to protect themselves from the natural tendency of their cellular machinery to break down over time

I find this all truly amazing