Thread by @JacquelynGill

Today’s #MakeAPlanet quiz: This essential element was a toxic product of early life that first caused extinction, but later allowed life to establish on land. Its production formed distinct rock layers. 50-80% comes from the oceans today.

A) Carbon
B) Oxygen
C) Iron
D) Nitrogen

Adding a little buffer so folks can guess without seeing the comments.

I tried to mislead you with some sneaky references (the reddish rocks and the ocean), so this was a fun one.

Remember coloring in the parts of a cell in school? Cyanobacteria are bacteria that contain chloroplasts (an organelle used in photosynthesis). Like other organelles, it’s thought these were once a separate single-called organism that formed a symbiosis and got assimilated!

Around 3.5 billion years ago, those early cyanobacteria started forming mats called stromatolites, which are some of the oldest evidence of life we have. And guess what? There are still active stromatolites today! These are in Shark Bay, Australia.

Stromatolites were turning ☀️ into sugar—photosynthesizing, like plants. What’s the byproduct of photosynthesis? Oxygen, which is essential to almost every organism today. But! There was no oxygen Earth’s early atmosphere, so the first life evolved without it. Oxygen was toxic.

For tens of millions of years most of that dissolved oxygen reacted with iron-rich waters, forming red, rust-like layers in what eventually became sedimentary rocks. These “banded iron formations” are some of the oldest rocks on Earth, and hold up to 60% of global iron reserves.

Eventually, the iron levels in the oceans dropped, and oxygen started building up in the atmosphere, where some of it formed our ozone layer (ozone= 3 oxygen molecules). Ozone protects life from harmful UV radiation from the sun; without it, life was largely restricted to the 🌊.

And while we often credit 🌳 for producing our life-sustaining oxygen, more than half of it actually comes from phytoplankton and seaweed in the ocean (both of which are algae, and not technically even plants).

But back to that early life and toxic oxygen—did the Great Oxygenation Event really trigger a mass extinction? You’ll just have to back @MakeAPlanetPod to find out.

Great job, everyone!

kickstarter.com/projects/makea…

Correction: I misremembered how cyanobacteria do photosynthesis—they don’t have chloroplasts (they do photosynthesis via special pigments, including chlorophyll). Cyanobacteria are thought to have BECOME organelles like chloroplasts! They were the assimilated, not assimilators!

Thanks to @rustneversleepz for catching my mistake!

And this is why @MakeAPlanetPod will have a paid fact-checker, ha! 4.5 billion years’ worth of biology and geology information is a lot.