- This key that we see here is rusty.

And in order to form rust, we need three ingredients.

The first is iron, which is what this key is made of.

The second is water, or water vapor in the air.

And the third crucial ingredient is oxygen.

- [Narrator] And in Earth's early oceans, oxygen began to react more and more with iron and other metals as cyanobacteria spread across the oceans.

(light music) Below the surface it's raining.

As oxygen and iron interact, rust particles form in the water and sink down to the seabed, trapping the oxygen and iron to form solid rock at the bottom of the ocean.

Vast swabs of the water turn red.

- There are many, many ingredients in the ocean at that time that would readily react with oxygen.

- So there was no way for atmospheric oxygen to build up to any significant level.

(birds calling) - [Narrator] A record of the reactions that happened long ago can be seen in formations today.

- This is a sedimentary rock, meaning it's made up of many different layers that were all deposited on the sea floor over millions of years.

- [Narrator] And this sample is evidence of how iron can react to form solid rock.

- This particular sedimentary rock is a banded iron formation, and we can see red color in the rock here, and that red is evidence of iron oxidation, or rust.

- [Narrator] As these reactions continued, the metals in the ocean water started to diminish, locked away in the sea floor.

- Eventually the oceans would've been depleted of the things that would readily react with the oxygen that was being released.

- [Narrator] But cyanobacteria continued to produce oxygen.

(water bubbling) For up to half a billion years this oxygen has been trapped in the oceans.

(water bubbling) But now, increasing amounts are free to escape.

(light music) Over just a few million years, oxygen floods into the atmosphere.

(epic music) The very composition of the atmosphere itself is changing.

(light music)