Three billion years ago the land was lifeless and the air oxygen-free but rich in CO2.

The oceans were hot and loaded with nitrogen and phosphorus, and aquatic microbes called

cyanobacteria were loving it.

These microbes would later turn out to be our enemies, but at this point in time humans

didn't exist yet.

In fact, cyanobacteria actually helped make our existence possible in the first place.

But back to early life on Earth...

In addition to being heat tolerant, the cyanobacteria grew in thin mats that were good at soaking

up light and nutrients like nitrogen and phosphorus, and built nasty toxins to poison their competitors.

And in one of the most profound steps in all of evolution, they figured out how to combine

carbon dioxide with water to make tasty sugar - a process called photosynthesis.

But that fancy new photosynthesis also happened to release oxygen which was poisonous to organisms

that had evolved under oxygen-free conditions, which meant pretty much all life on Earth

at that time, including most of the cyanobacteria themselves.

But over time the surviving cyanobacteria evolved, to not just tolerate oxygen, but

to use it, with a sort-of reversal of photosynthesis, which we now call aerobic respiration.

This helped cyanobacteria survive, and in fact, grow to dominate the Earth's oceans

for another billion years.

Eventually though, as the Earth began to cool and nutrient supplies got used up, some algae

well adapted to those conditions also stole cyanobacteria's metabolic secrets.

The algae out-competed cyanobacteria, pushing them into the shadows across much of Earth's

waters for the next billion years or so.

This long interval also saw the evolution of more complicated life-forms, including

oxygen-breathers like us … and we have held center stage from cyanobacteria ever since.

But our success today is now making things awesome again for cyanobacteria.

We've done this by pumping CO2 into the air, which has warmed the atmosphere and oceans,

which cyanobacteria like.

Also, because we over-fertilize our farm fields, the rain washes a lot of that fertilizer into

rivers and oceans, providing a level of delicious nutrients that cyanobacteria haven't seen

for perhaps billions of years.

And that's bad for us, because these heat-loving, nutrient-gobbling microbes are once again

forming sludgy gross-smelling mats, that release nasty toxins that keep their algal competitors

at bay - but also make animals and people sick.

And since cyanobacteria live short lives and die in large groups, floating mats of their

dead bodies serve as food for oxygen-breathing decomposers, who temporarily use up all the

available oxygen in the water, killing fish, shrimp, insects, and plants in sometimes dangerously

massive dead zones.

To keep cyanobacteria at bay, we need to stop warming the planet and to farm in a way that

doesn't send nutrients into waterways.

Until we do, the little creatures that first gave us oxygen are going to keep on blooming.

And dying.

And turning our oceans and lakes to the dark side.

This video was sponsored by the St. Croix Watershed Research Station, with financial

support from the McKnight Foundation.

The Research Station is part of the Science Museum of Minnesota, and its scientists study

lakes and rivers across Minnesota to better understand when, where, and why cyanobacterial

blooms occur — and how we can prevent cyanobacteria from harming lakes and rivers and the people

and wildlife that depend on them.

Thanks, St. Croix Watershed Research Station!