Oxygen. Without it, life on Earth would look very different. Because only plants and a few types of microorganisms can live without it.

Fortunately, plants originated 3.5 billion years ago. They inhaled so much carbon dioxide and exhaled so much oxygen that they changed Earth’s atmosphere. With more oxygen in the air, Earth could sustain animal life -- which would eventually become us.

Those first plants looked a lot different than you might imagine. They were phytoplankton, single-cell algae adrift in the surface layers of oceans. The largest were just one millimeter in diameter, and many far smaller.

Those same kinds of phytoplankton still exist today, in massive quantities, in every ocean. They were the base of the marine food web then and still are today. Without them, ocean ecosystems would collapse.

And, they still provide a great deal of Earth’s oxygen.

Just one type of phytoplankton—called cyanobacteria, or blue-green algae—makes a whopping 20% of our oxygen. That’s more than all of Earth’s rainforests put together!

Another 30% comes from other kinds of phytoplankton and marine plants, meaning fully half of Earth’s oxygen comes from the ocean. Every other breath you take!

We talk a lot about ocean health on EarthDate, and this is another reason why. The ocean and its trillions of microscopic algae literally make the air we breathe.