The circulation of fluids evolved around 3.5 billion years ago in single-celled organisms to distribute nutrients and remove waste. Blood, as a specialized fluid carrying these essentials, evolved much later, around 1.2 billion years ago. In multicellular organisms, blood developed to nourish internal cells and carry away waste.

The big game-changer arrived about 450-500 million years ago, during the Silurian period, with the evolution of hemoglobin. This protein, found in red blood cells, allowed for the efficient transport of oxygen throughout the body. Hemoglobin’s unique ability to bind with oxygen and release it as needed revolutionized life on Earth.

And here’s a fun fact: the myth that your blood is blue until exposed to air is wrong! While blood appears a brighter red when exposed, inside your body, it’s actually a darker red. However, some animals, like crabs and mollusks, do have blue blood.

So there you have it – a brief, bloody history of the evolution of blood on Earth!