Single-celled creatures' size spiked as oxygen levels rose
MINNEAPOLIS — Giant armor-clad amoebas that once swam Paleozoic seas may have owed their monstrous size to something in the water: oxygen.
A new look at the fossil record suggests that a spike in oxygen levels supersized many species of these fusulinids, a now-extinct type of single-celled microbe called foraminifera. About 300 million years ago, when the atmosphere contained almost enough oxygen to spontaneously combust, some of these critters grew to be 10 centimeters long. They would have been visible to the naked eye.