Fungal reproductive spores vary greatly in size, shape, color, and surface texture. They range from 1 to 100 microns (1 micron = 3.93700787 × 10-5 inch) in diameter—the largest approximately one-tenth the thickness of a dime—with the majority about 2 to 20 microns in size. Yet another comparison is that twenty million 5-micron spores could fit into the space of a postage stamp.

Although not as old as bacteria (fossil evidence suggests they may be at least 3.5 billion years old), fungi have been on the Earth for hundreds of millions of years. The earliest fungi fossil evidence is from the Ordovician period, about 460 to 455 million years ago. Some scientists suggest that fungi may have played an important part in allowing vascular plants to colonize land about 425 million years ago. Even when the dinosaurs roamed the planet over sixty-five million years ago, fungi were there, with some ninety-four-million-year-old fossils displaying mushroom-forming fungi similar to those that exist today.