They are the oldest form of life on earth. Microbe fossils date back more than 3.5 billion years to a time when the Earth was covered with oceans that regularly reached the boiling point, hundreds of millions of years before dinosaurs roamed the earth.

Bacteria
Often dismissed as “germs” that cause illness, bacteria help us do an amazing array of useful things, like make vitamins, break down some types of garbage, and maintain our atmosphere.

Fungi
From a single-celled yeast to a 3.5-mile-wide mushroom, fungi do everything from helping to bake bread to recycling to decomposing waste.

Protista
Plant-like algae produce much of the oxygen we breathe; animal-like protozoa (including the famous amoeba) help maintain the balance of microbial life.

Viruses
Unable to do much of anything on their own, viruses go into host cells to reproduce, often wreaking havoc and causing disease. Their ability to move genetic information from one cell to another makes them useful for cloning DNA and could provide a way to deliver gene therapy.