The cell theory is the concept that all living things are made up of essential units called cells and that they are the fundamental components of all life. The cell is the simplest collection of matter that can live. They represent diverse forms of life, existing as single-celled organisms or more complex organisms, such as multicellular plants and animals. They also include diverse specialized cells that cannot survive for long on their own. In addition, everything an organism does occurs fundamentally at the cellular level. Finally, all cells come from preexisting cells, and they are related by division to earlier cells—all of which have been modified in various ways during the long evolutionary history of life on Earth.