For two billion years (nearly two thirds of our planet's history) single cells were the only forms of life on earth.

Today, single-celled organisms are the most widely distributed life forms. They thrive almost everywhere, from arctic ice to hot sulphur springs.

The earliest cells are known from fossils. Some of these divided to form ball-shaped or thread-like structures. By 3.5 billion years ago, many forms of single-celled life existed.

Life: a system that is able to convert raw materials from its environment into identical, or nearly identical, copies of itself. Occasionally, slight changes occur during the copying process, resulting in offspring with individual differences -- from which new features can arise, and upon which evolutionary change depends.

New features:

• Structured cell organizes energy to feed, grow, and reproduce
  
• DNA: Chemical gene coding ensures that offspring are identical -- or almost identical