Primordial Earth
Life on Earth began about four billion years ago, when volcanism spewed forth gases to create a thick atmosphere, and water vapor clouds condensed to form the oceans. Crashing meteorites and comets brought organic molecules to the mixture. Electrical storms, volcanism, and radioactivity provided the energy needed to spark the mixture to life.

Life Begins to Assemble
In modern cells, leafy veins of RNA follow the genetic plan supplied by the central strand of DNA. However, DNA, on which all living things store their genetic information, seems too elaborate a starting point for early life. It may have relied instead on RNA, which is able to make crude copies of itself and interact readily in a world pervaded with chemical activity.

Cells Take Form
Nestled in a cavity etched from 3.86-billion-year-old rock, this tiny ball of carbon may be the oldest evidence of life on Earth. Many scientists believe that meteorites supplied the early Earth with amino acids that could have turned into layered structures similar to cell walls. According to one scenario, these acids accumulated as foam on the oceans and were carried by wind and waves onto ancient tidal flats, where they formed into the cell membranes of Earth's first microorganisms.

Global Gas Warfare
Stromatolite reefs still flourish in Shark Bay, Australia, as they did throughout the globe 3.5 billion years ago. Stromatolites are made up of different layers of bacteria. The first bacteria thrived in an oxygen free environment. Then mutant offspring began to produce oxygen, poisoning the parents. As the air became richer in oxygen, the offspring took over the surface; burying the parents and creating the layers that make up the stromatolite.

Triumph of Oxygen
Sea water once contained dissolved iron. The oxygen produced by bacteria reacted with the iron, which resulted in rust on the ocean floor. Millions of years later this rust turned into deep beds of banded rock. Eventually, all the iron in the oceans was oxidized, making the atmospheric oxygen levels rise sharply, benefiting larger organisms. 

Sex is Invented
Until now all cells reproduced by self replicating, or creating identical clones of themselves. Then, eucaryotes, or cells with a true nucleus, began to emerge. The nucleus, which stores genetic material, allowed these organisms to reproduce with another cell for the first time. By creating a third individual and unique cell, the variety of living things increased dramatically.