During the Proterozoic eon life began to develop from simple bacteria into complex plants and animals, including humans. Proterozoic eon started 2500 MYA. At this time all of life still lived under water. The atmosphere had plenty of carbon dioxide, but little oxygen, making dry land a very uncomfortable place to live. However, as time went by, bacteria, that lived during the Proterozoic began to generate oxygen. This excess oyxgen was released into the atmosphere and by 2000 MYA, Earth's atmosphere actually contained a high amount of that gas.