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.
In turn, the mass appearance of oxygen resulted in evolution of new animals known as eukaryotes. Unlike the earlier bacteria, these creatures, though still single-celled, had something new - a DNA-containing nucleus in its center. Having first appeared around 1500 MYA, eukaryotes then began to evolve into multicell creatures instead, similar to modern sponges and jellyfish.
Then, between 750 and 600 MYA the world was hit by an ice age. The temperature fell to -40 degrees Celsius and the formerly warm oceans began to freeze, creating what is unofficially called 'Snowball Earth'. When it ended, live had survived and had began to evolve into more complex animals, such as the possible ancestors of trilobites, called Spriggina.