Life began on Earth about 3.8 billion years ago, a mere 700 million or so years after the Earth was formed. However, these simple life forms remained mostly microscopic until about 570 million years ago.
The first life forms were single-celled prokaryotic beings similar to bacteria. For over a billion years, these were the dominant life form on Earth until, with the accumulation of oxygen and a protective atmospheric ozone layer, simple multicellular life began to appear. By 500 million years ago, the current phyla of all life forms had appeared. About 434 million years ago, primitive plants began living on land. By 395 million years ago, some sort of animal had moved onto land as well.