The document summarizes the history of life on Earth based on evidence from the fossil record. It describes how early Earth's atmosphere allowed for the abiotic synthesis of organic molecules around 4 billion years ago. The earliest life forms were single-celled prokaryotes that formed around 3.5 billion years ago. Oxygen began accumulating in the atmosphere around 2.7 billion years ago due to cyanobacteria. Eukaryotic cells developed around 2.1 billion years ago through endosymbiosis. Multicellular life emerged around 1.5 billion years ago. The Cambrian explosion saw a rapid diversification of life around 535 million years ago. Plants and fungi first colonized land around 500 million years ago