The document summarizes information from a textbook chapter about the origin of life on Earth. It discusses how Earth's early atmosphere differed and was less oxygenated. The first life forms on Earth were likely single-celled organisms that lived in the oceans around 3.6 billion years ago. Scientists have conducted experiments that suggest simple organic molecules could have spontaneously formed in early Earth conditions and eventually combined to form the building blocks of life. Fossil evidence also supports the hypothesis that primitive single-celled life existed billions of years ago.