The origin of life is one of the great mysteries in the Universe. We know that life began at least 3.5 billion years ago, because that is the age of the oldest rocks with fossil evidence of life on earth. These rocks are rare because subsequent geologic processes have reshaped the surface of our planet, often destroying older rocks while making new ones. Nonetheless, 3.5 billion year old rocks with fossils can be found in Africa and Australia. They are usually a mix of solidified volcanic lavas and sedimentary cherts. The fossils occur in sedimentary cherts.
Chemical traces of life have also been detected in slightly older rocks. In Greenland, a series of ancient metamorphosed sediments have been found. Analysis indicates the sediments were deposited about 3.8 billion years ago. They also revealed carbon isotope signatures that appear to have been produced by organisms that lived when the sediments were deposited.
In all cases, life as we understand it must have water. This general rule is true on Earth and is thought to be true elsewhere in the solar system. Currently, life is being sought on Mars where water may have once flowed on the surface and Europa where a subterranean sea of water may exist beneath its icy surface.