From fossil evidence, it appears that life may have existed on Earth as early as 3.5 billion years ago. This suggests that life must have evolved sometime during Earth's tumultuous first billion years. How did life evolve? What did early forms of life look like? Topics covered include protocells, endosymbiosis, prokaryotes, eukaryotes, evolution, heredity, variation, natural selection, genetic drift, and gene flow. Part of the "Biology" series.