The evolutionary history of life on Earth traces the processes by which living and fossil organisms have evolved since life appeared on the planet, until the present day. Earth formed about 4.5 Ga (billion years) ago and life appeared on its surface within 1 billion years. The similarities between all present-day organisms indicate the presence of a common ancestor from which all known species have diverged through the process of evolution. More than 99 percent of all species, amounting to over five billion species, that ever lived on Earth are estimated to be extinct. Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described.