The Evolutionary History of Life

The evolutionary history of life on Earth traces the processes by which both living organisms and fossil organisms evolved since life emerged on the planet until the present. Earth formed about 4.5 billion years (Ga) ago and evidence suggests life emerged prior to 3.7 Ga. Although there is some evidence to suggest that life appeared as early as 4.1 to 4.28 Ga this evidence remains controversial due to the non-biological mechanisms that may have formed these potential signatures of past life. The similarities among 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 Earths current species range from 10 million to 14 million of which about 1.9 million are estimated to have been named and 1.6 million documented in a central database to date. More recently in May 2016 scientists reported that 1 trillion species are estimated to be on Earth currently with only one-thousandth of one percent described. This book is a compilation of high quality articles from the Internet.