Evolution of life on Earth has been ongoing for about four billion years, covering 90% of Earth's history. Only the last 500 million years (less than 15% of evolution's time span) are documented by fossils, while more than 3 billion years cover the 'dark era' [1] . If the time period from the ' Cambrian explosion' of life to the present is imagined as a clock's movement from 12 AM to 12 PM, then homo sapiens appeared less than one minute (i.e. 20 seconds) before noon and has left written records of his history only during the last half second. Life's evolution happened in tree-like branching of lineages, marked by the evolvement and extinction [2] of ever new species. As more complex forms evolved over time, there is the appearance of an advancing process, but in reality there is no direction. In his groundbreaking book On the Origin of Species, Darwin explains evolution as a process of random genetic variations and adaptive natural selection. Modern molecular genetics is discovering new ancestral relations and lineages. The enormous time span involved, the randomness of the process, the briefness of human history, and the abundance of planets around other stars, can trigger intriguing thoughts of what our planet looked like eons ago, how the process could have gone differently, how it might develop in future, and what the situation might be on countless other planets.
More on verifying evolutionary events
Extinctions of species, larger groups, and lineages during the course of evolution are normal events (scientists believe that more than 99 % of all species that ever lived on Earth are extinct). Geologists/paleontologists have also identified mass extinctions, rapid global disappearances of many species, that contributed to the classification of geologic times. Three of the largest such events occurred: (i) about 450 million years ago, at the transition Ordovician-Silurian, when about 50% of all species went extinct; (ii) about 250 million years ago, at the transition Permian-Triassic, when about 90% of all species went extinct; and (iii) about 65 million years ago, at the transition Cretaceous-Paleogene, when about 50% of all species (including all dinosaurs) went extinct. The causes are believed to be meteoric hits, vulcanism, and climate changes.