The solar system in which the Earth lies came into being 4.7 - 4.6 billion years ago out of the gas clouds and dust left over from the formation of the Sun. The Earth itself began to form 4.5 - 4.4. billion years ago, subject to heat and pressure so intense that it was at first a molten planet. The earliest stage of Earth's history is known as the pre-Cambrian period, divided into three separate sections: the Hadean, Archean, and Proterozoic.
[Earth's formation] It was during the firt period, the Hadean, that Earth was, formed, began to cool, and fell into a regular orbit around the Sun. The cooling process gave rise to the Earth's first atmosphere, which consisted of gases produced by the constant eruptions on the surface, but did not contain any free oxygen. The cooling of the Earth's crust also produced vast clouds that gave out the enormous volume of rainwater requerid to create the oceans of the word. This led to the Archean period, from 4.0 to 2.5 billion years ago, which saw the first appearance of life on Earth, initially in the form of simple bacteria. By the beginning of the Proterozoic period, simple algae were beginning to photosynthesize and create oxygen; this period also saw the formation of the first super-continent, known as Rodinia, about 1.1. billion years ago.
[Life forms] The end of the Proterozoic period and beginning of the Paleozoic era (542 to 251 million years ago) saw a massive upsurge in new forms of life, as shown in the fossil record. This "Cambrian explosion" led to the development of most of the major animal groups that survive today. While many of these species were to become extinct at the end of the Cambrian period, ca 488 million years ago, the Paleozoic period saw the increased specialization of organisms, with the separate development of fish, amphibians, and then mammals, birds, and reptiles.
It was this last group of creatures which was to provide the best-known life forms of the following period, the Mesozoic (250 - 65.5 million years ago). During it, reptiles developed into dinosaurs, the dominant species of the era, which came to an abrupt end with a mass extinction some 65.5 million years ago. It marked the end of the Paleozoic era and the commencement of the Cenozoic era, which continues to the present day. This period saw Earth's continents drift to the positions they currently occupy, while the disappearence of the dinosaurs led to the rise of the mammals - most notably our own species, Homo sapiens - as the dominant class of creatures.
[Demise of the dinosaurs] Around 65.5 million years ago, many of the species that had developed during the "Cambrian explosion" were wiped out, including the dinosaurs. This event, known to scientists as the Cretaceous-Tertiary extinction event (or K-T Extinction, from the German), wiped out some 85 percent of the species alive on Earth. Its causes are uncertain, but one plausible theory is that a massive asteroid or comet hit the Earth, creating a mass cloud of vaporized rock and steam in the atmosphere, blocking out the Sun, and causing a catastrophic drop in temperature. This would have caused many plants to die out, discrupting the food chain and possibly causing the mass extinction.
[The earliest humans] The human race belongs to the genus Homo, which first appeared around 2.5 million years ago. The earliest member of this genus, Homo habilis, developed in Africa and used simple tools. Homo ergaster, who existed some 1.65 million years ago, developed more complex tools, including axes and cleavers, and also spread beyond Africa for the first time. It was not, however, until the time of Homo erectus, some 1.8 to 1.5 million years ago, that man spread throughout Asia in any kind of numbers. The first known Europeans were Homo antecessor or Homo heidelbergensis, dating from some 800.000 years ago. Another species of Homo, the Neanderthais, also thrived in Europe from 200.000 to 25.000 years ago, when they finally became extinct, with their place being taken by Homo sapiens, modern humans.
COWPER, Marcus. History book: an interactive journey. National Geografhic. Carlton Books Limited: Washington D.C., 2010. p. 6-7.