User:Mahat Bashir Mohamud

The history of Earth concerns the development of planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to the understanding of the main events of Earth's past. The age of the Earth is approximately one-third of the age of the universe. An immense amount of geological change has occurred in that timespan, accompanied by the emergence of life and its subsequent evolution.

Earth formed around 4.54 billion years ago byaccretion from the solar nebula.Volcanic outgassing probably created the primordial atmosphere and then the ocean, but the early atmosphere contained almost nooxygen and so would not have supported known forms of life. Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. A giant impact collision with a planet-sized body named Theia while Earth was in its earliest stage, also known as Early Earth, is thought to have been responsible for forming the Moon. Over time, the Earth cooled, causing the formation of a solid crust, and allowing liquid water to exist on the surface.

The geological time scale (GTS) depicts the larger spans of time, from the beginning of the Earth to the present, and it chronicles some definitive events of Earth history. The Hadean eon represents time before the reliable (fossil) record of life beginning on Earth; it began with the formation of the planet and ended at 4.0 billion years ago as defined by international convention. The Archean and Proterozoic eons follow; they produced the abiogenesis of life on Earth and then the evolution of early life. The succeeding eon is the Phanerozoic, which is represented by its three component eras: the Palaeozoic; the Mesozoic, which spanned the rise, reign, and climactic extinction of the non-avian dinosaurs; and the Cenozoic, which presented the subsequent development of dominant mammals on Earth.

Hominins, the earliest direct ancestors of the human clade, rose sometime during the latter part of the Miocene epoch; the precise time marking the first hominins is broadly debated over a current range of 13 to 4 million years ago. The succeeding Quaternary period is the time of recognizable humans, i.e., the genusHomo, but that period's two million-year-plus term of the recent times is too small to be visible at the scale of the GTS graphic. (Notes re the graphic: Ga means "billion years"; Ma, "million years".)

The earliest undisputed evidence of life on Earth dates at least from 3.5 billion years ago, during the Eoarchean Era after a geological crust started to solidify following the earlier molten Hadean Eon. There aremicrobial mat fossils such as stromatolites found in 3.48 billion-year-old sandstone discovered in Western Australia.Other early physical evidence of a biogenic substance is graphite in 3.7 billion-year-oldmetasedimentary rocks discovered in southwestern Greenland as well as "remains of biotic life" found in 4.1 billion-year-old rocks in Western Australia.According to one of the researchers, "If life arose relatively quickly on Earth … then it could be common in the universe."

Photosynthetic organisms appeared between 3.2 and 2.4 billion years ago and began enriching the atmosphere with oxygen. Life remained mostly small and microscopic until about 580 million years ago, when complexmulticellular life arose, developed over time, and culminated in the Cambrian Explosion about 541 million years ago. This event drove a rapid diversification of life forms on Earth that produced most of the major phyla known today, and it marked the end of the Proterozoic Eon and the beginning of the Cambrian Period of the Paleozoic Era. 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 currentspecies range from 10 million to 14 million,of which about 1.2 million are documented, but over 86 percent have not been described. Scientists recently reported that 1 trillion species are estimated to be on Earth currently with only one-thousandth of one percent described.

The Earth's crust has constantly changed since its formation. Likewise, life has constantly changed since its first appearance. Species continue to evolve, taking on new forms, splitting into daughter species or going extinct in the process of adapting or dying in response to ever-changing physical environments. The process of plate tectonics continues to shape the Earth's continents and oceans and the life they harbor. Human activity is now a dominant force affecting global change, adversely affecting thebiosphere, the Earth's surface, hydrosphere, and atmosphere, with the loss of wild lands, over-exploitation of the oceans, production ofgreenhouse gases, degradation of the ozone layer, and general degradation of soil, air, and water quality.