It allowed cells to colonize the surface of the ocean and eventually the land: without the ozone layer, ultraviolet radiation bombarding land and sea would have caused unsustainable levels of mutation in exposed cells. [citation needed], This article is about scientific evidence concerning the history of Earth. The history of Earth is divided into four great eons, starting 4,540 mya with the formation of the planet. [41] Nevertheless, detrital zircon crystals dated to 4.4 Ga show evidence of having undergone contact with liquid water, suggesting that the Earth already had oceans or seas at that time. [8] This definition would serve as the basis of logical positivism, with its assumption that there is exactly one world, consisting of the totality of facts, regardless of the interpretations that individual people may make of them.

[24] A shock wave would have also made the nebula rotate. Thus, this glaciation, known as the Huronian glaciation, may have been global. [142] After each extinction pulse, the continental shelf regions were repopulated by similar life forms that may have been evolving slowly elsewhere.

[187] The World Wide Web became a part of everyday life in the 1990s, and since then has become an indispensable source of information in the developed world. [125]:374 It is at least certain that by the end of the Proterozoic eon, most of the continental mass lay united in a position around the south pole.

[183] However, among those civilizations that did adopt agriculture, the relative stability and increased productivity provided by farming allowed the population to expand.

[59]:130–132, As the Earth cooled, clouds formed. Examples of temple mounds include Monks Mound located at the Cahokia site in Collinsville, Illinois,[23] and Mound H at the Crystal River site in Citrus County, Florida.

[101] Some organisms, including purple bacteria and green sulfur bacteria, use an anoxygenic form of photosynthesis that uses alternatives to hydrogen stripped from water as electron donors; examples are hydrogen sulfide, sulfur and iron. Although a process similar to present-day plate tectonics did occur, this would have gone faster too. In philosophy, the term world has several possible meanings.

75% of all life, including the non-avian dinosaurs, became extinct,[165] marking the end of the Cretaceous period and Mesozoic era. The new forms of life, called Ediacara biota, were larger and more diverse than ever. An alternative hypothesis is that enough carbon dioxide escaped through volcanic outgassing that the resulting greenhouse effect raised global temperatures. According to the alternative Slushball Earth theory, even at the height of the ice ages there was still open water at the Equator.

[25], Military earthworks can result in subsequent archaeological earthworks. [22] The American Plains also hold temple mounds, or platform mounds, which are giant pyramid-shaped mounds with flat tops that once held temples made of wood. Some of these Cambrian groups appear complex but are seemingly quite different from modern life; examples are Anomalocaris and Haikouichthys. Shallow earthworks are often more visible as cropmarks or in aerial photographs if taken when the sun is low in the sky and shadows are more pronounced.