Metabolism first: iron–sulfur world The replicator in virtually all known life is deoxyribonucleic acid. His work was generally ignored until the 1920s, though in 1917 Joseph Barrell, a professor of geology at Yale, redrew geological history as it was understood at the time to conform to Holmes's findings in radiometric dating. According to the alternative Slushball Earth theory, even at the height of the ice ages there was still open water at the Equator. By measuring the concentration of the stable end product of the decay, coupled with knowledge of the half life and initial concentration of the decaying element, the age of the rock can be calculated. Early Paleozoic climates were warmer than today, but the end of the Ordovician saw a short ice age during which glaciers covered the south pole, where the huge continent Gondwana was situated.

In the 1930s, isotopes would be shown to have nuclei with differing numbers of the neutral particles known as " neutrons". He calculated the amount of time it would have taken for tidal friction to give Earth its current 24-hour day. At the time, Rutherford was only guessing at the relationship between alpha particles and helium atoms, but he would prove the connection four years later. The land bridge allowed the isolated creatures of South America to migrate over to North America, and vice versa. Between 8500 and 7000 BC, humans in the Fertile Crescent in the Middle East began the systematic husbandry of plants and animals: agriculture.billion years ago, the amount of time which passed since the last universal ancestor of all living organisms as shown by geological dating. Boltwood did the legwork, and by the end of 1905 had provided dates for 26 separate rock samples, ranging from 92 to 570 million years. After Henri Becquerel's initial discovery in 1896, [30] [31] [32] [33] Marie and Pierre Curie discovered the radioactive elements polonium and radium in 1898; [34] and in 1903, Pierre Curie and Albert Laborde announced that radium produces enough heat to melt its own weight in ice in less than an hour. The center of the nebula, not having much angular momentum, collapsed rapidly, the compression heating it until nuclear fusion of hydrogen into helium began. Although short, self-replicating RNA molecules have been artificially produced in laboratories, [84] doubts have been raised about whether natural non-biological synthesis of RNA is possible.

In a lecture in 1869, Darwin's great advocate, Thomas Henry Huxley, attacked Thomson's calculations, suggesting they appeared precise in themselves but were based on faulty assumptions. Present life forms could not have survived at Earth's surface, because the Archean atmosphere lacked oxygen hence had no ozone layer to block ultraviolet light. Methane is a strong greenhouse gas, but with oxygen it reacts to form CO 2, a less effective greenhouse gas. Although a process similar to present-day plate tectonics did occur, this would have gone faster too.The democratic capitalist United States and the socialist Soviet Union became the world's dominant superpowers for a time, and they held an ideological, often-violent rivalry known as the Cold War until the dissolution of the latter.

In the 1790s, William Smith hypothesized that if two layers of rock at widely differing locations contained similar fossils, then it was very plausible that the layers were the same age. The first shows the entire time from the formation of the Earth to the present, but this gives little space for the most recent eon. The Cryogenian period was followed by the Ediacaran period, which was characterized by a rapid development of new multicellular lifeforms. The large amounts of water held in the ice allowed for various bodies of water to shrink and sometimes disappear such as the North Sea and the Bering Strait. It captures the energy of sunlight in energy-rich molecules such as ATP, which then provide the energy to make sugars.

Surplus food allowed a priestly or governing class to arise, followed by increasing division of labor.

years ago (with an uncertainty of 1%) [28] [29] [4] and was largely completed within 10–20millionyears. The first atmosphere, captured from the solar nebula, was composed of light ( atmophile) elements from the solar nebula, mostly hydrogen and helium. John Perry's neglected critique of Kelvin's age for the Earth: A missed opportunity in geodynamics".As transportation and communication improved, the economies and political affairs of nations around the world have become increasingly intertwined. About 20million years later (340Ma [126] :293–296), the amniotic egg evolved, which could be laid on land, giving a survival advantage to tetrapod embryos. Archaeans, bacteria, and eukaryotes continued to diversify and to become more complex and better adapted to their environments. Complex life, including vertebrates, begin to dominate the Earth's ocean in a process known as the Cambrian explosion. The oldest such minerals analyzed to date—small crystals of zircon from the Jack Hills of Western Australia—are at least 4.