Earliest human migrations and expansions of archaic and modern humans across continents began 2 million years ago with the migration out of Africa of Homo erectus. This was followed by the migrations of other pre-modern humans including H. heidelbergensis, the likely ancestor of both modern humans and Neanderthals. Finally, Homo sapiens ventured out of Africa around 100,000 years ago, spread across Asia around 60,000 years ago and arrived on new continents and islands since then.
Knowledge of early human migrations, a major topic of archeology, has been achieved by the study of human fossils, occasionally by stone-age artifacts and more recently has been assisted by archaeogenetics. Cultural and ethnic migrations are estimated by combining archaeogenetics and comparative linguistics.
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Patents protected inventors and let them profit from their invention
A common factor that triggered the hot wars in Asia was based on communism. The United States wanted to halt the spread of communism while the Soviet Union wanted to influence every region and make them communists.
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Mohs hardness, rough measure of the resistance of a smooth surface to scratching or abrasion, expressed in terms of a scale devised (1812) by the German mineralogist Friedrich Mohs. The Mohs hardness of a mineral is determined by observing whether its surface is scratched by a substance of known or defined hardness.
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To give numerical values to this physical property, minerals are ranked along the Mohs scale, which is composed of 10 minerals that have been given arbitrary hardness values. The minerals contained in the scale are shown in the Table; also shown are other materials that approximate the hardness of some of the minerals. As is indicated by the ranking in the scale, if a mineral is scratched by orthoclase but not by apatite, its Mohs hardness is between 5 and 6. In the determination procedure it is necessary to be certain that a scratch is actually made and not just a “chalk” mark that will rub off. If the species being tested is fine-grained, friable, or pulverulent, the test may only loosen grains without testing individual mineral surfaces; thus, certain textures or aggregate forms may hinder or prevent a true hardness determination. For this reason the Mohs test, while greatly facilitating the identification of minerals in the field, is not suitable for accurately gauging the hardness of industrial materials such as steel or ceramics. (For these materials a more precise measure is to be found in the Vickers hardness or Knoop hardness) Another disadvantage of the Mohs scale is that it is not linear; that is, each increment of one in the scale does not indicate a proportional increase in hardness. For instance, the progression from calcite to fluorite (from 3 to 4 on the Mohs scale) reflects an increase in hardness of approximately 25 percent; the progression from corundum to diamond, on the other hand (9 to 10 on the Mohs scale), reflects a hardness increase of more than 300 percent.
