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The Olmec colossal heads are at least 17 monumental stone representations of human heads sculpted from large basalt boulders. The heads date from at least before 900 BC and are a distinctive feature of the Olmec civilization of ancient Mesoamerica. All portray mature men with fleshy cheeks, flat noses and slightly crossed eyes; their physical characteristics correspond to a type that is still common among the inhabitants of Tabasco and Veracruz. The boulders were brought from the Sierra de los Tuxtlas, a mountain range in Veracruz. It is thought that the finished monuments represent portraits of powerful individual Olmec rulers. Each is given a distinctive headdress. The heads were variously arranged in lines or groups at major Olmec centres. Dating the monuments remains difficult due to the movement of many from their original context prior to archaeological investigation. Most have been dated to the Early Preclassic period (1500–1000 BC) with some to the Middle Preclassic period (1000–400 BC). (Full article...)
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In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. In biochemistry, this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent.[1] In the alpha amino acids, the amino and carboxylate groups are attached to the same carbon, which is called the α–carbon. The various alpha amino acids differ in which side chain (R group) is attached to their alpha carbon. They can vary in size from just a hydrogen atom in glycine through a methyl group in alanine to a large heterocyclic group in tryptophan. Beyond the amino acids that are found in all forms of life, many non-natural amino acids have vital roles in technology and industry. For example, the chelating agents EDTA and nitrilotriacetic acid are alpha amino acids that are important in the chemical industry. (more...) Recently highlighted: Two-level utilitarianism – Islamic view of Ezra – Weapon dance |
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October 4: Cinnamon Roll Day in Sweden and Finland
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Hübnerite is a mineral consisting of manganese tungsten oxide, with a chemical formula MnWO4. It typically occurs in association with high-temperature hydrothermal vein deposits and altered granites and in alluvial deposits. Hübnerite is the manganese endmember of the wolframite solid solution series, with ferberite (FeWO4) the opposite iron endmember. Color differences between members of the wolframite family are clear and marked, with the color of hübnerite varying from yellowish brown to reddish brown. It was first described in 1865 for an occurrence in the US state of Nevada and was named after the German mining engineer and metallurgist Adolf Hübner. This stacked image, composed of 38 individual photographs, shows red hübnerite crystals with quartz extracted from the Pasto Bueno mine in Pallasca Province, Peru. Photograph credit: Ivar Leidus Recently featured: Leonhard Euler - Australasian shoveler - Passion fruit |
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- ^ Proline is an exception to this general formula. It lacks the NH2 group because of the cyclization of the side chain.