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Evolution of modern human beings from extinct nonhuman and humanlike forms. Genetic evidence points to an evolutionary divergence between the lineages of humans and the great apes on the African continent 85 million years ago (mya). The earliest fossils considered to be remains of hominins (members of the human lineage) date to at least 4 mya in Africa; they include the genus Australopithecus and other forms. The next major evolutionary stage, Homo habilis, inhabited sub-Saharan Africa about 21.5 mya. Homo habilis appears to have been supplanted by a taller and more humanlike species, Homo erectus, which lived from c. 1,700,000 to 200,000 years ago, gradually migrating into Asia and parts of Europe. Between c. 600,000 and 200,000 years ago, Homo heidelbergensis, sometimes called archaic Homo sapiens, lived in Africa, Europe, and perhaps parts of Asia. Having features resembling those of both H. erectus and modern humans, H. heidelbergensis may have been an ancestor of modern humans and also of the Neanderthals (H. neanderthalensis), who inhabited Europe and western Asia from c. 200,000 to 28,000 years ago. Fully modern humans (H. sapiens) seem to have emerged in Africa only c. 150,000 years ago, perhaps having descended directly from H. erectus or from an intermediate species such as H. heidelbergensis.
Cross section of an adult molar. The crown (the part of the tooth above the gum) is protected by a © Merriam-Webster Inc.Any of the hard structures in the mouth used for biting and chewing and in speech. Each consists of a crown above the gum and one or more roots below it, embedded in the jaw. Its inner pulp contains the blood and nerve supply for the bonelike dentin, covered in the crown by enamel, the hardest tissue in the body. Twenty primary (baby) teeth come in by age 2 1/2 and fall out between ages 5 and 13 to be replaced by 32 permanent teeth. The incisors, in front, are shaped mostly for biting, the pointed canines for tearing, and the premolars and molars for grinding food. The teeth are subject to caries (decay), caused by acid from bacteria in plaque, a yellowish film that builds up on teeth. Misalignment of teeth between the upper and lower jaws can grind down the teeth and cause problems in chewing. Elsewhere, it is a cosmetic problem. Both can be treated with braces. See also dentistry.
© 2010 Encyclopædia Britannica, Inc.


