written records, the shift from nomadic life to settlement and agriculture can be inferred from a range of archaeological evidence. Such evidence includes ancient tools, cave paintings, and other prehistoric art, such as the Venus of Willendorf. Human remains also provide direct evidence, both through the examination of bones, and the study of mummies. Scientists and historians have been able to form significant inferences about the lifestyle and culture of various prehistoric peoples, and especially their technology. Ancient Copper and Bronze Ages Metallic copper occurs on the surface of weathered copper ore deposits and copper was used before copper smelting was known. Copper smelting is believed to have originated when the technology of pottery kilns allowed sufficiently high temperatures. The concentration of various elements such as arsenic increase with depth in copper ore deposits and smelting of these ores yields arsenical bronze, which can be sufficiently work hardened to be suitable for making tools. Bronze is an alloy of copper with tin; the latter being found in relatively few deposits globally caused a long time to elapse before true tin bronze became widespread. (See: Tin sources and trade in ancient times) Bronze was a major advancement over stone as a material for making tools, both because of its mechanical properties like strength and ductility and because it could be cast in molds to make intricately shaped objects. Bronze significantly advanced shipbuilding technology with better tools and bronze nails. Bronze nails replaced the old method of attaching boards of the hull with cord woven through drilled holes. Better ships enabled long-distance trade and the advance of civilization. This technological trend apparently began in the Fertile Crescent and spread outward over time. These developments were not, and still are not, universal. The three-age system does not accurately describe the technology history of groups outside of Eurasia, and does not apply at all in the case of some isolated populations, such as the Spinifex People, the Sentinelese, and various Amazonian tribes, which still make use of Stone Age technology, and have not developed agricultural or metal technology. Iron Age Before iron smelting was developed the only iron was obtained from meteorites and is usually identified by having nickel content. Meteoric iron was rare and valuable, but was sometimes used to make tools and other implements, such as fish hooks. The Iron Age involved the adoption of iron smelting technology. It generally replaced bronze