Agriculture is the production of food, feed, fiber and other goods by the systematic growing/harvesting of plants, animals and other life forms. "Agriculture" may commonly refer to the study of the practice of agriculture (also, "agronomy" or "agricultural science").Agriculture encompasses many subjects, including aquaculture, cultivation, animal husbandry, and horticulture. Each of these subjects can be further partitioned: for example, cultivation includes both organic farming and intensive farming, and animal husbandry includes ranching, herding, and intensive pig farming. Agricultural products include fodder, (starch, sugar, alcohols and resins), fibers (cotton, wool, hemp, silk and flax), fuels (methane from biomass, ethanol, biodiesel), cut flowers, ornamental and nursery plants, tropical fish and birds for the pet trade, and both legal and illegal drugs (biopharmaceuticals, tobacco, marijuana, opium, cocaine).The history of agriculture is a central element of human history, as agricultural progress has been a crucial factor in worldwide socio-economic change. Wealth-building and militaristic specializations rarely seen in hunter-gatherer cultures are commonplace in agricultural and agro-industrial societies—when farmers became capable of producing food beyond the needs of their own families, others in the tribe/village/City-state/nation/empire were freed to devote themselves to projects other than food acquisition.As of 2006, an estimated 36 percent of the world's workers are employed in agriculture (down from 42% in 1996), making it by far the most common occupation. However, the relative significance of farming has dropped steadily since the beginning of industrialization, and in 2006 – for the first time in history – the services sector overtook agriculture as the economic sector employing the most people worldwide. Also, agricultural production accounts for less than five percent of the gross world product (an aggregate of all gross domestic products).