In economics and sociology, the activities and labour necessary for the survival of society. As early as 40,000 BC, hunters worked in groups to track and kill animals, while younger or weaker members of the tribe gathered food. When agriculture replaced hunting and gathering, the resulting surplus of food allowed early societies to develop and some of its members to pursue crafts such as pottery, weaving, and metallurgy. Historically, rigid social hierarchies caused nobles, clergy, merchants, artisans, and peasants to pursue occupations defined largely by hereditary social class. Craft guilds, influential in the economic development of medieval Europe, limited the supply of labour in each profession and controlled production. The establishment of towns led to the creation of new occupations in commerce, law, medicine, and defense. The coming of the Industrial Revolution, spurred by technological advances such as steam power, changed working life profoundly. Factories divided the work once done by a single craftsman into a number of distinct tasks performed by unskilled or semiskilled workers (see division of labour). Manufacturing firms grew larger in the 19th century as standardized parts and machine tools came into use, and ever-more-specialized positions for managers, supervisors, accountants, engineers, technicians, and salesmen became necessary. The trend toward specialization continued into the 21st century, giving rise to a number of disciplines concerned with the management and design of work, including production management, industrial relations, personnel administration, and systems engineering. By the turn of the 21st century, automation and technology had spurred tremendous growth in service industries.