Division of labour
In ancient history, a division of labour (or specialisation) was when some people in a city stopped growing crops and took other jobs. This usually happened because of a surplus of food. This means that there was more than enough food for everyone, so some people did not have to grow crops anymore.
"Just as the various trades are most highly developed in large cities, in the same way food at the palace is prepared in a far superior manner. In small towns the same man makes couches, doors, ploughs and tables, and often he even builds houses, and still he is thankful if only he can find enough work to support himself. And it is impossible for a man of many trades to do all of them well. In large cities, however, because many make demands on each trade, one alone is enough to support a man, and often less than one: for instance one man makes shoes for men, another for women, there are places even where one man earns a living just by mending shoes, another by cutting them out, another just by sewing the uppers together, while there is another who performs none of these operations but assembles the parts, Of necessity, he who pursues a very specialised task will do it best."
- Cited in The Ancient Economy by M. I. Finley. Penguin books 1992, p 135.