There is a strong correlation between manual labour and unskilled or semiskilled workers, despite the fact that nearly any work can potentially have skill and intelligence applied to it (for example, the artisanal skill of craft production, or the logic of applied science). It has always been the case for humans that many workers begin their working lives lacking any special level of skill or experience. (In the past two centuries, education has become more important and more widely disseminated; but even today, not everyone can know everything, or have experience in a great number of occupations.) It has also always been the case that there was a large amount of manual labour to be done; and that much of it was simple enough to be successfully (if not masterfully) done by unskilled or semiskilled workers, which has meant that there have always been plenty of people with the potential to do it. These conditions have assured the correlation's strength and persistence.
Throughout human prehistory and history, wherever social class systems have developed, the social status of manual labourers has, more often than not, been low, as most physical tasks were done by peasants, serfs, slaves, indentured servants, wage slaves, or domestic servants. For example, legal scholar L. Ali Khan analyses how the Greeks, Hindus, English, and Americans all created sophisticated social structures to outsource manual labour to distinct classes, castes, ethnicities, or races.1
Khan 2006. - Khan, Ali (2006-10-12) [2001], "The dignity of manual labor", Columbia Human Rights Law Review, Social Science Research Network, SSRN 936890. https://papers.ssrn.com/sol3/papers.cfm?abstract_id=936890 ↩