Manual labour or manual work is physical work done by humans, in contrast to labour by machines and working animals. It is most literally work done with the hands and, by figurative extension, it is work done with any of the muscles and bones of the human body. For most of human prehistory and history, manual labour and its close cousin, animal labour, have been the primary ways that physical work has been accomplished. Mechanisation and automation, which reduce the need for human and animal labour in production, have existed for centuries, but it was only starting in the 18th and 19th centuries that they began to significantly expand and to change human culture. To be implemented, they require that sufficient technology exist and that its capital costs be justified by the amount of future wages that they will obviate. Semi-automation is an alternative to worker displacement that combines human labour, automation, and computerisation to leverage the advantages of both man and machine.