Full Idea
For millenia people didn't know how to convert one type of energy into another, …and the only machine capable of performing energy conversion was the body.
Gist of Idea
For millenia people didn't know how to convert one type of energy into another
Source
Yuval Noah Harari (Sapiens: brief history of humankind [2014], 17 'Intro')
Book Reference
Harari,Yuval Noah: 'Sapiens: a brief history of Humankind' [Vintage 2014], p.375
A Reaction
Hence the huge and revolutionary importance of the steam engine and the electricity generator.