Noun

1. The theory that mankind originated with a single ancestor or ancestral couple.

   • 2003: Some held that negritude was a product of living under the tropical sun, a perhaps beneficial adaptation to a fierce climate – an environmentalist solution chiming with monogenesis and Lockean malleability models. — Roy Porter, Flesh in the Age of Reason (Penguin 2004, p. 248)

2. The theory that all languages, or a particular set of languages, originated from a single source.
3. The emergence from a single cause.