Full Idea
Essentialism has its source in the cognitive requirement of categorization in certain domains - particularly as they affect the young learner.
Gist of Idea
Essentialism comes from the cognitive need to categorise
Source
Susan A. Gelman (The Essential Child [2003], 01 'Essentialist')
Book Reference
Gelman,Susan A.: 'The Essential Child' [OUP 2005], p.7
A Reaction
I think the phenomenon is better understood as part of the cognitive requirement to understand and explain. Categorisation is just one way to aid explanation. Children try to understand (essentially) a new animal without categorisation.