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The reptilian brain is considered to be the oldest part of the human brain. Reckoned to be hundreds of millions of years in development, it handles very basic things:

The following components perform functions including those associated with the reptilian brain:

Compare with the mammalian brain and neocortex, which presumably developed as our distant ancestors evolved from reptiles into mammals.

Portions of this are derived from http://www.brainchannels.com/evolution/reptiliantable.html