[N]ew research led by Stanford Medicine reveals that what we call the brain is two distinct organs that evolved independently over hundreds of millions of years.

The discovery overturns a prevailing model of brain development…The new research finding shows that the human brain consists of two ancient nervous systems cleverly packaged together — a more primitive part that regulates our hearts’ beating, our breathing and other functions, and another that makes us distinctly human, capable of poetry, mathematics and wondering about our own origins.

— “Human brain is two separate organs, Stanford Medicine-led research finds,” Stanford Medicine News Center