Stanford University neuroscientists believe the brain was originally two distinct organs that evolved independently over hundreds of millions of years. Studying the earliest stages of brain development in mice, they isolated two cell types that never overlap. One, linked to the Otx2 gene, forms the forebrain and midbrain; the other, linked to Gbx2, forms the hindbrain. Martin Parent, of the CERVO Brain Research Centre, notes that these cells are already committed to a specific lineage by the second embryonic day. He finds the work very interesting for brain research, but says the researchers must now confirm this predetermination.

See CBC article here (in French).