600-555 million years ago
THE NERVOUS SYSTEM
Ancestral coelomates evolved a number of fundamental features which would be retained in their vertebrate descendants such as a brain divided into 3 regions (forebrain, midbrain, and hindbrain with an important midbrain/hindbrain boundary) and homologous gene expression patterns.
Are the divisions of vertebrates and advanced invertebrates homologous?
In other words, were some of the divisions of the brain of advanced vertebrates
inherited from invertebrate ancestors? Molecular and developmental evidence
supports the conclusion that the brains of bilateran animals are homologous
organs which are derived from a common ancestral structure (Reichert,
2001). Both vertebrates and higher invertebrates (such as Drosophila)
have the common feature of a brain which is divided into a forebrain,
midbrain, and hindbrain with an important midbrain/hindbrain boundary
characterized by the expression patterns of Otx, Hox, and Pax2/5/8 genes
(Hirth, 2003). The brain of urochordates seems to be divided into three
regions: a forebrain/midbrain (as indicated by the expression of Hroth,
the homolog of vertebrate Otx), an anterior hindbrain (as indicated by
the tunicate homolog of vertebrate Pax genes), and a posterior hindbrain
(as indicated by the expression of a tunicate Hox gene) (Wada, 1998).