630-600 million years ago

acoel flatworm

Evolutionarily, brains began as a concentration of neural tissue from the nerve net comparable to that which existed in cnidarians. The acoel flatworms which possessed such nervous systems retained a number of features which were radially symmetrical, similar to the condition of the ancestral cnidarians.

Brains had humble beginnings (as evidenced by the fact that many neuroanatomists would insist that the term "cerebral ganglion" be used instead of "brain" when referring to primitive bilateran animals). Acoelomate worms (particularly a group called Acoela) are the most primitive bilateral animals (other than some elongated comb jellies) and possess the most primitive brains. At first this brain had little connection with or control over the diffuse nerve net throughout the rest of the body (Beklemishev, vol. 2, p. 50-1; Raikova, 2000). Acoels such as Convoluta stylifera and Otocoelis gulmariensis (and even turbellarians such as Xenoturbella) lack longitudinal nerves and their nervous system consists of a diffuse subepithelial plexus which is more developed at the anterior end (similar to the situation in coelenterate larvae)(Beklemishev, vol. 2, p. 80). These acoels retain traces of radial symmetry: there are four radially arranged brain rudiments and four roots of ventral and dorsal nerves (Beklemishev, vol. 1, p. 112-3).