Given the major importance of the capability to generate defined
types of neurons in a defined succession, it may be that no complex
nervous system could evolve without the ability to program and
control asymmetric divisions. In flies, where the mechanism of this
asymmetry has been best analysed, one of its major protagonists
is the product of the gene numb. The Numb protein is localized at
one pole of the dividing cell as a cortical crescent, and determines
the fate of the daughter cell by which it is inherited (Rhyu et al.,
1994). There is now good evidence that this process has been
conserved in vertebrates (Shen et al., 2002, Cayouette and Raff,
2002), indicating that mechanisms to program cell diversity already
existed in the urbilaterian central nervous system.