A complex central nervous system (CNS) is integral to vertebrate anatomy. Such complexity is largely achieved by a combination of more neuronal cells, a greater diversity of cell types, and sophisticated patterning events. How complex nervous systems evolved in vertebrates is an important question that has primarily been investigated by comparing patterning mechanisms between vertebrates and invertebrate chordates. In this study, Sebastian Shimeld and colleagues take a different approach to address vertebrate CNS complexity using the lamprey – a distant relative of jawed vertebrates – to study how vertebrates evolved to develop more neurons. Using in situ hybridisation to identify proliferating cells, the authors discover a conserved ventricular progenitor zone, similar to that observed in vertebrates, throughout the spinal cord of the lamprey. In addition, small molecule inhibition of Notch signalling reduced expression of proliferative cell markers and caused premature expression of differentiation markers. These findings demonstrate a conserved role for Notch signalling in regulating proliferation and differentiation of the ventricular progenitor zone in vertebrates, including lamprey. Taken together with evidence from invertebrate chordates, this work places the Notch-dependent ventricular progenitor zone as an essential vertebrate innovation contributing to CNS complexity.