In a fundamental sense, time represents the continuous progression of existence and events. It unfolds moment by moment and permits a structured chronology where events occur in a specific sequence, not randomly or all at once. This framework allows events to be ordered from the past through the present into the future, which in turn helps us understand causal chains where causes precede effects.

In developmental biology, the concept of time is closely intertwined with the pace of embryonic development. This connection can be viewed in at least two ways: the specific sequence of events that occur in a particular chronological order as well as the total amount of time this sequence occupies. Both of these dimensions are found in one of the most-studied and evolutionarily conserved biological mechanisms, neurogenesis, and the generation of neurons from neural progenitor cells (NPCs). Not only are different neuronal subtypes generated at different time points in a specific order but the total amount of time dedicated to neurogenesis is itself a species feature of brain development. Intriguingly, while the sequential stages of brain development share ancestral blueprints, the temporal dynamics can diverge considerably among species. The human brain, for example, undergoes a particularly extended period of neurogenesis compared with rodents and even other primates, allowing for a disproportional increase in the number of neurons, especially in the cerebral neocortex 1, 2. Thus, there is a correlation between differences in timing and brain size, presumably intimately linked to differences in cognitive capabilities.

This review will discuss the temporal dynamics of neurogenesis and the factors that drive NPCs to their transition into postmitotic neurons. We will explore a conceptual framework for understanding how and why changes in timing arose during evolution and what this framework predicts as possible future experimental avenues.