What underlies the ability to deal with numbers and where did it come from? It has been hypothesized that our ability to accurately represent the number of objects in a set (numerosity), and to carry out numerical comparisons and arithmetic, developed from an evolutionarily conserved system for approximating numerical magnitude. Non-symbolic number cognition based on an approximate sense of magnitude has been documented in a variety of species. However, we know little about its origins (i.e., to what extent experience would shape it) and of its neural and molecular bases. To address the first issue we performed single cell recordings in awake young domestic chicks. We found neurons selective to number in the caudal nidopallium (a higher associative area with functional similarities to the mammalian prefrontal cortex), which suggest that an approximate sense of magnitude can be an inborn feature in the avian brain. To address the issue of circuitry and molecular bases of the sense of magnitude we made use of zebrafish, that in recent years became established as ideal developmental and behavioral genetic model system. Using a combination of early gene expression and in-situ hybridization we identified for the first time a small region in the caudal part of the dorso-central division of the zebrafish pallium that shows selective activation upon change in numerosity of visual stimuli. As pallial regions are implicated in number cognition in mammals and birds, these findings support the existence of an evolutionarily conserved system for approximating magnitudes and provide an avenue for exploring its underlying molecular and genetic correlates.