Cellular development and evolution of the mammalian cerebellum.

The expansion of the neocortex, a hallmark of mammalian evolution 1,2 , was accompanied by an increase in cerebellar neuron numbers 3 . However, little is known about the evolution of the cellular programs underlying cerebellum development in mammals. In this study, we generated single-nucleus RNA-sequencing data for ~400,000 cells to trace cerebellum development from early neurogenesis to adulthood in human, mouse, and the marsupial opossum. We established a consensus classification of the cellular diversity in the developing mammalian cerebellum and validated it by spatial mapping in the fetal human cerebellum. Our cross-species analyses revealed a largely conserved developmental dynamics of cell type generation, except for Purkinje cells, where we observed an expansion of early-born subtypes in the human lineage. Global transcriptome profiles, conserved cell state markers, and gene expression trajectories across neuronal differentiation show that cerebellar cell type-defining programs have been overall preserved for at least ~160 million years. However, we also identified many orthologous genes that gained or lost expression in cerebellar neural cell types in one of the species, or evolved new expression trajectories during neuronal differentiation, indicating widespread gene repurposing at the cell type level. Altogether, our study unveils shared and lineage-specific gene expression programs governing the development of cerebellar cells, and expands our understanding of mammalian brain evolution.