26Primates and rodents, which descended from a common ancestor more than 90 million years 27 ago, exhibit profound differences in behavior and cognitive capacity. Modifications, 28 specializations, and innovations to brain cell types may have occurred along each lineage. We 29 used Drop-seq to profile RNA expression in more than 184,000 individual telencephalic 30 interneurons from humans, macaques, marmosets, and mice. Conserved interneuron types 31 varied significantly in abundance and RNA expression between mice and primates, but varied 32 much more modestly among primates. In adult primates, the expression patterns of dozens of 33 genes exhibited spatial expression gradients among neocortical interneurons, suggesting that 34 adult neocortical interneurons are imprinted by their local cortical context. In addition, we found 35 that an interneuron type previously associated with the mouse hippocampus-the "ivy cell", which 36 has neurogliaform characteristics-has become abundant across the neocortex of humans, 37 macaques, and marmosets. The most striking innovation was subcortical: we identified an 38 abundant striatal interneuron type in primates that had no molecularly homologous cell population 39 in mouse striatum, cortex, thalamus, or hippocampus. These interneurons, which expressed a 40 unique combination of transcription factors, receptors, and neuropeptides, including the 41 neuropeptide TAC3, constituted almost 30% of striatal interneurons in marmosets and humans.
42Understanding how gene and cell-type attributes changed or persisted over the evolutionary 43 divergence of primates and rodents will guide the choice of models for human brain disorders and 44 mutations and help to identify the cellular substrates of expanded cognition in humans and other 45 primates. 46 47 53 54 Brain structures, circuits, and cell types have acquired adaptations and new functions along 55 specific evolutionary lineages. Numerous examples of modifications to specific cell types within 56 larger conserved brain systems have been discovered, including hindbrain circuits that control 57 species-specific courtship calls in frogs 3 , the evolution of trichromatic vision in primates 4 , and 58 neurons that have converted from motor to sensory processing to produce a novel swimming 59 behavior in sand crabs 5 . Evolution can modify brain structures through a wide range of 60 mechanisms, including increasing or reducing production of cells of a given type, altering the 61 molecular and cellular properties of shared cell types, reallocating or redeploying cell types to 62 new locations in the brain, or inventing entirely new cell types (Fig. 1a). 63 64 Single-cell RNA sequencing, which systematically measures gene expression in thousands of 65 individual cells, has recently enabled detailed comparisons of cell types and expression patterns 66 between homologous brain structures separated by millions of years of evolution 4,6,7 (non-single 67 cell approaches have also yielded important insights in this domain, e.g. 8 ). For example...