My research aims to understand how the brain functions by taking a comparative approach orthogonal to most contemporary systems neuroscience studies. Instead of focusing on a single model species, we engineer new technologies to efficiently map and compare brains of many vertebrates at a resolution that can resolve the relevant constituent brain parts: single cells, cell types, and brain regions. In this effort, we pioneer the use of DNA sequencing to map neuronal connectivity for millions of neurons and spatial RNA sequencing technologies to read out gene expression in 3D. By comparing gene expression patterns, we establish the correspondence of brain regions and cell types across species. We then use this insight to align and compare connectivity, revealing conserved and re-used circuit architectures. Through this comparative connectomics approach, we will learn how brain connectivity changed to support new behaviors and define fundamental principles of brain architecture.