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Charles Darwin developed his theory of natural selection by comparing features between individuals and species. A comparative approach is also crucial to establish the cellular taxonomy that underlies human physiology. Technological advances in single-cell genomics have facilitated the production of numerous cell atlases that, through comparative analysis, define the full set of cells that constitute a system of interest — usually a whole organ1. Extending the scope of an atlas from organ to whole organism increases the power of this approach by capturing data across physiological systems. To this end, two papers in Science present the comprehensive molecular characterization of cell types across nearly all organs during human fetal development2,3. They reveal previously unidentified cell subtypes, and define cell-differentiation pathways through analysis of gene expression and chromatin (the DNA–protein complex into which a cell’s genetic material is packaged).