The origin and molecular evolution of the mammalian liver cell architecture
The liver is a central organ with essential roles in processes such as nutritional metabolism, detoxification, and immune defense1-6. It has been instrumental in the adaptation of mammalian species to diverse environments, as reflected by its rapid molecular evolution7,8. However, the origins and evolutionary dynamics of liver cell types and their structural organization remain largely unexplored. Here we report evolutionary analyses of transcriptome and chromatin accessibility data for liver cells from 17 species, spanning all major feeding strategies (herbivory, omnivory, carnivory, insectivory), great apes (including humans), placental clades (Afrotheria, Xenarthra, Laurasiatheria, Euarchontoglires), major mammalian lineages (placentals, marsupials, monotremes), and a bird as outgroup. Integrated with spatial transcriptomics, our data reveal that liver zonation--the compartmentalization of hepatocyte functions along the lobule, the livers fundamental anatomical and functional unit--is conserved across mammals but absent in other vertebrates. We find that zonation originated in the mammalian ancestor, driven by the emergence of WNT and R-spondin signaling from central vein endothelial cells, which activate central hepatocyte gene expression via the transcription factor TCF7L2. Despite this conserved architecture and signaling, genes with zonated expression exhibit rapid evolutionary turnover. Consistently, hepatocytes evolve fast, likely due to reduced selective constraints, enabling adaptive changes under positive selection. Alongside immune cells, hepatocytes are therefore key drivers of the livers rapid evolution and functional innovations. In great apes, we identify human-specific shifts in zonation and cell-type-specific expression linked to recent cis-regulatory changes, particularly in genes involved in lipid metabolism, likely contributing to human-specific metabolic traits. Our study uncovers the origins of a mammal-specific liver cell architecture, within which reduced constraints facilitated molecular changes underlying ecological adaptations.