Ancient whole-genome duplications (WGDs) characterize many large clades of angiosperms, including angiosperms themselves. Prominently, the core eudicot clade accommodates 70% of all angiosperms and shares ancestral hexaploidy, termed gamma. Gamma arose via two WGDs that occurred early in eudicot history; however, the relative timing of these is unclear, largely due to the lack of high-quality genomes among early-diverging eudicots. We provide complete genomes for Buxus sinica (Buxales) and Tetracentron sinense (Trochodendrales), representing the lineages most closely related to core eudicots. We show that Buxus and Tetracentron are both characterized by independent WGDs, resolve relationships among early-diverging eudicots and their respective genomes, and use a new pipeline (RACCROCHE) to reconstruct ancestral genome structure at three key phylogenetic nodes of eudicot diversification. Our reconstructions indicate genome structure remained relatively stable during early eudicot diversification, and reject hypotheses of gamma arising via inter-lineage hybridization between ancestral eudicot lineages, involving, instead, only stem lineage core eudicot ancestors.