The article reviews the evolution of modern eukaryotic phytoplankton, focusing on the three principal clades—dinoflagellates, coccolithophores, and diatoms—that dominate contemporary marine ecosystems. These groups, which emerged during the Mesozoic Era (251 to 65 million years ago), are characterized by their possession of plastids derived from secondary symbiosis with red algae. The authors examine the geological, geochemical, and biological processes that contributed to their rise, including the ecological advantages of acquiring and retaining plastids, the impact of environmental changes such as the End-Permian mass extinction and Mesozoic Oceanic Anoxic Events (OAEs), and the coevolution with terrestrial ecosystems, particularly the rise of grasses. The expansion of these red-lineage phytoplankton has significantly influenced marine ecosystem structure and biogeochemical cycles, leading to increased organic matter burial and a gradual depletion of CO2 in the ocean-atmosphere system.The article reviews the evolution of modern eukaryotic phytoplankton, focusing on the three principal clades—dinoflagellates, coccolithophores, and diatoms—that dominate contemporary marine ecosystems. These groups, which emerged during the Mesozoic Era (251 to 65 million years ago), are characterized by their possession of plastids derived from secondary symbiosis with red algae. The authors examine the geological, geochemical, and biological processes that contributed to their rise, including the ecological advantages of acquiring and retaining plastids, the impact of environmental changes such as the End-Permian mass extinction and Mesozoic Oceanic Anoxic Events (OAEs), and the coevolution with terrestrial ecosystems, particularly the rise of grasses. The expansion of these red-lineage phytoplankton has significantly influenced marine ecosystem structure and biogeochemical cycles, leading to increased organic matter burial and a gradual depletion of CO2 in the ocean-atmosphere system.