The evolution of modern eukaryotic phytoplankton is central to marine ecosystems, despite their numerical inferiority to cyanobacteria. These organisms drive organic matter flux to higher trophic levels and the ocean interior. Photosynthetic eukaryotes evolved over 1.5 billion years ago, but the three dominant phytoplankton clades (dinoflagellates, coccolithophores, and diatoms) rose to prominence during the Mesozoic. These groups have plastids derived from red algae via secondary symbiosis. The fossil record shows their rise in the Middle Triassic, following the end-Permian mass extinction. Dinoflagellates, coccolithophores, and diatoms became dominant in the modern ocean, contributing to most organic matter export. Their success is linked to red plastid lineage, which evolved from red algae and retained key genes. The red lineage's success is influenced by environmental factors like nutrient availability and oceanic conditions. The rise of diatoms in the Cenozoic is tied to grass evolution and silica weathering. The fossil record indicates that these three groups largely replaced other eukaryotic algae in the oceans. The red lineage's dominance is attributed to their ability to adapt to changing environmental conditions and their genetic traits. The evolution of these phytoplankton groups has significantly shaped marine ecosystems and biogeochemical cycles. The fossil record, though incomplete, provides insights into their diversification and ecological roles. The red lineage's success is linked to their ability to utilize specific nutrients and their genetic adaptability. The coevolution of terrestrial and marine ecosystems, particularly grasses and diatoms, has influenced silica availability and phytoplankton diversity. The long-term impact of these phytoplankton groups on the carbon cycle and oceanic conditions is evident in the geological record. The study of these groups highlights their critical role in marine ecosystems and the factors driving their evolutionary success.The evolution of modern eukaryotic phytoplankton is central to marine ecosystems, despite their numerical inferiority to cyanobacteria. These organisms drive organic matter flux to higher trophic levels and the ocean interior. Photosynthetic eukaryotes evolved over 1.5 billion years ago, but the three dominant phytoplankton clades (dinoflagellates, coccolithophores, and diatoms) rose to prominence during the Mesozoic. These groups have plastids derived from red algae via secondary symbiosis. The fossil record shows their rise in the Middle Triassic, following the end-Permian mass extinction. Dinoflagellates, coccolithophores, and diatoms became dominant in the modern ocean, contributing to most organic matter export. Their success is linked to red plastid lineage, which evolved from red algae and retained key genes. The red lineage's success is influenced by environmental factors like nutrient availability and oceanic conditions. The rise of diatoms in the Cenozoic is tied to grass evolution and silica weathering. The fossil record indicates that these three groups largely replaced other eukaryotic algae in the oceans. The red lineage's dominance is attributed to their ability to adapt to changing environmental conditions and their genetic traits. The evolution of these phytoplankton groups has significantly shaped marine ecosystems and biogeochemical cycles. The fossil record, though incomplete, provides insights into their diversification and ecological roles. The red lineage's success is linked to their ability to utilize specific nutrients and their genetic adaptability. The coevolution of terrestrial and marine ecosystems, particularly grasses and diatoms, has influenced silica availability and phytoplankton diversity. The long-term impact of these phytoplankton groups on the carbon cycle and oceanic conditions is evident in the geological record. The study of these groups highlights their critical role in marine ecosystems and the factors driving their evolutionary success.