Phylogenetic comparisons suggest that plastid primary endosymbiosis, in which a single-celled protist engulfs and "enslaves" a cyanobacterium, likely occurred once and resulted in the primordial alga. This photosynthetic cell diversified, through vertical evolution, into the ubiquitous green (Chlorophyta) and red (Rhodophyta) algae, and the more scarce Glaucophyta. However, some modern algal lineages have a more complicated evolutionary history involving a secondary endosymbiotic event, in which a protist engulfed an existing eukaryotic alga (rather than a cyanobacterium), which was then reduced to a secondary plastid. Secondary endosymbiosis explains the majority of algal biodiversity, yet the number and timing of these events is unresolved. Here we analyzed a five-gene plastid data set to show that a diverse group of chlorophyll c-containing protists comprising cryptophyte, haptophyte, and stramenopiles algae (Chromista) share a common plastid that most likely arose from a single, ancient (about 1260 million years ago, Ma) secondary endosymbiosis involving a red alga. This finding is consistent with Chromista monophyly and implicates secondary endosymbiosis as a driving force in early eukaryotic evolution.

Key words: Chromista, phylogeny, plastid, red algae, secondary endosymbiosis