[6] It also includes the non-photosynthetic lineage Rhodelphidia, a predatorial (eukaryotrophic) flagellate that is sister to the Rhodophyta, and probably the microscopic picozoans.

[7] The Archaeplastida have chloroplasts that are surrounded by two membranes, suggesting that they were acquired directly through a single endosymbiosis event by phagocytosis of a cyanobacterium.

The main evidence that the Archaeplastida form a monophyletic group comes from genetic studies, which indicate their plastids probably had a single origin.

[11][12] Based on the evidence to date, it is not possible to confirm or refute alternative evolutionary scenarios to a single primary endosymbiosis.

[14] The glaucophytes have typical cyanobacterial pigments, but their plastids (called cyanelles) differ in having a peptidoglycan outer layer.

[15][16] The consensus in 2005, when the group consisting of the glaucophytes and red and green algae and land plants was named 'Archaeplastida',[1] was that it was a clade, i.e. was monophyletic.

[21][22][23][12][24] To date, the situation appears unresolved, but a strong signal for Plantae (Archaeplastida) monophyly has been demonstrated in a recent study (with an enrichment of red algal genes).

[5] Another name applied to this node is Plastida, defined as the clade sharing "plastids of primary (direct prokaryote) origin [as] in Magnolia virginiana Linnaeus 1753".

The name Archaeplastida was proposed in 2005 by a large international group of authors (Adl et al.), who aimed to produce a classification for the eukaryotes which took into account morphology, biochemistry, and phylogenetics, and which had "some stability in the near term."

A sister of Gloeomargarita lithophora has been engulfed by an ancestor of the Archaeplastida, leading to the plastids which are living in permanent endosymbiosis in most of the descendant lineages.

Mesostigmatophyceae Chlorokybophyceae Spirotaenia Klebsormidiales Chara Coleochaetales Zygnematophyceae Hornworts Liverworts Mosses Lycophytes Ferns Gymnosperms Angiosperms All archaeplastidans have plastids (chloroplasts) that carry out photosynthesis and are believed to be derived from endosymbiotic cyanobacteria.

Because the ancestral archaeplastidan is hypothesized to have acquired its chloroplasts directly by engulfing cyanobacteria, the event is known as a primary endosymbiosis (as reflected in the name chosen for the group 'Archaeplastida' i.e. 'ancient plastid').

In 2013 it was discovered that one species of green algae, Cymbomonas tetramitiformis in the order Pyramimonadales, is a mixotroph and able to support itself through both phagotrophy and phototrophy.

[49] The presence of such genes in the nuclei of eukaryotes without chloroplasts suggests this transfer happened early in the evolution of the group.

[56] The oldest fossil that can be assigned to a specific modern group is the red alga Bangiomorpha, from 1200 Ma.