Every land plant descends from a cell that swallowed a cyanobacterium
Green algae, red seaweeds, tiny grey algae and every tree and flower belong to one proposed lineage, the Archaeplastida, meaning ancient plastid. Its founding ancestor engulfed a cyanobacterium and kept it as a permanent solar panel. That merger probably happened in fresh water, long before these organisms colonised the oceans.
Archaeplastids, sometimes called plants in the broad sense, include three groups: the Viridiplantae, meaning green algae plus land plants; the Rhodophyta, or red algae; and the small Glaucophyta, freshwater single-celled grey algae. Almost all photosynthesise, though some predatory flagellates called Rhodelphidia, and probably the microscopic Picozoa, have abandoned it. Their cells usually lack centrioles, often have cellulose walls and store the sugars they make as starch.
The key evidence for kinship is the chloroplast. It is wrapped in two membranes, the signature of a primary endosymbiosis in which an ancestral cell took in a cyanobacterium directly rather than stealing a chloroplast from another alga. The closest living relative of that captured partner appears to be a sister of Gloeomargarita lithophora, and because both it and the most primitive archaeplastids live in fresh water, the group likely only reached the oceans in the late Proterozoic. Glaucophyte chloroplasts, called cyanelles, still carry a peptidoglycan wall like free-living cyanobacteria. Algae with second-hand chloroplasts, such as diatoms, brown algae and dinoflagellates, sit elsewhere on the tree of life.
Colour follows chemistry. Red algae use phycobiliproteins, especially phycoerythrin, as extra light-harvesting pigments, and most are multicellular marine seaweeds. The green lineage lost those pigments. Multicellularity arose independently several times: in red algae, in one branch of green algae, and in the green algae that led to stoneworts and land plants. In 2013 researchers found that the green alga Cymbomonas tetramitiformis can both photosynthesise and engulf prey.
Naming has been messy. Plantae has been applied to narrower groups, so alternatives like Primoplantae in 2004 and Plastida were floated before an international team led by Adl proposed Archaeplastida in 2005, dropping formal ranks. Whether the group is truly a single lineage is still debated, though a recent study with many red algal genes found a strong signal for it. A 2019 phylogeny drew on genomes and transcriptomes from 1,153 plant species.
Source: Archaeplastida
