Usually the endosymbiosis event is considered to have occurred in the Archaeplastida, within which the glaucophyta being the possible earliest diverging lineage. The glaucophyte chloroplast group is the smallest of the three primary chloroplast lineages, being found in only 13 species, and is thought to be the one that branched off the earliest. Glaucophytes have chloroplasts that retain a peptidoglycan wall between their double membranes, like their cyanobacterial parent. For this reason, glaucophyte chloroplasts are also known as 'muroplasts' (besides 'cyanoplasts' or 'cyanelles'). Glaucophyte chloroplasts also contain concentric unstacked thylakoids, which surround a carboxysome – an icosahedral structure that glaucophyte chloroplasts and cyanobacteria keep their carbon fixation enzyme RuBisCO in. The starch that they synthesize collects outside the chloroplast. Like cyanobacteria, glaucophyte and rhodophyte chloroplast thylakoids are studded with light collecting structures called phycobilisomes. For these reasons, glaucophyte chloroplasts are considered a primitive intermediate between cyanobacteria and the more evolved chloroplasts in red algae and plants.