Support for the endosymbiotic theory for the evolution of eukaryotic groups includes that chloroplasts and mitochondria are surrounded by two membranes, one that derives presumably from the bacterial symbiont and one that derives from the host cell’s vacuole that originally surrounded it. Interestingly, some algal phyla, for instance the Euglenophyta, contain chloroplasts composed of three membranes. Given your understanding of endosymbiosis, how do you think this extra membrane may have evolved?