Endosymbiosis is important because it allows us to understand the evolution of eukaryotes from the common ancestor. This essay will focus on: the early evolution of our eukaryotic ancestor during the Precambrian period, the origin of plastids along the algae family due to the second endosymbiosis; discuss evidence supporting the theory, including additional examples of endosymbiosis. The theory, as discussed by Lynn Margulis, states that mitochondria originated from an α-proteobacterium that was phagocytosed by the ancestral anaerobic eukaryotic cell, through endocytosis, and retained within the cytoplasm due to increased atmospheric oxygen. The prokaryotic organism produces ATP, through oxidative phosphorylation, by receiving organic compounds from the eukaryote, causing the eukaryote to become dependent on the prokaryote for ATP production and the prokaryote to become dependent on the eukaryote for other cellular functions. As a result, both organisms evolved in symbiosis with each other, and most of the genes of a unicellular organism were transferred into the host genome, enclosed in the nucleus. Due to the advantageous relationship between host and symbiont, prokaryotic organisms lost the ability to survive independently and were reduced to mitochondria that were vertically transmitted to the future generation (Debashish et al., 2003). The evolutionary history of plants involves at least two independent endosymbiotic events (as shown in Figure 1); because plastids such as the chloroplast evolved when a primary endosymbiotic event caused the engulfment of photosynthetic cyanobacteria by some non-photosynthetic host cells (Dyall et al., 2004). Chlorophyta, Rhodophyta and Glaucophyta are three clades, belonging to the Archaeplas group... ... middle of document ...... n, PJ, 2004. Ancient invasions: from endosymbionts to organelles. Science, 304 (5668), pp. 253-257.Lake, J.A., 2009. Evidence for early prokaryotic endosymbiosis. Nature, 460, pp. 967-971.McFadden, G.I., 2001.Primary and secondary endosymbiosis and the origin of plastids. Journal of Phycology, 37(6), pp. 951–959. Rumpho, M.E., Worful, J.M., Lee J., Kannan, K., Tyler, M.S., Bhattacharya, D., Moustafa, A. and Manhart, J.R., 2008. Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chronotica. PNAS, 105(46), pp. 17867-17871.Smith, A.M., Coupland, G., Dolan, L., Harberd, N., Jones, J., Martin, C., Sablowski, R. and Amey, A., 2010. Plant biology. New York: Garland Science; Taylor & Francis distributor. Tomitani, A., 2006. Origin and early evolution of chloroplasts. Paleontological Research, 10 (4), pp. 283-297.