Secondary Endosymbiosis:
A mutualistic and eventually codependent relationship developed between the symbiont and its host. This explains why the organism, once engulfed, was not simply digested and used as a food sources. Overtime, the codependence evolved into a more complex and perminant relationship. One in which the organism engulfed (symbiont), became a part of the original organism. These new pieces became known as membrane bound organelles, and appear in the forms of Mitochondria and Chloroplasts, as well as all other types of plastids. The differentiation among the algae was then a result of a secondary series of endosymbiosis taking place. The separation occurred due to the differentiation among what was engulfed during the secondary endosymbiosis.