EndoSIMBAyosis

This article gives a brief history/overview of what endosymbiosis is. So, there is this preconceived notion that the first photosynthetic organisms were single-celled bacteria. About 1.5 million years ago, something phenomenal occurred. Non-photosynthetic plants (way back in the day), took in tiny green bacteria. They maintained a mutual relationship. The host cell provided a healthy/safe environment for the bacteria; and the bacteria provided energy for the cell that was harvested from sunlight. Eventually these two became so dependent on each other that they pretty much got to a point where one couldn't live without the other. Plastids are remnants of organisms inside a cell. Chloroplasts are the green plastids inside plant cells.
More recently amoebas also decided to take advantage of the sunlight. Now, amoebas are usually not photosynthetic but, there is a species that has plastids that are closely related to cyanobacteria. This leads scientists to suggest that this new species must be relatively new.
Endosymbiosis can actually happen again and again with the same organism. For example, Hatena arenicola (a unicellular organism) hosts algae; which itself is a product of endosymbiotic events. There are also some red and green algae that have been used by other cells to become plastids.
This article also mentions a couple other organisms that have been trying to "go green" such as the: green sea slug and the photosynthetic salamander!