How is the evolution of photosynthesis connected to the endosymbiotic theory?

Question

Does anybody know how is the evolution of photosynthesis connected to the endosymbiotic theory?

I think evolution and photosynthesis are two theories about origins of life on the Earth, but I don't understand how they could be connected.

Answer 1

First let me correct your last statement.  Evolution is a theory about the origins of biological diversity on earth.  Photosynthesis is a process by which plants and other organisms convert light energy into chemical potential energy that organisms can use for cellular processes.  As we usually think about it, photosynthesis allows plants to "fix" the carbons from carbon dioxide into sugars.  Fortunately for us, we have a way to break down the sugars in starch, and convert that chemical potential energy back into something we can use directly for stuff we want to do (e.g. ATP to allow you to move, think, build stuff, and reproduce).

Endosymbiosis starts out with one cell engulfing another.  Usually when a cell does this, it digests the cell it has engulfed (see videos of amoebae on YouTube).  But in endosymbiosis, that doesn't happen.  Somehow the cell that has been engulfed continues to live inside the other one, providing it with some benefit.  Over time and many generations, each becomes so dependent on the other, that they could no longer live apart.

In the history of plants, there are thought to be two endosymbiotic events.  The first of these is believed to have been when an early eukaryote (a cell with a nucleus and some other internal compartments) took in a bacteria that was really good at breaking down and using food.  This gave rise to the mitochondrion, which allows us (and plants) to do cellular respiration.  

For photosynthesis, we are talking about an event that gave them the chloroplast.    It is thought that a plant ancestor took in a bacteria that was really good at photosynthesis, and over time it became the chloroplast.  Now we have over 300,000 species of plants, the vast majority of which depend on chloroplasts for their food.

This is the connection between evolution (change in organisms over time, which leads to the biodiversity we that exists and has existed on earth), photosynthesis, and endosymbiosis.