Endosymbiosis theory | Cell structure and function | AP Biology | Khan Academy

When we look inside of eukaryotic cells, we see membrane-bound organelles. Some of these membrane-bound organelles are particularly interesting. For example, here is a diagram of a chloroplast that are found in plant or algal cells. We know that this is where photosynthesis takes place.

But what's really interesting, above and beyond that, is that it seems that chloroplasts have a lot of the machinery necessary for being a prokaryotic cell on its own. We don't see it acting on its own, but it has its own DNA. It has ribosomes, which we know are the site where we go from messenger RNA to protein.

Similarly, another interesting membrane-bound organelle that we see in eukaryotic cells—this would include even animal cells, like the cells in your and my bodies—are mitochondria. Mitochondria are often viewed as the energy factories of eukaryotic cells, where we can leverage oxygen in order to produce ATP. Like chloroplasts, mitochondria have their own DNA; they also have mitochondrial ribosomes.

Here are some diagrams of how mitochondria might look inside of a larger eukaryotic cell. Evolutionary biologists for many decades looked at this and said, "Well, why do these things exist? Why do they almost look like prokaryotic cells on their own?" There are even examples of independent prokaryotic bacteria that live in symbiosis inside of other cells, and they look an awful lot like mitochondria and chloroplasts.

If we fast forward to the 1960s, someone named Lynn Margulis comes on the scene with endosymbiosis theory. Her view is that these membrane-bound organelles, like mitochondria and chloroplasts, if we go deep into our evolutionary past—say, two and a half billion years ago—were actually independent prokaryotic organisms.

These organisms could produce energy aerobically, or using oxygen, and were precursors to what we would consider today to be modern eukaryotic cells. These modern cells might have already had some membrane-bound structures, like a nucleus, and maybe some other things that they could only metabolize anaerobically; they couldn't leverage oxygen.

While these other prokaryotic organisms could leverage oxygen, they could have become symbionts, where the one that could leverage oxygen to produce more energy would get engulfed into the larger cell. That larger cell is able to provide nutrients and protection, while the smaller cell that's engulfed inside of it could better metabolize the nutrients and leverage oxygen to produce more energy.

Over time, this symbiotic relationship became even more connected so that the smaller organism could not operate by itself; it lost some of its DNA that was necessary to act independently. Some of it might have gotten incorporated into the DNA of the larger cell, and those smaller organisms are what eventually evolved into what we consider today to be mitochondria.

This is a fascinating theory, and it's actually been proven out. When Lynn Margulis first published this in the late 1960s, she wasn't taken that seriously.

However, in the decades since, it's been validated as we've looked at the DNA structures of mitochondria and chloroplasts. This actually is the most likely theory of how they emerged in our cells. It's just a fascinating glimpse of evolution in general.

We talk a lot about natural selection and the role of variation in mutations, but Lynn Margulis introduces another idea that could catalyze evolution, and that's that of symbiosis. We see symbiosis throughout the natural world, and her argument is sometimes those symbionts can become so co-dependent on each other that they merge into one organism.