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Endosymbiosis theory

Certain characteristics of mitochondria and chloroplasts support the endosymbiotic theory, which posits that eukaryotic cells evolved from simpler prokaryotic cells that merged together. Lynn Margulis was a key figure in developing this theory. Margulis's work was at first met with skepticism, but now the endosymbiotic theory is widely accepted in the scientific community.

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Video transcript

- [Instructor] When we look inside of eukaryotic cells, we see membrane-bound organelles, and 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, and we know that this is where the 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, and this would include even animal cells and the cells in your and my bodies, are mitochondria, and mitochondria are often viewed as the energy factories of eukaryotic cells, where we can leverage oxygen in order to produce ATP, and like chloroplasts, mitochondria has its own DNA. It also has mitochondrial ribosomes. Here are some just diagrams of how mitochondria might look inside of a larger eukaryotic cell. And so, 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? And there's even examples of prokaryotic cells, independent prokaryotic bacteria, that live in symbiosis inside of other cells, and they look an awful lot like mitochondria and chloroplasts. And so, if we fast-forward to the 1960s, someone named Lynn Margulis comes on the scene with endosymbiosis theory, and her view is, 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, their ancestors were actually independent prokaryotic organisms that could produce energy aerobically, or using oxygen, and precursors to what we would consider today to be modern eukaryotic cells that might have already had some membrane-bound structures, like a nucleus and maybe some other things, that they could only metabolize things anaerobically. They couldn't leverage oxygen, while these other characters could leverage oxygen, and then they could have become symbionts, where the one that could leverage oxygen to produce more energy would get engulfed into the larger cell, and that larger cell is able to provide nutrients and protection, while the smaller cell that's engulfed inside of it is able to better metabolize the nutrients and leverage oxygen to produce more energy, and that over time, this symbiotic relationship became even more connected, so that the smaller organism could not operate by itself, that it lost some of its DNA that was necessary to act independently, and 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, but in the decades since, it's been validated as we've looked at the DNA structures of mitochondria and chloroplasts, that this actually is the most likely theory of how they emerged in our cells. And so, it's just a fascinating glimpse of evolution in general. We talk a lot about natural selection and the role of variation and mutations, but Lynn Margulis introduces another idea that could catalyze evolution, and that's that of symbiosis, and we see symbiosis throughout the natural world. And her argument is, sometimes those symbionts can become so codependent on each other that they merge into one organism.