 There's obviously lots of questions about where life came from, where prokaryotic life like bacteria came from, where more complex cells like our eukaryotic cells came from. So we're going to talk about a few of these theories, but they all revolve around the endosymbiotic theory, which you can see here. So an endosymbiant would be an organism that lives inside another organism or inside another cell. So the endosymbiotic theory basically says that some of our more complex organelles like mitochondria in us and then chloroplastin plant cells would have been their own organisms and learn to co-evolve with other cells to become more complicated cells. So that's basically the basis of the theory. I'll walk you through the steps here and then show you a little bit more detailed information as far as why this matters. So number one, you see that just infoldings of the plasma membrane may have given rise to the endomembrane system, which is what we talk about with the nucleus. That's the real tricky spot is where the nucleus actually came from. But the rough and smooth endoplasmic reticulum, the Golgi, those would be some examples of some organelles that may have just come from this infolding of the plasma membranes. So that actually, there are some organisms, there are some prokaryotes that have like double membranes inside their cells and stuff, so there's some evidence to believe that. Number two, in a first endosymbiotic event, the ancestral eukaryote cell consumed an aerobic bacteria that would have evolved into the mitochondria. And I'll show you why we believe that in another image here in just a moment. Number three, we don't care about as much in this class, but obviously we have algae and these kind of things. But in the second event, the same type of thing, an early eukaryotic organism that was photosynthetic would have been taken up and became the chloroplast. And that's what we're seeing here at the end. We are seeing cells that have chloroplasts and mitochondria inside of them that have evolved from cells that would have first taken them up. So let me show you, we're going to focus on the mitochondria because obviously a lot of what we're talking about is human related. But imagine the story with the chloroplasts being the same thing. So some of the evidence that the mitochondria used to be its own organism comes from the fact that it has its own DNA, as you can see there, and it has its own ribosomes. So very interesting, the DNA in the mitochondria is not the same as DNA from the nucleus. So it definitely is separate. At this point I believe there's about 13 chromosomes that are still actually inside the mitochondria, most of them. So the mitochondria couldn't survive on its own, neither could the chloroplasts because most of the genes that actually control the mitochondria are now in the nucleus. So there's only a handful of genes left, but there is. The mitochondria does have its own DNA, and it looks more like bacterial DNA than other eukaryotic DNA than the ribosomes. So this is the most significant thing to me, mitochondria have their own ribosomes just like chloroplasts do. So our ribosomes, eukaryotic ribosomes are usually ADS ribosomes, but in the case of the mitochondria, they are 70s ribosomes, which is the same structure as bacterial ribosomes. So the reason this matters to me is because I think this is probably where part of the side effects of antibiotics come into play. If you're taking antibiotics that are attacking bacteria and their 70s ribosomes, well your mitochondria have the same ones. So chloroplasts and mitochondria have their own DNA, have their own ribosomes, and this is where a lot of the evidence for the fact that they would have been a different organism early in our evolutionary history comes from. The other thing to note about them is they both have a double membrane, and it's believed that the inner membrane would have been the first organisms, the prokaryotes, the outer membrane would have came as it joined this symbiotic relationship. So all right, one more thing here, it's just kind of interesting. I will never ask you to know this, but at least not in microbiology and anatomy, we'll talk about it. Mitochondria, all the mitochondrial DNA comes from mom. So it's believed that one of two things happened, either the mitochondria from the sperm degrade after the egg is fertilized or the mitochondria actually don't make it into the egg during fertilization or into the OO site. So all mitochondrial DNA comes from mom. This is kind of interesting. All right, another theory here is there's a few more of these theories that all kind of relate is where the gram-negative bacteria came from, come from. Just like mitochondria and chloroplasts have double membranes, gram-negative bacteria have an inner membrane very similar to gram-positive bacteria with a second outer membrane made of lipids. So it's believed that gram-negative bacteria would have evolved from gram-positive bacteria that then took up the outer membrane of an archaea, like I like to call them prehistoric bacteria. So that's another theory. I will not ask you to know any of these. They're just kind of interesting. I love the thought experiments. But this could be this could be where gram-negative bacteria came from and I have to say could be because there's lots of contention around all these theories. And speaking of that, these are actually three separate models for where the different domains came from. The archaea, the prokaryotes and the eukaryotes. So first out of the nucleus, first hypothesis. That is the trickiest one. No matter where you think cells come from, did the nucleus evolve on its own? Did it come from the DNA somewhere else? Very, very tricky. So some theories say that the nucleus is what showed up first and then these prokaryotic cells would have built around it and made it a more complex eukaryotic cell. The mitochondrion first hypothesis actually says that the key there would be the mitochondria powering this process and making these simple cells much more complex and adaptable. The third one's actually the most interesting just because it actually is the eukaryote first hypothesis what actually says that more complex eukaryotic cells were here first and then they degraded and became prokaryotes like bacteria. So most people would not agree with that. So this is very interesting. So this is the endosymbiotic theory or the different theories for where cells, more complex cells came from over time. So it doesn't talk at all about the origins of cells in the beginning. Obviously where life came from and those kinds of things. But this is just trying to show how cells became more and more complex over time. All right, hope this helps. Have a wonderful day. Be blessed.