[MUSIC] My name is Martin Sorensen. I'm a zoologist and I'm a specialist in microscopic organisms and in evolutionary biology. Today we're going to talk about one of the most significant events in the evolution of life. Namely, the transition from prokaryotic life to eukaryotic life, or I could say the transition from bacterial life to all the varieties of life that we see around us today. Subsequently we'll talk about the major lineages of the eukaryotic organisms and see how we today are able to group life into six super kingdoms. In previous lectures, you've heard about how life began and how it developed in the earliest bacterial life. You also heard how all bacteria belong to the prokaryotes, which are rather simple cells that basically are characterized by cell membranes with some free-floating DNA inside. A simple design doesn't mean that it's a bad design, though. Today, we can basically find bacterial life everywhere, so it's hard to claim that a prokaryotic design was not successful. But it has its limitations though. Because the prokaryotic cells are so simple as they are, it is unable to differentiate and form more complex organisms. Also interaction with other cells, and hence becoming a multicellular organism, is not an option for the prokaryotic cell. For this, we need a much more complex design and that's what we are going to find in the eukaryotic cell. Let's just recap and take a look at the prokaryotic cell that you already learned about. What we see here is a rather simple cell design. We have the plasma membrane with the capsule on the outside and eventually a flagellum also. Inside we find the cytoplasm and free-floating in the cytoplasm, we have the ribosomes for protein synthesis and the DNA. If we compare this with the eukaryotic cell, we'll realize that this type of cell is much more complex. We can still find the DNA, but in the eukaryotic cell it is contained in a nucleus. And together with the nucleus, we see several other units inside the cell. Together these units are called organelles. For instance, we see the Golgi apparatus, where proteins can be stored, and we see mitochondria that are responsible for the aerobic metabolism. We also see all these folded membranes here that form an almost labyrinth-like mesh. That's a transport system called the endoplasmatic reticulum, or simply the ER, which ensures that different molecules like RNA or amino acids can be transported inside the cell. If you look closer at the endoplasmatic reticulum, you'll see these little dots. That's the ribosomes. They receive copies of DNA from the nucleus, the copies come in shape of RNA, and use this as a template to synthesize proteins. You can also take a look at another kind of eukaryotic cell. We see several similarities between the two cells, such as the nuclei, and the mitochondria, but we also find differences. For instance, we have these green organelles here. They are called chloroplasts, and they are used to make photosynthesis or, in other words, to turn sunlight into energy. So, this also tells us that the eukaryotic cell to the left would be from the plant or an algae. Whereas the one to the right, for instance, could be from an animal. So how did this transition from prokaryotic to eukaryotic cell take place? Actually, we have a pretty good idea about this and the transition can be explained by membrane movements and by the endosymbiont theory. At the top of this figure, we see a prokaryotic cell. There's basically only the membrane and the free-floating DNA inside it. What we imagine, then, is that this cell membrane could make some infoldings. This would, for instance, make sense if the cell wanted to increase the ratio between its surface area relative to its volume. We can then imagine also that these infoldings from the outer membrane can get pinched off so that we in this way get membranes inside the cell. These membranes could, for instance, fold and make up the system of tubes and in this way we could have the endoplasmatic reticulum, this network I talked about that eukaryotic cells use for intracellular transport. Or the membranes could wrap around the DNA and in this way we would have formed a nucleus. In this way we would have a very simple eukaryotic cell. This process didn't happen overnight of course. The membrane foldings and movements probably developed through numerous generations of cells. But at some point, this new design was finally established. But it doesn't stop here. After the formation of the nucleus and the endoplasmatic reticulum, it seems like some other prokaryotic organisms has entered our newborn eukaryotic cell. It could be as prey for instance, because the eukaryotic cell simply wanted to eat the other prokaryote. But it could also be because this prokaryotic cell was a parasite or a commensal that wanted to take advantage of living inside another cell. However, instead of getting digested or instead of harming the host cell, the two cells started to live together in a mutual symbiosis, which is an intimate relationship where both organisms benefit from it. And then again, over many generations, of course, this symbiosis became more and more intimate until the endosymbiotic prokaryote no longer could be considered an independent organism. Instead, it had become a part of the eukaryotic cell and hence a cell organelle. Now, we know that some prokaryotes actually went through this process. And some of them resemble the so-called proteobacteria. Proteobacteria are aerobic bacteria, which means that they use oxygen when they produce energy. And in an oxygen-rich environment, such endosymbiotic bacteria would give the host cell a great advantage. If we look at the mitochondria in eukaryotic cells, we see that they have much in common with these proteobacteria. They are structurally, very similar. And mitochondria are, in fact, the organelles that are responsible for our cell metabolism, because they use oxygen to produce energy. Based on this, we have pretty good evidence supporting that the mitochondria which are found in most eukaryotic cells originated from proteobacteria that started out as endosymbionts and subsequently became integrated cell organelles. And if we take a look at another quite important organelle, namely the chloroplasts that we find in photoautotrophic organisms, those that makes the photosynthesis, we see the same story repeated. A photoautotrophic, prokaryotic cell, very similar to a cyanobacteria, started out as an endosymbiont and was then gradually integrated into the cell and became a chloroplast organelle. This may, of course, sound like some freaked out theory, but as I said, the similarity between, on one hand, proteobacteria and mitochondria, and on the other, cyanobacteria and chloroplasts is striking. And, we even have more evidence. I said previously that all DNA in the eukaryotic cell was contained in the nucleus, but it would be more correct to say almost all DNA. Actually, we find a little DNA in two other organelles, which are the mitochondria and the chloroplasts. And if you sequence this DNA and compare it with DNA from other organisms, I guess you can figure out where we find the greatest resemblance. Mitochondrial DNA is very close to proteobacterial DNA, whereas the chloroplast DNA resembles the DNA of the cyanobacteria. So in other words, we have pretty good evidence supporting that cell evolution through endosymbiosis actually took place. And in fact, we can even show that it wasn't just a single event. It has happened many times. Here on this graphic, we see our eukaryotic cell again. We can see that a cyanobacterium has been engulfed and that it, over time, has turned into a chloroplast. If we look closely at it, we can also see that the chloroplast actually has two membranes, the original membrane from the cyanobacterium and the membrane from the prokaryotic host cell that originally engulfed it. However, sometimes we find chloroplasts with three or even four membranes. Now, how can this happen? Well, the answer is simple. It's not only prokaryotes that may become endosymbionts and subsequently turn into organelles. The same may happen with a eukaryotic cell. In the picture to the left, you see a eukaryotic cell that is about to be engulfed in another eukaryotic cell. So this eukaryotic cell has already been through one event of endosymbiotic evolution and now it happens a second time. On the next picture, you see how the eukaryotic cell has become an endosymbiont. And now we can identify four membranes around the chloroplast: the two original ones, the outer membrane of the eukaryotic cell, and the membrane from the prokaryotic cell or eukaryotic cell that engulfed it. When this endosymbiotic eukaryote becomes an organelle, the third membrane will often disappear, together with its nucleus. But what we are left with is a chloroplast with three membranes, instead of two. This kind of chloroplast has been through two events of endosymbiotic evolution. And accordingly, we call it secondary endosymbiosis. And it doesn't even end here. Even a eukaryotic cell that evolved through secondary endosymbiosis may become an endosymbiont again if a new cell, takes it in. And again, it may become integrated in the cell and turn into an organelle. We call this tertiary endosymbiosis, and we typically find no less than four membranes around the chloroplast then. This shows that evolution through endosymbiosis happened several times and as we'll see in a moment, this may also helps us to understand how the tree of life, looks, and how the first eukaryotic groups evolved.
