 So in this video I'd like to talk about some clues as to how we think oxygenic photosynthesis may have evolved. This work comes from collaborations with a lot of people in my lab, especially people whose names are in bold, as well as really important collaborators from around the world. So let's first take a look at oxygenic photosynthesis. It contains two photosystems, one of which absorbs photons, and it is the one that takes the water and splits it, it takes two waters and splits it into an O2 molecule and produces a bunch of protons which are used for energy. The electrons from this process get bumped up in energy, and they go through a series of molecules that again pump protons across the membrane which allows ATP synthesis. This is a very energetic molecule that cells use to store energy. And those go into what's called a quinone pool which then provides electrons to photosystem one, and that photosystem absorbs more photons and jumps energy up even more, and as those electrons lose energy going through enzymes, it converts NADP plus to NADPH, another energy storage molecule. So this process of photosynthesis has these two photosystems and hundreds of genes, and it's really interesting to think about how this metabolic process first evolved. We can look at it in terms of the phylogeny of different organisms. So this is part of the bacterial tree, and the lines represent the evolutionary similarity and differences between different organisms that are at the tips of all of these branches. So this is all other bacteria, and up here we have the cyanobacteria which are the ones that are photosynthetic, and they have these two photosystems, one and two, and they all tolerate oxygen. In the past decade, a number of authors have found these bacteria, first named melanobacteria, that are very closely related to the cyanobacteria, but they are not photosynthetic. They are heterotrophs and fermenters, and some can't even tolerate oxygen. None of them have any evidence of photosynthetic genes within them, and so if we're thinking about evolution and the inheritance of properties, we have these two closely related groups of organisms, one of which contains photosynthesizers, the others don't. And so something very special happened in this evolutionarily, something very special happened that led to photosynthesis in this group, or if the common ancestor had photosynthesis it would be the loss of photosynthesis in all of these organisms. So what I'm going to do next is just show a tree of only photosynthetic bacteria. So we have our cyanobacteria here, and when we think of photosynthesis we usually think about oxygen producing photosynthesis, but there are other branches of photosynthesis that use different compounds and don't produce oxygen. So for example, these green sulfur bacteria use photons to oxidize sulfide, to sulfate, into reduce carbon. So they are photosynthetic as well, but they don't produce oxygen. Similarly, there are purple bacteria here that also again absorb light as part of their metabolism, but they don't produce oxygen. So we can look at the genes for all of these different organisms, and we can compare them to the genes that we see in the cyanobacteria themselves. And it turns out that the photosystem 1 and photosystem 2 have genes that are related to the green sulfur bacteria for photosystem 1 and the purple bacteria for photosystem 2. And the interesting thing is that these are not particularly closely related organisms and they're not particularly closely related to cyanobacteria, but if we use the genetic similarity to understand the process, the interpretation, one interpretation is that the green sulfur bacteria provided a whole bunch of genes that got transferred into the ancestor of cyanobacteria with a lateral gene transfer process, whereas photosystem 2 came from photosynthetic purple bacteria. So what this would mean is that the oxygenic photosynthesis is actually the merger of two different metabolisms that evolved in separate organisms and then were transferred into an ancestor. That lateral gene transfer can happen through a number of processes. In fact, there are known viruses that include some of the genes for photosynthesis and these viruses infect cyanobacteria and the genes function to keep photosynthesis going even when the cyanobacteria is dying because of the viral infection. And there are other ways that genes can be transferred between bacteria as well. So one possible model is that we have this merger of the proto-cyanobacteria, the ancestor of the cyanobacteria with at least some of the genes from two different organisms. And if we think about that in terms of the process of how the bacteria changed the chemistry of earth, we can look at the amount of oxygen through time. Today we're sitting here with a 20-some percent oxygen. The first animals evolved about 500 million years ago and when we go back to our key in time there was essentially no oxygen. The question is when did cyanobacteria start producing enough oxygen for it to accumulate in the atmosphere? And the idea that my group is working with is that the ability to produce oxygen evolved very early, but it took a long time and a lot of evolutionary selection to merge those two photosystems into one new metabolic process to get the breaking of water and the production of oxygen. That's one model that most people who are studying the process think is reasonable. There are others who think that the photosynthesis evolved right before we got oxygen in the atmosphere because it's such an incredible metabolic process that it would just take off as soon as it evolved. So it's a research question that we are working on in my group along with hundreds of other people around the world tackling it in different ways. Thanks for watching.