[MUSIC] You've now heard about the evolution of the eukaryotic cell. And with this cell we have the component that forms the basis for all the diversity and all the different shapes of life and all the millions and millions of species that we see around us. In order to understand the evolution that lies behind all this diversity, we like to make phylogenies, which are groupings of organisms that are more closely related to each other. When we're talking about the big kingdoms, like animals and plants or fungi, we begin to have a fairly good idea about how the different groups are related. However, when we're going back to the root of all the eukaryotes, we're digging so far back in time that we still have some gaps in our knowledge. We still don't know exactly how the major eukaryotic lineages are related to each other. But what we know though is that we can group all eukaryotes into six large groups, and we call them the six eukaryotic super kingdoms. As I just said, we don't really know yet how the six eukaryotic super kingdoms are interrelated or which lineage branched off as the first. However, if we should try and still to pick one the Archaeplastida would be a good choice. The Archaeplastida includes groups that most of us are familiar with. We have all the green plants, we have the green algae, the red algae, and some different single celled groups such as the glaucophytes. So, why do we think that the archaeplastids could be one of the oldest eukaryotic groups? Well, first of all, we have a fossil record going 1.5 billion years back in time. This is of course not evidence in itself, but it shows that we are definitely dealing with an old group. We can get another indication when we look at the chloroplasts or the plastids as we also call them. Already when you see the name of the group you'll get a hint that tells you that we could be dealing with something really old here. If you know a little Greek you'll know that the prefix archae means original or the beginning. In other words, archaeplastids are those with the original plastids or original chloroplasts. So what does that mean? Well, as you remember, we talked about the endosymbiont theory and how a cyanobacterium could be incorporated into a prokaryotic cell and evolved into a chloroplast inside the cell. This is what we call primary endosymbiosis. Alternatively, it could also be a eukaryotic cell that already had a chloroplast that got integrated into another new prokaryotic or eukaryotic host cell. That's what we call secondary or tertiary endosymbiosis. Whereas we find examples of secondary and tertiary endosymbiosis in several other super kingdoms, all chloroplasts among the archaeplastid organisms seem to originate from primary endosymbiosis. This would suggest that the archaeplastid lineages perhaps were the earliest eukaryotic lineage and that the other super kingdoms branched off, after the archaeplastid stem line. But let's take a closer look at some of the archaeplastid groups. First we have the red algae, the Rhodophyta as they're also called. If you have been walking along a beach you might have seen these fluffy, red algae that are attached to rocks. However, they are not always soft and fluffy. We also have encrusting calcareous red algae such as the Lithothamnion for instance. The encrusting red algae are reef builders, and together with the corals, they often make up great parts of the reef habitats, and they are therefore very important for the reef communities. As you can see on the photos we are dealing with macroscopic organisms here. And most red algae are, of course, multicellular. In fact, it seems like the very first examples of multicellular life evolved within the red algae lineage. Hence, it's an important group from an environmental point of view as well as an evolutionary point. Two other important archaeplastid groups are the green algae and the green plants. Among the green algae, we find both single-celled and multicellular organisms, which means that we again have experienced evolution of multicellularity. If we want to understand more about this evolution, we can look at a very interesting green algae called Volvox. Volvox is a single celled organism, but it likes to live in big colonies as those that you see in the photo. When you live in a big colony you can benefit from it in different ways, but you're still single celled, which means that your cell has to be able to take care of everything yourself. Like feeding, like locomotion, reproduction, eventually protection also. But now, you can imagine, if you live in a colony anyway, it might perhaps be a good idea to specialize a little bit, or you can say to share the work. Why spend energy developing both feeding, and locomotion, and reproductionary capabilities. If you can share the work a bit and perhaps even become more efficient in this way, why not let one cell become specialized in reproduction, and then let another one take care of the feeding and then share the energy with the rest of the cells in the colony afterwards. It is this specialization, or you can call it self-differentiation, that happened at the transition from single celled to multicellular life. And the organisms that evolved into multicellular green algae might very well have started with colonies that resemble the Volvox colonies that we know today. The benefits of this cell differentiation are obvious because the more specialized you are the more efficient you can do your job. The trade-off is of course that you become dependent on the other cells. You can feed, but you can't reproduce, and then you cannot live on your own anymore. In other words, you have changed from being an individual single cell in a colony to become a part of a multicellular organism. The green plants evolved from the green algae and all the green plants are multicellular. Plant lineages, like ferns and mosses and these horsetails that you just saw in the photo, they are ancient and they have fossils going all the way back to the Ordovician, the Silurian, and the Devonian. However, when many of you think of plants you probably think of flowers and they are much newer. To find the first flowering plants we have to move all the way up to the Jurassic, and we don't really experience the great radiation of flowering plants before we get into the Cretaceous. Hence, it seems like dinosaurs and flowers actually evolved side by side. [MUSIC]
