[MUSIC] We have now been through four of the six eukaryotic super kingdoms. And the two groups left are the Amoebozoa and the Opisthokonta. I said previously that we don't really know how the eukaryotic super kingdoms are interrelated, but we actually have a good molecular evidence that supports a closer relationship between the two latter ones here, the Amoebozoa and the Opisthokonta. So let's try to take a closer look at them. The first of the two super kingdoms left is Amoebozoa. And it consists, probably not that surprisingly, of amoebae. I'm not going to say too much about amoebae besides that they are single celled, amorphous animals that slowly move around and constantly change shape. By changing the inner pressure and the shape of the membrane, they can make these arm-like structures called pseudopods that, for instance, are used for feeding. All amoebae are heterotrophic, which means that they have to eat. They cannot get energy through photosynthesis. Hence, what they do is that they wrap one of these pseudopods around the prey and then they slowly start to unwrap it until they subsequently engulfs it. And at some point the prey will suddenly be inside the cell membrane instead of outside. But we'll leave the amoebas again and get to the last eukaryotic super kingdom. And this is also the most interesting one, if you ask me. And this is definitely the one where we find the greatest biodiversity. The super kingdom is called Opisthokonta, and it consists of some smaller groups and then two large ones and very, very well-known ones also. In the graphic you see here, I've shown three of the groups. We have the animals or metazoans and the fungi, that all of us know of course. There is no reason to say much more about the fungi. Instead, you should take a look at the third group I've listed, namely the choanoflagellates. It's probably not a group that very many people are familiar with, but it might be important if we want to understand the origin of the animal kingdom. So let's take a closer look at the choanoflagellates. Choanoflagellates are single celled organisms and roughly, they consist of a cell body, a flagellum, and then they have this characteristic collar around the flagellum as you see on this idealized illustration. They may be solitary, but many of them like to live in large globular colonies. When you look at this choanoflagellate colony, this might remind you of something I've showed you earlier. Maybe you remember these quite similar green algae colonies called Volvox. And maybe you remember I said that Volvox would be a kind of transitional stage between single celled, and multi-cellular life. These choanoflagellate colonies are exactly the same. We imagine that back in time, maybe 600 million years ago, or even more, if you had choanoflagellate-like colonies, as the one you see here, and perhaps the individuals in these colonies started acting as the Volvox individuals. They started to share the work between them. And they went through a differentiation of the cells, so that all cells were no longer able to do everything themselves, instead they got specialized into certain tasks such as feeding or reproduction. And this cell differentiation would mark the transition from single celled life in colonies to multicellular life and this is how we imagine that the first metazoans, or multicellular animals, evolved. Now if you ask a morphologist which metazoans were the first to evolve, he or she would probably say that it would be the sponges or the poriferens as those that you see lined up here in front of me. If you look at this sponge for instance, you can see that it doesn't really have any differentiated organs. They just inhale water through the surface and then inside the sponge you have these small ciliated chambers where all the food is collected, and then the water will be exhaled again through these chimney-like openings in the top of the sponge. So this design is completely different from what we see in other metazoans, but interestingly, the design of these ciliated chambers is actually quite similar to the choanoflagellate colonies. These ciliated chambers look like inverted choanoflagellate colonies. And just to make the comparison perfect, each flagellum in the chamber has a little collar at its base. It's a lot of cells and each cell has a single cilium or flagellum, and to make the comparison perfect, we find a collar around each flagellum. So, this is how a morphologist see the origin of the metazoans. At the earliest metazoan branch, we find the sponges, and then they are followed by three other clades with basal metazoans, inclusive the small group Placozoa, the Cnidarians that include jellyfish, hydroids, and corals, and then finally, the Ctenophora or comb jellies. After these, we have all the remaining metazoans branching out as one big group. It's those that we altogether refer to as the bilaterians. This phylogeny makes very good sense from a morphological point of view, and the first molecular analysis based on single or just a few select molecular loci actually showed a similar topology. However, if you look at this second graphic, within the last few years we have started to get conflicting results. If you don't focus on these small, restricted molecular datasets, but instead use much, much larger phylogenomic data, some of the groups begin to switch positions in the tree. If we analyse phylogenomic data, which are much more comprehensive, we get pretty good support for the Ctenophorans, or comb jellies as the first Metazoan group to branch out. Morphologically, this seems strange, because comb jellies in many ways are much more complex organisms. For instance, they have a well-developed nervous system, which is completely opposed to the sponges. So understanding the early evolution of Metazoans is one of the big challenges that we still need to solve.
