 So in this video I'm going to talk about the domains of life, which are their overall groupings, and also phylogeny, which is the process of looking at modern organisms and interpreting how they're related to each other through evolution in the past. So most paleontologists use the morphology of organisms to understand how they have evolved. And this is particularly good for things like animals and plants that have more complicated morphology. All of the organisms with backbones share a common ancestor that evolved that structure and then it was passed on to its descendants through time. Since the 1970s, we can now use the genes of the organisms to also understand those relationships and the phylogeny, and the genes are the encoders of how an organism works. And so they make the enzymes, they control the morphology of the organisms by the way the genes are expressed, as for an example a multicellular organism divides. Genes also work well for tracing the phylogeny or evolutionary history of single-celled organisms like bacteria where morphology isn't very useful. So I'm going to talk about genetic phylogeny and what that tells us about the domains of life or the major groupings of life that we've observed on earth. Okay, so when we look at the genes of organisms, they divide out with animals, plants, fungi, and organisms like this that we call eukaryotes. And these organisms have a very specific morphology, both the macroscopic scale but also in terms of their cellular structure. And then on the other hand, we have organisms that are single-celled mostly, sometimes the cells are grouped together with sheaves around them, but they don't have the differentiation the same way animals and plants do, and they have no nucleus. And that lack of a nucleus is what distinguished between the eukaryotes and the single-celled organisms and they used to be called prokaryotes. This first division was based on morphology here. But in the 1970s, when people started being able to characterize the genes, they realized that this group of prokaryotes is really two different genetically very different groups of organisms. We have the bacteria, which was a term from before, but we also have the archaea. And although these have a similar morphology, the genetic material in them is really quite different. And so this led to the idea that you'll have these three domains of life, the bacteria, the archaea, and the eukaryotes. So these are what we call the domains. They are the sort of largest scale divisions of life based on genetics. So we draw these lines that show the evolutionary relationship space that we've reconstructed from the genes. And each one of these branch points here means that the two groups of organisms share a common ancestor that's represented by the line at the bottom. So down here we would say that all life on earth has what we call a last common ancestor. And in some sense, there was some organism or community of organisms that existed that all life that we've characterized on earth descended from. And at this point, there was an evolutionary difference between them, and some ended up evolving to form bacteria, and some ended up evolving to form the archaea and the eukaryotes. There's another branch here, which represents an evolutionary event or separation between the organisms that evolved to form the archaea and those that evolved to form the eukaryotes. So as we know that there are just thousands and millions and billions of organisms in each one of these trees, and there are branching points that come out from each one of these branches. So we can look at the first phylogenetic tree that had these three domains of life. It was only published in 1977. Before Carl Woes looked at the genetic material of organisms, the bacteria and archaea shown here in purple and red were characterized as one type of organisms with the eukaryotes as another. And so this down here would represent the common ancestor for all life on earth, and these branches represent the division of different types of organisms. The length of each one of these branches represents the genetic difference between the different types of organisms. And so that genetic difference has to be measured in a specific way. So we can say that the length is proportional to the genetic difference. So that genetic difference needs to be measured in different ways, and the way Carl Woes chose to do it is the way we still do a lot of it now is to look at the genes coding for the ribosome. Ribosomes are critically important for life because they take the messenger RNA and convert it into the enzymes. And so by looking at the genetic difference of ribosomes, these evolve slowly, which means that you can actually look at the ribosome of an archaea on this branch and compare it to one, say a cyanobacter over here. And the genetic code is similar enough that you can actually compare them. So genetic code is too different the comparisons break down. So looking at the ribosomal RNA allows us to look at these really large-scale relationships.