[BLANK_AUDIO] One challenge of Astrobiology is to understand the relationship between particular organisms and how our early life on Earth gave rise to the multiplicity of life that we see on the earth today. And this is the job of finer geneticists. What we're going to talk about here is the Tree of Life. The Tree of Life is essentially a depiction of the relationship between organisms that evolutionary relationships and how they branch from one another. This is an example of the Tree of Life, and you can see it's made up of many groups of organisms. In fact, three groups or domains of organisms. On the left, you can see bacteria such as cyanobacteria, these are photosynthetic cyanobacteria that produce oxygen, and on the far right hand side, part of that Tree of Life includes animals, which includes, of course, ourselves, as well as, fungi and plants. How do we create this tree, and how do astrobiologists know the relationship between these different organisms? Well, Phylogenetics is the field of study that seeks to understand the evolutionary relationship between groups of organisms. It's based on comparisons of molecules within those organisms, such as the genetic storage information DNA or differences between their shapes. The Tree of Life can help us address several interesting questions in Astrobiology, such as which organisms appeared first? Who lived on the early Earth? What was the nature of those early organisms that inhabited the early Earth? And what is the history of life on Earth? How did those early organisms on Earth eventually branch out into the organisms that we're familiar with today? Well, let's have a look again at this Tree of Life and some of its features. Here's a more detailed Tree of Life, and you can see that it starts with a common branch, and this is the common ancestor. The common ancestor of life that gave rise to all life on Earth today. And it's the job of astrobiologists to try and find out what that common ancestor was. The common ancestor gave rise to three main domains of life. These three domains are bacteria, which include many of the bacteria living in your soil, in your garden, also bacteria that cause disease. And then another domain called archaea. Archaea are made up of some of the organisms that live in the most extreme environments on the Earth, such as microbes that live in hot springs in the deep oceans or in volcanic springs, in volcanic areas. And then on the right of that diagram, you can see the eukaryotes, eukaryota. Eukaryotes include all multi-cellular life, such as animals, plants, some fungi, but also some single-celled organisms, as well including algae that live in the oceans. These are the three domains of life, and they break up into smaller subdivisions of organisms. We call the bacteria and archaea, these single-celled organisms, the prokaryotes. Prokaryotes is a general name that's quite useful to classify microorganisms, bacteria and archaea, and it's a term one frequently sees when describing simple single-celled organisms. So how do we build this phylogenetic tree, this Tree of Life? Well, one way we can do this is to compare the shapes of different organisms. Most of us can tell the difference between a horse and a dog, for instance. So that might be one way which we could classify organisms. This is called Taxonomy. Another way in which we can do this is to look at molecules inside organisms and compare them. And the reason why this is useful is because it's sometimes very difficult to tell the difference between two organisms just based on their shape. This is particularly true with microorganisms. Here are a couple of images of microbes. On the left you can see Escherichia coli, the typical microbe grown in many microbiology labs and used to study the biochemistry of microbes, for instance. On the right you can see Vibrio cholerae, the microbe responsible for cholera. If you look at these two microbes under a microscope, they look very similar. They're both rod-shaped. They're both about the same sort of size. It would be very difficult to build a Tree of Life by looking at all the microbes in the world and comparing their shapes. So we have to use more sophisticated methods to tell the difference between different organisms. And one way we, in which we can do this is to look at molecules that are very important for fundamental cell processes. We saw earlier in this lecture course how cells need to replicate their DNA, the information storage system, and also read that DNA, read the instructions in the DNA to carry out basic cell functions. There's a particular molecule called ribosomal RNA. That's responsible for translating DNA into proteins for reading the DNA code and turning it into proteins and in fact reads the code on RNA, RiboNucleicAcid and because this Ribosomal RNA so crucial to cell function, it hasn't changed much over billions of years. So if we look two organisms that are very closely related, we find that Ribosomal RNA has not changed much, since those two organisms diverge in the evolutionary record. If we look at ribosomal RNA from two organisms that diverged a lot time ago, that split apart in the evolutionary record much greater time ago. Then we find that the ribosomal RNA has changed, to a much greater extent. So by looking at ribosomal RNAs in different organisms, and how much it's changed, we can start to build up a tree of the evolutionary distance between those organisms. And so these molecular methods, allow us to build, a Tree of Life. And another reason for looking at molecular structures, is that many micro organisms cannot be cultured in the laboratory. This is called the great plate count anomaly. And the reason for this is we really don't know how to grow these microbes from the natural environment. So if we really want to build a true Tree of Life, a Tree of Life that represents, all of life on Earth, going about trying to grow all these things in the laboratory can be very laborious and not very productive. It's much easier to extract these molecules from the environment and identify them without even growing those microbes in the laboratory. And if we do that, we find that some places on the earth such as deserts, actually turn out to be quite diverse in terms of the microbiology, the microbes that they harbor. And so using these molecular methods like ribosomal RNA, not only allows us to build the Tree of Life and to tell the evolutionary distances between organisms, it also allows us to get a better grasp of the full diversity of life on our planet, and what that common ancestor might have been like. Well, I've talked about a Tree of Life, that assumes that genetic material is just transferred from parent to offspring. One generation, after another. It turns out, that it's not quite that simple. Microbes can do remarkable things. If I told you, for instance, that you could walk up to a person in the street, and just by touching them, your eye color would become the same as their eye color, you would think that's a ridiculous idea. Some sort of weird science fiction idea. But in fact, bacteria can do exactly that. In the environment they can take up DNA from other bacteria, and they can change their characteristics by absorbing this DNA. One process by which they do this, is called conjugation. Whereby they directly transfer DNA from one bacterium to another. This process makes the Tree of Life a little bit more complicated, because genetic information is not just transferred vertically from one generation to another through time, but the material can also be transferred horizontally, as we say, between species, at any particular point in time. And this means that the Tree of Life is less like a Tree of Life and more like a web of life, or a ring of life. Or perhaps more amusingly, some people have referred to it as a shrub of life. But it's certainly more complicated than just branches. Branching off at different points in the history of life on Earth. We have to understand this horizontal gene transfer, between organisms, particularly micro organisms, in order to understand, how that, relationship between organisms has developed over time. So what've we learned? We've learned that phylogenetics studies the evolutionary relationships between groups of organisms. The Tree of Life can be divided into three domains: Bacteria, archaea, and then eukaryotes including us. Prokaryotes, which broadly include bacteria and archaea, are the oldest and most abundant of all organisms. Most prokaryotes are known only through their genetic material, the genetic sequences. The Tree of Life remains debated, and alternative phylogenetic models have been proposed. But this is yet another task of astrobiologists to understand what the earliest common ancestors to life on Earth were, how the phylogenetic tree developed, and the relationship between organisms on the Earth.
