Welcome back. In today's lecture, we will contemplate what will turn out to be a very difficult question to answer. What is life? Think we can all agree that these puffins, shown from my summer vacation picture, and the grass around them are alive. But when we go to sit down to make a definition of life, we see that's a very challenging problem. And we'll begin by discussing that, then turn to the Building Blocks of Life. And in the end, we'll talk about the efforts to search for life and give the example of Mars, and the current effort to detect methane on Mars. Well, first how do we, what do we need when we have a definition? We have a definition, we can describe the definition itself in different ways. We could have a stipulative definition, an electron is the smallest particle that carries a negative charge. That tells us the key properties of the electron that make it what it is. We can have an ostensive definition, and we can define dogs by saying, you know, a collie's a dog, and a schnauzer's a dog, and a terrier's a dog. And dogs are a great thing to define, because it gives me an opportunity to put on my slide cute dog pictures. And that's, that's a definition where I could list things that are in that group. Or, I can define objects or what things are by tests. So for example, if I talk about rocks and talk about how hard rocks are, there's a famous test called a Mohs test. And I can see, is it, if I take this rock, does diamond scratch it? Does gypsum scratch it? Does it scratch gypsum? I take some property it has and test to see whether it satisfies that. So we can attempt to come up with different kinds of definitions for life that might have different kinds of properties. Carl Sagan in his classic 1970 paper identified five different types of definitions that people put forward for life, and most attempts to do this really fall into these categories. And we'll look at all these types, the Physiological, the Metabolic, the Thermodynamic, the Biochemical, and finally the Genetic. Let's bring with, begin with the Physiological. A physiological definition would be for example, to define life as a system capable of carrying out a series of functions. You define it by its functionality. It's something that eats, it metabolizes, it excretes, it breathes, it grows, it moves, it;s responsive to external stimuli. Now when you look at that list, at first it seems very satisfying. But you start to try to apply it to some specific examples and you find that there are things that seem to satisfy this that most of us are agree aren't alive. If you look at a car and imagine yourself as an alien who's landed on this planet, you see these cars driving around, eating gas, converting it into energy so it can move, excreting from their tail pipes, interacting with their environments. In fact many cars today have sensors that respond to the environments. Headlights that go on when they're dark. They seem to be alive. They seem to satisfy these definitions. And even such abstract concepts as a city. When we think about a city, a city seems to be alive and, some of you may think well that's true, cities are alive. But most of us don't think of cities as living creatures. Yet they would seem to satisfy a lot of these physiological definitions. Another definition might be a Metabolic one. We think of life consuming energy in order to move or grow or reproduce. And often a metabolical definition takes advantage of the fact that things that we call alive tend to have boundaries. And we'll talk about this more in the second half of this, part of the lecture when we notice that cells all tend to have well defined boundary roles. And there's something inside and something outside. So a metabolic definition might be a well defined structure that consumes things from its external environment, and uses that to produce energy, grow and reproduce. Sounds like a good definition. But I would argue that fire basically satisfies that definition. If you have a flame on a candle, that flame will maintain the same shape for a very long period of time. It will consume the candle below. It will convert its food if you like, the wax, into energy and use it to move and grow. And those of you who've talked to people who fight fires, fires sometimes seem to be almost alive, as they jump from place to place and seem to search out almost, where there's a viable fuel for them to gain energy, grow and reproduce. Yet I think we would agree, fire's not alive. We can take a Biochemical definition, and think about life as something that contains a reproducible heredity material. We think about defining life in terms of something that has something like DNA. Now on one hand that definition may be, it depends how we formulate it, too limited. It would, might apply to life on Earth, but life elsewhere might not be based on DNA. There may be some other way of storing genetic material. In fact, early Earth was probably an RNA world as we'll talk about later in the course. And there may be other biochemistries out there. When we think about this definition, we also have to struggle about questions like, are viruses alive? A virus contains a bit of genetic material. A virus in isolation cannot reproduce. A virus has to be absorbed into the cell, and then use the mechanisms of that cell to reproduce. It's only alive when its inside a bacteria or some cell of a fungus or a human. Only then can the vi, virus reproduce. An even more dramatic example is a prion, and here's the discoverer of prions. A prion is a misfolded protein. And it's responsible for diseases scrapies in livestock, and Creutzfeldt-Jacob disease. And this single misfolded protein can cause other proteins in your cells to misfold, to misfunction and can produce devastating disease. But we don't think of a single misfolded protein as alive. It's almost like a piece of crystal, that stimulates growth. We can have a Thermodynamic definition. Let me remind you of the second law of thermodynamics. The second law of thermodynamics says, disorder or entropy always increases. That if you try to do something like take all the atoms and molecules in this room, all the air, and put it in one corner, that's a very ordered state. Everything's in one corner. And just because entropy will increase, those molecules will diffuse through the room. If you start with a cup of coffee, that's pure coffee. Add some milk in it. The milk will slowly spread through the coffee. It will become more disordered, and eventually become to your eye an effectively completely uniformly mixed. Now, one of the interesting features of life, it's a local reason of order, surrounded by disorder. And we can think of life as something that's the self-contained system that takes in energy, uses that energy to create order locally in an environment that grows increasingly disordered. To be more prosaic in my language, you could think of life as something that generates heat, so it could survive. Something and, by surviving it creates a local organized structure. The problem with that definition is that crystals, which grow locally and produce highly ordered structures would seem to satisfy this thermodynamic definition and would seem to be alive. One cannot think about a Darwinian Definition or a Genetic Definition. Something that is alive, is something capable of undergoing evolution. Now, this definition has many attractions but I think it has a number of potential problems, both in terms of things that it would seem to imply aren't almost alive. One part of evolution is you would need to reproduce to evolve. I think we'd all agree that someone who is incapable of having children is very much alive. They're just not going to be participating in future generations and the process of evolution. The other hand, there are things that seem to basically evolve. We can apply the laws of evolution to ideas, watch how ideas compete, and evolve in response to their environment. Companies, when we look at how companies behave, people have applied the concept of evolution and survival of the fittest in the marketplace. And I don't think corporations are people. And I don't think they're alive. So this actually quite a hard problem. And, since we don't know what life is, it's actually difficult to answer this question. Was there a distinct moment when the planet went from being lifeless, to having life? Because, what does it mean to be alive? It's a, turns out to be a very hard question. And it's one where, I think we all agree, you're alive, and I'm alive. But, as we'll see it's hard to write this down and figure out how to formulate this. At this point, I'm going to stop, and have you answer a question. And you'll see this question come up when you exit this lecture. And you're going to tell me, with fire which of those five definitions does fire satisfy? And then we'll turn back to the lecture. Thanks.