[MUSIC] Well, we didn't get very far in answering the question of when and where did life originate by looking at the rock record. So we're going to have to take a different approach, and try tackling the question from the other direction. If we can answer the how question first how did life originate then maybe we could back calculate the environment necessary to make life and then think about where and when on earth such an environment might have existed. Obviously, the how is not an easy question. In order to even begin to answer this question, we first have to be able to explain what is life. That is, we need to define exactly what it is we actually want to know how to make so that we know where we're going. Now, this in itself turns out to be a surprisingly difficult thing to do. I mean, we all inherently know whether something is alive or dead or inanimate, right? But what distinguishes these things from one another? Are there specific characteristics that describe, in a simple way, what is life? Is life something that is made up of organic material? Perhaps. But there are many organic molecules such as table sugar that are not alive. Is it something that uses energy in the environment to grow? It might do that, but minerals can grow by the same manner and they're certainly not alive. Could you identify life by it's ability to consume other things to take in energy while dispelling waste products, of what it doesn't need? Well, most certainly. Life does this, but then again, so does a fire. Which is not alive in quite the sense that we mean. Carol Cleland, a professor of philosophy at the University of Colorado Boulder has said that it is hard to find a definition for life because we do not yet have the correct language. One can only define what one understands. For example, before we understood molecular chemistry, we could not define water. Which we now know is a molecule containing two hydrogen atoms and one oxygen atom. Well perhaps Professor Cleland is correct, and we'll take a complete paradigm shift in the natural sciences before we can really understand what life is or have the proper language to describe it. But until then, the most accepted working definition of life is that living organisms are things that use energy to build molecular structures and to reproduce or replicate. And they do both of these activities following a specific set of instructions that are embedded within the organism. That is, they have a metabolism, the ability to manufacture biomolecular structures using a source of energy, and matter taken from the surrounding environment. And they have a self-replication. Or the ability to transfer biological information from one generation to the next using a blueprint or code for the information and some kind of mechanism for passing it on. Now in terrestrial life forms, the code or blueprint is DNA. And metabolism in life on earth is accomplished by proteins. These are the functional bits that make our cells work and drive metabolism. So the question is, if life evolved from a previous abiotic world, what was the first step in becoming a living world? Was it metabolism? Or was it the ability to replicate the blueprint? Now, you need a metabolism in order to produce the blueprint materials and then replicate them. But on the other hand, you need a blueprint to build the structures that turn energy and matter into the proteins that metabolize. So which came first, the ability to harvest the energy or the ability to reproduce the genetic memory of how to harvest the energy? Well, let's just start with one, with replication. Maybe that came first. In modern organisms, DNA is the molecule that encodes genetic instructions. It is the information storage. And it transfers that information to RNA which can then use it to form proteins. Which are the functional bits that make our cells work. And that is, drive metabolism. But DNA is an incredibly complex molecule. It's hard to figure out how such a molecule could evolve from an abiotic world, because it is made up of so many tiny complex bits, that would have to independently evolve and then come together in a way to make DNA. And each of these individual bits they can't actually do anything like replicate themselves until they are part of the larger DNA. So, let's try approaching the problem from the metabolism direction. There we end with the same result. Proteins are also large and incredibly complex molecules. It's hard to imagine how they would just abiotically form and self replicate themselves without a blueprint like DNA. A leading theory that tries to get around this chicken or the egg dilemma is the RNA World hypothesis. RNA is a simpler molecule than either a DNA or a protein, and it has the potential to both store information like DNA and code the productions of proteins, thus making it possible to synthesis the molecules need for metabolism. The theory is, then, that first life was a strand of RNA. It could replicate itself and it evolved to produce proteins that made the self-replication more efficient by forming a metabolism. And eventually, it evolved DNA, so that the self-replicating blueprint could be kept in a more chemically stable information storage system. But even with this solution, the complexity problem still stands. How do you make an RNA molecule from anything other than another RNA? Well simpler than DNA or protein, RNA is still extremely complex, made up of long strands of nucleotides, the organic molecules called cytosine, guanine, adenine, and uracil. So before we had an RNA world, there must have been a pre-RNA world, an environment where these organic molecules were somehow being formed and were energetically boding to one another to form strands. Now scientists called molecular biochemists try to solve this problem. They do laboratory and theoretical experiments in order to find out under what conditions these pre-RNA molecules might form. The concept is a bit like trying to figure out the recipe to your favorite cake. You have a general idea of the ingredients and the processes of putting them together and you know what the final product is, but to make your cake, or to make life, you need just the right proportions of just the right ingredients. And they have to be mixed together and cooked in just the right way to get the proper outcome. Well, we at least know how to guess about what we want to make. The nucleotides that will join together to form RNA and then evolve into life. But what are the ingredients that we need to use to make them? I'll give you a hint. It's probably going to be the same things that modern living organisms need to stay alive. For modern terrestrial lifeforms, those are three main things. We need liquid water. All known forms of life on earth require it. And all living cells are composed predominantly of water. We need a source of energy. Animals like ourselves are heterotrophs, we obtain our energy by consuming organic matter from other organisms. Plants and algae get their energy from the sun via photosynthesis. The prebiotic world must have had an energy source as well. And likely culprits include the radiant energy of the sun, bolts of lightning, asteroid impacts, the earth's inner heat, or the chemical energy released from redox reactions in minerals. Finally, we need the ingredients themselves, the building blocks for life and the prebiotic organic molecules we want to make. So that's carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus. Now if we have all of these ingredients then all we need to do is figure out the right conditions for them to form together into a pre-RNA nucleotide. And if we can do that, then at least theoretically we should be able to experimentally reproduce life in the laboratory. The first scientists to attempt this in earnest were the famous University of Chicago professor Harold Urey and his graduate student Stanley Miller. Their experiments to form organic molecules were first performed in 1953. The definitive results of their work was published in the journal Science in 1959. What these researchers did was place molecules of methane, ammonia, hydrogen, and carbon monoxide in a simple glass chamber with water in it. They boiled it gently and added some electric sparks that might represent lightning pulses or some electric charges in an early Earth atmosphere. And after a few days of running the experiment, eureka. The water in the chamber turned yellow. And residues began to build up on the electrodes that were sending in the electric sparks. These residues it turned out were organic molecules. Very simple molecules, but organic nonetheless. And this experiment opened the floodgates and chemists ever since have been develop-, developing new ways to abiotically create organic molecules in an attempt to create the right conditions for making nucleotides, the precursors to an RNA world. And the possible answers to the origin of life. Needless to say, this approach has remained unsuccessful so far. Well it turns out it is very easy to make simple biomolecules experimentally. Researchers still haven't come close to synthesizing the four RNA base nucleotides from scratch. Nor do we understand what geochemical mechanism could've linked those nucleotides to make them into a functional RNA strand. With the aim of better focusing these experiments to mimic the natural conditions under which life actually did form, there's renewed interest in going back to the where and when questions. Trying to characterize the environmental setting in which life arose so that it could be experimentally reproduced. Now that means we find ourselves back to the first question. What did earth look like when life arose? What kind of biologically hospitable environments existed in the Hadean? As you know, we have no direct evidence of Hadean environments, but we can certainly hypothesize what could have been. Again, by using the law of uniformitarianism and of course what we know about physical processes in the early Earth. The Hadean prebiotic environments that have been proposed are about as diverse as the environments in which life thrives today. But there are most certainly some most favorite scenarios that are consistent with what we know about the Hadean world. For example, we know that the Jack Hill zircons and gneiss. We know from these that there was liquid water on the Earth. Most likely in the form of oceans. We know from the high density of craters on the Moon that there were frequent meteor impacts. We can guess that since we know atmospheric oxygen to be a trace fossil of photosynthetic activity. Free oxygen probably wasn't in the atmosphere, and thus prebiotic environments existing in reducing, or non-oxidizing conditions. And we know that at the beginning of the Hadean after the moon forming impact, the Earth was incredibly hot, and there were likely high concentrations of volcanic and hydrothermal environments associated with the release of that heat from the Earth's interior. Well, the Miller, the Miller–Urey experiments were designed following Russian biochemist Alexander Oparin's 1924 proposal that life formed in a primordial soup. A warm pond or ocean of water in contact with the highly reducing atmosphere of methane and ammonia, and being stimulated by electrical energy from lightning. Now while all the right ingredients are predominantly there, this actually might not have been the most ideal environment for the origin of life. It is not actually certain that ammonia was a large constituent of the Hadean atmosphere. And the ionizing effects of lightning, cosmic radiation, or ultraviolet light from the sun has the tendency to cause as much destruction of small biomolecules as it does their creation. In addition, a more attractive venue has been discovered in hydrothermal systems of the oceans. Now, these environments, while shielded from both the energy source and the radiation source of the sun, have their own modern biosphere. That is sustained by the chemical energy produced from the disequilibrium between cold seawater and venting hydrothermal fluids. Particularly promising locations for early life is in deep sea hydrothermal systems that are hosted by the mineral serpentine, this mineral right here, as well as hydrothermal systems that form in craters which are left by impacts. Now serpentine is a water rich mineral that the alteration product of peridotite, which is an intrusive igneous rock that is even more iron and magnesium rich than basalt and is what makes up Earth's mantle. Serpentine is rich in the elements nickel and cobalt, which might be useful in stabilizing organic molecules so that they can combine to form more complex molecules. And hydrothermal fluids flowing through serpentine have a very high pH, providing a chemical potential gradient with seawater that could be an important energy source. On the other side, water filled impact craters also concentrate metals like nickel and cobalt and often have hydrothermal systems that form during the impact. Sustaining a warm environment for many thousands of years. The advantage of the Impact Crater Hypothesis, is that such an environment concentrates pre-biotic molecules. So they have a greater probability of interacting than in a very dilute ocean. The disadvantage is that is, there is no way to spread life to other environment's if the crater lake is isolated. In contrast, life origin, life originating in hydrothermal environments on the ocean, has the potential to disperse and colonize other hydrothermal vents. Another tantalizing hypothesis is that life never even formed on Earth. Bio-molecules have been discovered on asteroids and comets, provoking the idea that life is not even a terrestrial phenomenon, but in fact might have hitched a ride on a meteor from an asteroid or a comet or another planet to Earth. In the end one of the most baffling paradoxes of the hunt for the origin of life, is that organic matter appears to be so ubiquitous in our solar system that life almost seems to be an inevitable consequence of Earth as ocea, as much as Ear, oceans and continents. And yet, the transition from organic material to life remains so completely enigmatic that the act of the origin of life remains one of the biggest unexplained phenomenons in our universe. [MUSIC]
