The Science of Everything Podcast - Episode 4: The Origin of Life

Episode Date: November 20, 2010

An overview of current theories about how life arose from non-living matter, including an overview of the conditions on early earth, definitions of life, the source of organic monomers, the mechanisms of polymerisation of these monomers, and the possible means by which the first proto-cell developed. If you enjoyed the podcast please consider supporting the show by making a paypal donation or becoming a patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

and welcome to the Science of Everything podcast. I'm your host, James Fodor. In this podcast, I discuss a wide variety of topics in both the natural and social sciences, exploring the many fascinating scientific discoveries that help us better understand the world around us. This is episode number four, and the topic for today is the origin of life. So in this episode, I will cover some basic introductory topics to life and the study of the origin of life, And then I'll move on to talking about the two main areas that we need to understand in examining the origin of life, the source of monomers, and then the origin of polymers from those monomers. And then I'll have a bit of an overview in summary at the end.

First of all, we need to ask the question, what is life? And this is not really the focus of this podcast, because it's rather a controversial topic. but for the purposes of this episode, I'll just list a fairly conventional list of the criteria of what constitutes life. First of all, cellular organization, so all life is composed of cells. Second of all, a high degree of order. That seems fairly obvious. Third, response to stimuli. Fourth, growth, development and reproduction. Fifth, energy utilization.

Sixth, evolutionary adaptation. And seventh, maintenance of homeostasis. And homeostasis refers to the constant internal conditions of temperature and pH and so on within an organism that allow the chemical reactions, the sustain metabolism, and to continue properly. It should be noted that the concept of life is rather a fuzzy one, and there's no clear line dividing life from non-life, because, as we'll see throughout this episode, it's somewhat of a continuum. You go from something that's gradually a chemical system that's more and more complicated. it becomes self-replicating, it does more and more different things,

and it sort of gradually transitions into something that we would call alive. So it's really just how many properties of life does something have, and not is it alive or not alive? So having introduced the concept of life, now we'll move on to the origin of life. Now, there are many different theories for the origin of life. The field is still rather in its infancy, and there's no, if you like, standard model of how life emerged. that in this podcast what I want to do is present plausible narrative of how life could have come to pass.

This is not the only possible story, but I think it's better to present one possibility rather than, as many other sources do, they say, well, there's this theory and that theory and the other theory, and by the end of it, it's just rather confusing, and you don't really seem to get much out of it. The theory that I propose here is based on what I think is a fair, generic presentation of the evidence and just focuses on the main elements and not the specific mechanisms and I think it represents to some extent the consensus of at least the majority of people who work in this field although by no means is the issue decided completely. So that said, I just want to briefly address the question of carbon. Now carbon is the

basis of all life that we know. All organisms that we know are predominantly made up of so-called organic molecules, which means based on carbon. Carbon forms the basic structure of virtually all of the molecules that make up people, including our DNA, the proteins that form our structure, our bones, our organs, hormones, pretty much everything is based on carbon. Now, why carbon? What's so special about it? Well, carbon has a valence of four, which means that of the eight electron spots that comprise the outer shell of the carbon atom, only four of them are filled. So the outer shell of a carbon atom has eight spots for electrons,

only four of them have something in them, have an electron in them. And an atom like carbon is most stable when it has a full outer shell. So that's a full eight electrons in its outer shell. As carbon only has four, it either has to lose four or gain four electrons in order to reach this level of stability. Now that means it has to form four atomic bonds with other atoms in order to become stable. And that ability to form four different bonds is what enables carbon to form so many different and very complicated molecules, which of course are essential for life, which is a very complicated system. So that's essentially why carbon forms the basis of all life that we know. Another important fact about carbon is that it's a relatively light molecule so that it can react fairly quickly.

Heavy molecules are more sluggish in chemical reactions and so it's harder for them to perform all of the reactions that are necessary to sustain life. Now the current evidence indicates that life first formed on the Earth around 3.5 to 4 billion years ago. And at this time the Earth was still very hot, it was very geologically active, so lots of volcanoes and that sort of thing, and there were still many collisions with meteors. And it's believed that many of these early collisions of comets and meteors actually brought the water to the Earth that formed the oceans, which are believed to have begun to cover the Earth within the first few hundred million years, and are believed to be the location of the origin of life.

The initial atmosphere of the Earth was very different from the atmosphere today. It contained very little oxygen and was probably mostly nitrogen and carbon dioxide, though the exact composition of the early Earth atmosphere is still controversial. Now, it should be noted that this lack of oxygen is very important because oxygen is very reactive, and it would have reacted with any organic molecules that did exist and were beginning to form on the early earth and prevented them from forming any really complicated things

like proteins and amino acids and so on that would have led to life. And that's one of the reasons why life does not spontaneously emerge on the earth today. It's because we have too much oxygen in the atmosphere, which is too reactive, and would prevent any really complicated organic molecules from forming spontaneously. Another reason we don't have life forming spontaneously today

is because there's too much life that already exists and it would quickly eat up any organic molecules that began to form. So that would be animals, bacteria, plants, anything. They would eat up any organic molecules that began to form before they could get anything near, as complicated as they would need to be for life. Now it's believed that the very first thing that we could call life, we might call it a protocell, all it needed really is a membrane surrounding a self-replicating molecule.

Now a membrane is basically just like a bag of molecules or a sphere of molecules that separates one interior area from its external environment. So as you may know, membranes surround all known cells that exist today and they just keep them separate from their environment. Membranes are made of special organic molecules called lipids and we might talk a bit more about that later. So we have this lipid membrane and inside the membrane is a self-replicating molecule. molecule that was probably RNA or something similar and we expected that it's something like RNA because we know RNA is able to contain genetic information that can be passed on from one molecule to another so the RNA makes a copy of itself and in so doing it makes a copy of the molecule that is able to make a

copy of itself so you see that in making a copy of itself it preserves the information necessary to for the for its daughter molecule to keep making copies of itself. And that's how life would have been able to propagate itself and continue to exist. And it's also how evolution would occur, because errors in the copying would have occurred just by natural chance. Most of those areas would have been disadvantageous, but a few of them would have been beneficial, helping the organism to copy itself, sorry, helping the molecule to copy itself more quickly or better. And then natural selection would have acted to select those few beneficial mutations,

and from there, this very simple protocell would have been able to have evolved into more and more complicated forms of life. Now, just a quick note on terminology. DNA, as you may know, is the molecule that stores the genetic information that makes up an organism. RNA is a very similar molecule, which does a slightly different job, but it's quite similar to DNA. Proteins are molecules that form the structure of,

most of our bodies and hormones, skin bones, all that kind of stuff. Basically, pretty much everything is made of proteins. Lipids are another type of organic molecule, which their most important function in humans is forming fat cells and other similar things. Also membranes around cells are made of lipids. And there are also carbohydrates, which store energy. Those are the four main classes of organic molecules. Now, all of these organic molecules, particularly the two were mainly focused on proteins and nucleic acids, RNA and DNA, they're all polymers, which are long chains of molecules formed of subunits called monomers. So you have monomers, and many monomers bonded together make up a polymer. Now, monomers themselves are actually quite complicated.

Generally, they are composed of maybe 30 or 40 atoms all bonded together. in a particular arrangement. Now generally these are mostly carbon atoms forming the backbone of the monomer but you'll also have things like hydrogen, nitrogen, oxygen, phosphorus and so on bonded there as well. So a monomer is basically just a clump of carbon atoms with a few other things bonded together in a particular arrangement. And before we can even think about getting polymers like proteins and RNA we need first to form the monomers that make up the polymers and so that's the first question that I will now turn to, how did these monomers arise out of inanimate matter?

They need to be formed somehow from very simple molecules that we know do exist and would have existed on early Earth, like ammonia, nitrogen, oxygen, water, things like that, very simple molecules that form spontaneously just in space and the formation of planets, which only have a few atoms. How do these very simple molecules come together to form very complex monomers? and that is the question to which I shall now turn. The most famous experiment in the studies of the origin of life is definitely the Yuri-Miller experiment,

which was conducted in 1953 to simulate conditions thought to have existed on early Earth. Basically, a hydrogen-rich reducing gas, and that's just a gas that reacts in a particular way, with no oxygen in it, which was thought to be similar to the Earth's early atmosphere, was placed in an apparatus, above liquid water and the liquid water was maintained at a temperature just under boiling points so they kept it pretty hot. The gas was then exposed to discharges of electrical energy

to simulate lightning which was thought to be one of the sources of energy on the early earth that could have contributed to the origin of life and then they ran this for a few days and what they found was that after only a few days a wide variety of relatively complex organic molecules had spontaneously formed. I should say that in the liquid water that they had, it wasn't just water, it was water mixed with a few other very simple organic molecules like nitrogen and ammonia, methane. Those are really, really simple molecules like just a couple of atoms put together, things that definitely would have existed on the early Earth. And as I said, they just brought the water, the gas together, kept it at a high temperature and simulated some lightning through electrical strikes. and after only a few days, a wide variety of complex organic molecules spontaneously formed.

Similar experiments conducted since have been able to synthesize a number of amino acids and nucleotide bases in a wide variety of different conditions. Amino acids are the monomers that make up proteins, and nucleotide bases are the monomers that make up RNA and DNA which store the genetic code. And as I said, these monomers, amino acids, nucleotide bases are quite complicated, 30-40 atoms in just the right way. But as we can see, the Yuri Miller and similar experiments have proven that they can form spontaneously in conditions that probably existed on early Earth. Now I should point out that it is, as I mentioned before, controversial as to exactly what was the composition of Earth's early atmosphere. So it's not really known how well these experiments simulate that atmosphere,

but as I said, these experiments have been done in various, somewhat different conditions, and they've all managed to produce similar results. One problem though is that this idea that the Uri-Miller experiment was based on was the idea of the primordial soup, which is basically that the early Earth was covered with a large ocean or large areas of ocean, which contained simple organic molecules like methane, ammonia, nitrogen and other things, that then helped to form the basis of life. The trouble is there's actually no real fossil evidence for this primordial soup, and we would expect to find some.

Also, monomers that did form in these conditions would tend to break down relatively quickly. And also, there's another problem, which is how, even if the monomers did form in such an ocean, how they would become concentrated in sufficiently large concentrations in order to form polymers, because if you just have a very small number of monomers floating around in isolation, that doesn't help you very much,

they need to become concentrated. So these are some problems with the Uri-Miller experiment itself, which then led to later developments, which I'll talk about in a second. But the point is that we can see from the Uri-Miller experiment that complex monomers can form spontaneously from relatively simple ingredients. And by today, all 20 amino acids that are used by life forms, so those all the 20 different types of monomers used in proteins, have been formed spontaneously in these types of aminoes. of experiments. So that's pretty cool. So we've basically got a good chunk of the monomers that we need to form spontaneously from reasonable conditions that they probably existed on early Earth.

Now, to address some of the problems with the Eurymuller experiment, as I mentioned, the concentration problem, the lack of evidence for the primordial soup, and the issue of monomus tending to break down quickly in these conditions, it has been proposed that hydrothermal vents may have actually played a very important role in the origin of monomus. Now, hydrothermal vents are basically just fishes found in the ocean floor, out of which is emitted very hot water, which is being heated to high temperatures by nearby Magna. So they're kind of like volcanic vents in the ocean floor, except hot water comes out of them rather than lava. And the water you're issuing from these vents generally contain significant concentrations of minerals and other materials that are dissolved from surrounding rocks in the crust. Now, interestingly, rather recently it's been discovered that near these hydrothermal ventures, vents exist highly complex ecosystems with very hardy forms of life, including microorganisms, which are called extremophiles, that can exist in very high temperatures and other extreme environments, which we previously thought would be devoid as life. These extremophiles use the energy that comes

from the vents, the heat, and also the minerals that come from the water issued from the vents as sources of food, so they can survive without any sunlight in very hot conditions. Other extremophiles have been found to exist in very high pressures, living inside of rocks, miles under the earth in extremely cold conditions, so extremely acidic or alkaline solutions, or in extremely salty water. And these extremophiles are important because they show that life can exist in conditions that we previously had no idea that it could, which indicates that life could have formed in more potential locations than we originally.

thinking. Now one important discovery that has been made is that the high pressures and temperatures that exist around hydrothermal vents actually makes water much less likely to react with and break apart organic molecules. And so this solves the problem of the monomers that would have formed in the ocean spontaneously breaking apart before they can form polymers and more complicated molecules. Experiments with this have managed to produce complex monomers and even some polymers in these conditions around hydrothermal vents. Another discovery has been that the high concentrations of minerals found near hydrothermal vents could also help to catalyze chemical reactions needed to form organic molecules,

and I'll talk more about that later. But the basic idea is that the crystal and mineral structures that surround the hydrothermal vents helped to attract the monomers that did form and other, well, not just the monomers, but the simpler organic molecules that existed, they were attracted to these crystal and mineral surfaces, and these surfaces helped to bond them together and bring them together to form monomers, and then later the monomers together to form polymers.

So this bringing together of the organic molecules into these relatively concentrated environments helps to solve the concentration problem that played earlier iterations of this idea of the formation of molecules, whilst, as I just said, the high temperatures and pressure helps to solve the water breaking down problem. We also know that hydrothermal events exist and did exist on the early Earth, and this solves the problem of the lack of fossil evidence for the primordial soup. So it seems that the hydrothermal vents existing on the ocean floor

may have been the most likely location for the synthesis of monomers from simple organic molecules. molecules using the heat, also using the heat and minerals that came from the hydrothermal vents to help in the formation of the monomer molecules. Because also in order to form these complex organic molecules, you need energy as well. And as mentioned before, the Yuri Miller experiment assumed that that came in the form of lightning, but the hydrothermal vents theory proposes that it came from the energy from the hot water that issued from the ocean fishes. So that's a second possibility. So as we've seen there is a reasonably good theory for the source of monomers as to how these complicated organic molecules arose from relatively simple molecules

like ammonia and water and so on. The next question is how did these monomers come together to form polymers, which are, as I said, big long chains of very complicated molecules, like proteins and RNA, which are essential for life to exist. Now, one of the biggest problems in understanding polymerization is that in ordinary cells, things like DNA and proteins don't form spontaneously. The monomers do not polymerize spontaneously. They need to be catalyzed by special molecules. whose job is to catalyze these reactions. And catalyze just basically means allow a reaction to occur, or speed a reaction up.

Now, in ordinary cells, this action of catalyzation is carried out by proteins, but obviously in the origin of life there were no proteins to carry out this catalyst. So the question is, how did it occur? How can we have polymerization without specific catalysts to carry it out? And one, I think, quite compelling answer to this, is clay-based polymerization. The idea of a clay-based polymerization is that mineral surfaces acted as catalysts and scaffolds

upon which monomers bonded and then they were brought together by the clay which catalyzed their polymerization into big long molecules. So clays are sedimentary particles comprised of fine grains of different types of minerals. And clays are thought to have played an important part in the development of life because they contain large amounts of nutrients,

They form regular repeating structures, and they have an electrically charged surface. So this electrically charged surface is important because it provides a means for them to attract nearby organic monomers that have formed. Okay, so you've got your clay sedimentary particles with the charged surfaces. They attract the monomers as a result of their charged surface. The monomers bonds to the clay particles, and then the clay particles sort of collect up monomers, and concentrate them on the surface of the clay. And the theory is that then the clay could have also acted as to catalyze the reaction of these monomers together so that they formed a polymer.

And this scaffolding was used, not used, but it allowed the polymerization of the original polymers until more efficient means of replication, like using proteins, RNA and DNA, until those evolved, at which time this form of clay-based scaffolding was lost, and that's why we no longer see it today. Now, I should point out that the clay-based model, there are two models for this. One is that you just have small particles of clay, probably in the hot water surrounding the hydrothermal vents.

So these loose particles of clay are going around, collecting up the monomers and forming polymers. Another model is that you actually have larger structures of mineral compartments that are, developing and would have attracted the monomers in that way. So either of these are possible, and quite possible there was a mixture of both the particles and the larger rock-type structures of these minerals. It's also hypothesized that semi-porous mineral compartments could have served a similar

function to cell membranes, so they would concentrate the monomers produced near hydrothermal vents and protect them from the outside world. This concentration is very important because near the hydrothermal vents, the monomers would have been too dilute to have had enough of them in high enough concentration to have polymerized to any significant degree. So there needs to have been some way of concentrating them, and these clay mineral compartments could have played a role in that. Another advantage of this polymerization by clay particles near the hydrothermal vents model is that the monomers produced near the hydrothermal vents would have been chemically activated by the high temperatures,

which would have made them more reactive and therefore increased the ability of them to form, to polymerize and to join together. Also, as I noted before, clay contains nutrients, which would have made it more likely that more of these monomers could have formed and that these early self-replicating mechanisms could have developed. So I think that the clay-based model is not fully proven. It's still largely speculative, but it does have, it is a fairly good model that describes one possible mechanism that polymerization could have occurred. Okay, so now we've proposed a mechanism by which all these monomers can be brought together

to form big long chains called polymers. But still, we've got polymers, but that's still a long way away from life. How do we go from having polymers to having self-replicating organisms that we can call alive? Another aspect of this problem is that in modern day organisms, DNA holds the genetic information, codes for life, and this genetic information is then translated into RNA, which is a similar molecule to DNA. The RNA acts as a sort of messenger and then transfers that information again to create the proteins, which are sort of the action molecules. They actually do stuff, they form the structures of cells, they act as hormones, etc. But the trouble is DNA cannot catalyze itself. DNA cannot make more DNA.

It requires proteins in order to catalyze itself and form new DNA molecules. But proteins do not store any genetic information. So proteins are dependent upon DNA to store the information about how to make proteins, but DNA is dependent upon proteins to actually do the replication to make more DNA. So you see there's this mutual dependence here. And it's sort of like a chicken and egg paradox, which came first. You could, of course, say that they both arose at the same time, but that is incredibly unlikely that you would have these two separate forms of very complicated molecules

that would just come together in this relationship, and there's no plausible mechanism by which that could happen, and it's incredibly unlikely. So the answer to this paradox may lie in RNA. Remember, that's the messenger intermediate molecule, which sort of transfers information between the DNA to the protein. Recently, it has been discovered that some forms of RNA can self-catalyze, and this is very important. This means that these forms of RNA do not require proteins to act upon them so that they can replicate themselves. Basically, these RNA molecules can spontaneously self-replicate themselves,

while simultaneously storing genetic information, because RNA does store genetic information like DNA does. It doesn't do quite as good a job as DNA. It's not quite stable, that's why we still have DNA. but RNA does store genetic information and as we discovered, it can self-catalyze itself, and this has been done in the laboratory. So it seems that RNA can sort of solve this chicken and eggs paradox of the DNA and the proteins. It can serve as both storage of genetic information and as a way of catalyzing and replicating itself. I should also note at this point that there are some problems with the origin of RNA,

which I won't go into right now, but basically these problems have led some to propose that the very first self-replicating polymer was not RNA, but something like RNA, which we no longer see today. And that is possible because there are just literally millions of different types of organic molecules that exist. And it is possible that a different one that's easier to form, but perhaps less efficient than RNA, was the original source of life, the original one to evolve, and then later on RNA evolved and was selected for because it was more efficient. just as DNA was later evolved out of RNA because it's a more stable molecule to hold that information. So it is possible that the original form of self-replicating life was an RNA,

or maybe it was something similar to RNA, but not quite the same. So the next question then is exactly how did these original RNA molecules form? And I think the best model that we have for that is if we go back to the hydrothermal vents at the bottom of the ocean. So it's known that nucleotide bases or similar molecules could have formed near these hydrothermal vents, spontaneously, we've already talked about that, and they could have been concentrated by mineral lattices, which we have also talked about. Now, either clay particles within those mineral lattices, or the surfaces of the mineral lattices themselves, could then act as catalysts. They would collect up the monomers onto the surfaces and catalyze them together into polymers. and these nucleotide bases, which spontaneously formed, would then join to form what is essentially RNA, or something similar to RNA.

Now, this has been tested in the laboratory, and it has been found that clay particles have helped to polymerize RNA of up to 50 bases long after only a few weeks of running the reaction, in similar conditions to, thought to have existed on the bottom of the ocean near these hydrothermal vents. Okay, now most of the resulting RNA that we produce as a result of this process is just going to be randomly ordered. the bases, the monomers that make up the RNA are just going to be in whatever order, and hints are going to be useless. It's still storing information, but the information is just gibberish. It's just like if you wrote a whole bunch of random letters down on the paper,

it's information sort of, but it doesn't mean anything, so it's not very helpful. But just by chance, a few of them would have been just the right order required to have been self-catalizing. Now remember, before I mentioned that, we have discovered self-catalizing RNA molecules. And this basically means that there are certain RNA molecules whereby if the bases are lined up in just the right order, they will self-catalyze. They will undergo a reaction whereby they make a copy of themselves. Basically, the way they do that is by taking nucleic acids, which are the monomers that make up RNA. They take them from the environment and put them together in a certain order so that a copy of the original molecule is made. And only certain orders of the nucleotide bases will do that.

Most of them won't. They'll just sit around and be a molecule. So by chance, some of these RNA molecules that were produced, as a result of the process just mentioned, would have been of the type that is self-catalizing. And therefore, they would replicate themselves using monomers formed in the hydrothermal vent environment. And over time, natural selection would have acted so that the, obviously, the polymers, the RNA polymers that replicated themselves more quickly and more efficiently would have become more numerous, and so they would have been favoured, and those that were less capable of replicating themselves would have died out.

And there would have also been competition for resource, because there would have been a limited number of nucleotide molecules available for these self-replicating molecules to use. And so we can see that natural selection would have come into action even at a very early stage in the origin of life. I should point out that the probability of just the right sequence of base pairs coming together to form a self-replicating RNA is fairly low. But remember, we have hundreds of millions of years for this process to be occurring over essentially the entire surface of the Earth, because at this early time there were virtually no continents, pretty much the whole Earth was just covered by sea. So with hundreds of millions of years and so much space for it to occur, certain calculations that I have seen indicate that the formation of self-replicating molecules

purely by chance is quite within the realms of possibility. Okay, so now we've got a self-replicating RNA basically, but it's still essentially stuck in these mineral compartments down in the bottom of the ocean, with the mineral compartments keeping it concentrated and protected enough for it to exist. Obviously life can't stay there because we know that life does not just live in hydrothermal events. so how did it get out of there? Basically, the answer is phospholipid membranes.

Now, phospholipid membranes are the membranes that surround all cells that we know. Phospholipids are molecules that have one hydrophobic end and one hydrophilic end. And the hydrophobic end tends to stay away from water. The hydrophilic end tends to bond strongly with water. And so it has been found that these phospholipid molecules, we know that they could have spontaneously formed in the hydrothermal vent environment, just as the other nucleotide molecules and amino acids could have formed. The conditions were all there, and this has been replicated in the laboratory.

So we can get these phosphorylipid molecules, which are basically like monomers, they're just organic molecules. They could have formed spontaneously. But what's really interesting is that we have found that when you get a whole bunch of these phosphory lipid molecules together in water, they spontaneously arrange themselves into a spherical structure called a vesicle, or sometimes it co-assivate, which is basically where you just have a double-layered sphere of these molecules surrounding a bunch of water. So it's kind of like a bubble, but made up of these phosphory lipid molecules. And this occurs because of the dual nature of the

lipid molecules, with the hydrophobic and the hydrophilic end. Basically, the hydrophilic parts of the molecule pointed towards each other in the middle, if you like, of this membrane, and the outer ends are the hydrophilic ends, and they like to bond with water. So basically, you've got the outside, if you can picture a sphere, the outside of the sphere is water, and you've got the hydrophilic end bonding to that, then you've got the hydrophobic tail pointing inwards, and then you've got another hydrophobic tail, and then you've got the hydrophobic end of the second lipid molecule, and it's bonding with water, which is inside the sphere. And the really cool thing, as I said, is that phospholipids spontaneously form this structure,

and this has been performed in the laboratory. And they form this structure because it's the lowest energy state that they can reach. Hydrophobic ends will naturally migrate, so they point towards the center of this membrane, and the hydrophilic ends will naturally move to the outside and bond with the water. It just happens. And this is really cool because it essentially means that phospholipid membranes, which are basically just cell membranes, can form spontaneously near the hydrothermal events.

And this is very important because these phospholipid membranes can act as to concentrate organic molecules, and they can protect the molecules, separate them from the outside environment, basically perform all of those functions necessary for life to form. And that's why we have cell membranes today. Okay, so basically now we have all the elements necessary to form a relatively autonomous self-replicating protocell, as it's called. So first of all, we start off with the simple organic molecules like methane, water and so on, near the hydrothermal vents at the bottom of the ocean. The energy from the hydrothermal vents, along with other nutrients coming up from the Earth's core and mantle,

causes these simple organic molecules to combine together to form more complicated organic monomers, like a nucleotide basis and amino acids and so on, and this has been demonstrated in the laboratory. Clay particles, or possibly clay surfaces, mineral surfaces, near the hydrothermal vents, concentrated these organic molecules, and also attracted the monomers to their surfaces, where they catalyzed the reaction of these monomers together to form polymers, possibly proteins or RNA or something similar. By chance, some of these polymers, possibly an RNA or RNA predecessor, were self-replicating, and such self-replicating polymers have been observed in the laboratory and created in the laboratory.

These self-replicating polymers would have taken in monomers from their environment and other particles that they needed as well, and use them to create copies of themselves. Meanwhile, phospholipid membranes formed spontaneously near the hydrothermal vents. By chance, some of these self-replicating RNA molecules would have ended up inside the phospholipid membranes. either the phospholipid membranes formed around them or they just moved into that particular area just as the phospholipid membranes were forming or something like that. Phosphilipid membranes permit small molecules to pass through, so like monomers could probably be passed through and simple organic molecules that the self-replicating RNA could use, but the RNA itself and other big molecules couldn't pass through this membrane. So once the self-replicating RNA was inside the phospholipid membrane, it would stay in there and but nutrients could still get inside it to it to it. so that it could continue to replicate itself.

It has been observed that as RNAs replicate, the amount of stuff inside the phospholipid membrane increases, effectively increasing the internal pressure of the membrane and causing it to grow. And the membrane grows by absorbing additional phospholipid molecules from its environment. Once again, this has been observed in the laboratory. As the membrane grew to a sufficiently large size,

it began to elongate along one direction. and this has been observed in the laboratory as well. Basically, the membrane, once the internal pressure gets too great, the phospholipid membrane elongates and eventually split into two separate membranes. And it looks very much like bacterial cell division. And so it's possible that the RNA that's replicated itself whilst inside the phospholipid membrane, one of these RNAs ends up in one of the phospholipid membrane daughters, if you like, and the other RNA copy ends up in the other one.

And so essentially, you've just had an RNA inside a phospholipid membrane, replicate itself, and now you have two RNAs inside phospholipid membranes, and you can see this process continues. Over time, natural selection would have acted to increase the efficiency with which this proto cell replicated itself, and over time it would have eventually acquired the ability to produce its own amino acids, to produce its own proteins, to produce its own phospholipids, and to become more complicated and more efficient.

and so effectively, therefore we have a proto cell, which for all intents of purposes is alive. And that is probably the best story that we have at the moment for how the origin of life could have occurred. As you can see, most of the elements in this story have been replicated in the laboratory, not quite all of them, and they certainly haven't all been replicated at the same time. We have not produced artificial life in the lab yet, but I think we probably will within a few decades. So, although many questions remain, and the science of abiogenesis is still fairly speculative, hopefully from this podcast you have at least gained some idea as to how something as amazingly complicated as life could have emerged spontaneously out of just a whole bunch of inanimate matter.

And I think, and certainly the thing that I learnt from doing this research is that, although we don't know exactly how it occurred, there are certainly many possibilities of how it could occurred and some pretty compelling stories that we can tell, And it just does go to show that systems that look simple in it of themselves, like for example, just a whole bunch of very simple organic molecules, can come together in ways that you would never imagine and form things as amazingly complicated as life. So that's all for this podcast.

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