Appendix One-- Primordial Earth, Primordial Soup

What was the earth like, back before life appeared? During the very earliest stages of its existence, the Earth was still ‘under construction’ and simply too hot and too chaotic for the formation of life. In fact, current theory suggests that a Mars-sized planet collided with the Earth about 4.5 billion years ago , blasting away a large amount of crust material which then fused into the Moon. A collision that huge in scale would have also propelled most of the Earth’s oceans and atmosphere into space. Needless to say, any life that might have been present before such a giant impact event was probably eradicated. Sometime between 4.4 and 3.8 billion years ago, the early Earth began to grow relatively calmer and cooler, and we can start thinking about possible life-friendly conditions. Primordial Soup Current theory suggests that the early Earth was quite a bit like today-- with continents and oceans composed of about the same materials as we have now. The biggest difference was the atmosphere, which probably did not contain any oxygen . Most likely it consisted mainly of carbon dioxide, possibly mixed with methane, ammonia and other ‘reducing’ gasses. There was no life, but almost certainly there were many simple organic compounds in the oceans, including hydrogen cyanide, formaldehyde, methanol, methane and ammonia . All of those compounds are found in interstellar space and on other bodies in our Solar System-- so it seems very likely they were present here as well. The famous Miller and Urey experiments of the 1950’s showed that fairly complex organic compounds are easily synthesized when simple molecules such as methane and ammonia are exposed to an energy source. Recent research has shown that many complex organic compounds can also form in directly in space, perhaps via catalytic activity present in interstellar dust particles . Meteorites have been found that include amino acids , nucleic acids , complex hydrocarbons and sugars . The early earth was barraged with comets and meteorites at a much greater rate than today, and many of those compounds would have survived impact and ended up in the seas. So, one way or another, we can imagine an ocean 4 billion years ago that contained a wide variety of simple organic compounds— including many of the key components of modern life such as amino acids, purines, pyrimidines, fatty acids and simple sugars. This is the primordial soup. What was in the Soup? There is some argument among exobiologists about the contents of the primordial soup. Of course, all of the original soup contents have long since been eaten by later life forms, and there are no surviving rocks from that period— so about the best anyone can do is draw conjectures from chemical experiments using simulated conditions, or look at spectroscopic data from other planets and their moons. It seems likely the soup contained a variety of amino acids— they are created readily in most prebiotic simulations (the original Miller/Urey experimental setup converted about 2% of its carbon into amino acids ). Adenine (a purine used in DNA and RNA) is also produced readily in prebiotic simulations . It’s a particularly important compound for life, especially when it is combined with ribose sugar and phosphate to become AMP (adenosine monophosphate), ADP (adenosine diphosphate) or ATP (adenosine triphosphate). It is less certain whether other important life compounds were found in the primordial soup. For example, cytidine (a pyrimidine used in DNA and RNA) is produced only rarely in ‘soup’ experiments . Ribose (part of the external skeleton of DNA and RNA) can be synthesized easily from 5 molecules of formaldehyde, but it is questionable whether that would happen under prebiotic conditions, and whether the ribose would be stable . As we’ll see later, the exact contents of the early soup is not very critical for the origins of life, so we won’t worry too much right now about what it contained. How Thick was the Soup? There is no direct evidence that helps us determine the density of the primordial soup, but we can take some known figures and make some fairly wild guesses about it. The Earth today contains about 1.2 x 1021 kilograms of surface water. It seems reasonable to guess that there was approximately the same amount of water 4 billion years ago . The Earth today also contains about 1.8 x 1015 kilograms of biomass . It also seems a good guess that a similar mass of organic matter was present prebiotically (though in the form of simpler molecules, rather than living organisms). Dissolve those organic materials in the oceans, and the result is about 1 milligram of organic material per kilogram of water. That is a very dilute solution (about .000001 molar, or the equivalent of dissolving one lima bean in a bathtub full of water). In the past few billion years, vast quantities of carbon and hydrocarbons have been deposited underground in the form of coal, petroleum and kerogen. Our current atmosphere also contains a high percentage of diatomic nitrogen, some of it created by bacterial action on ammonia, nitrates and other organic materials. If that ‘lost’ carbon and nitrogen were in organic form in the early Earth, it’s possible that the amount of organic molecules in the primordial soup would have been significantly larger. That means the soup may have been thicker by a couple of orders of magnitude. However, as we’ll see later, the exact concentration of the early soup is not very critical for the origins of life, so we won’t speculate any further about it. Hydrocarbons Was there oil in the soup? Maybe. There are three possible sources for hydrocarbons on the early Earth— UV-initiated synthesis from atmospheric methane and ethane , input from incoming comets and carbonaceous meteorites , and welling up of hydrocarbons created deep under the surface . It is possible that the early earth contained many hydrocarbons at the time of its birth, similar to present-day conditions on Titan and some other bodies in the Solar System. Depending on conditions, oil in water will form a surface film, droplets suspended in the water itself, or a coating on solid surfaces. Those oil/water interfaces tend to gather non-polar organic compounds, including some of the amino acids. Under the right conditions, phospholipids and other amphiphilic hydrocarbons can even form natural membranes . Hydrocarbons may have played an important role in the development of life, and we’ll mention them in a few places, later on. However they were not critical to biogenesis, and it is possible that they were not formed in quantity until the earliest life forms started to create them enzymatically. Soup Chemistry There are three basic chemical properties of the ‘primordial soup’ that would have made biogenesis much more difficult. First of all, the soup was probably very dilute. Because it was so thin, there would have been strong thermodynamic forces in favor of any hydrolyzing reactions (adding water). That means, for example, that any long-chain proteins that might happen to form would quickly break down into individual amino acids. Likewise, RNA or DNA would not form chains, and probably would break down into individual components (nucleic acids, ribose and phosphate ions). In fact ribose is not particularly stable in a dilute solution, so it probably didn’t exist in quantity. Secondly, the primordial soup was very random-- it probably contained a huge range of different organic compounds, just whatever happened to come in on comets or synthesize naturally. There were probably hundreds of different amino acids present, not just the 20 found in modern life. Likewise, at least some of the nucleic acids and other organic compounds used in modern life were probably present, but many small variations and unrelated compounds would also have been in the soup with them. That means there would have been many ‘poisons’ in the soup— similar compounds that would interfere with the formation of early biochemistry. For example, even if by some rare circumstances the soup managed to randomly assemble a few complete DNA nucleotides (nucleic acid, deoxy-ribose and phosphate) and then link them into chains, it could easily have run into a ‘terminator’ nucleotide that included a dideoxy-ribose component. After that, the chain would be unable to link to another nucleotide, and chain formation would end. Modern life is based on precise, enzyme-controlled synthesis of a narrow range of compounds, but the soup would have been more random, and far less friendly for life processes. Finally, the soup was racemic. That means that any compounds which can exist in mirror image forms (called enantiomers) would have contained equal quantities of both forms (usually expressed as levo-rotary and dextro-rotary, depending on the way they rotate a beam of polarized light). That is very difference from current life, which is built almost exclusively from levo-rotary amino acids, and dextro-rotary sugars. Even if the soup contained the correct chemicals, half of them would have been completely wrong for any emerging life forms. Chaos and Catastrophe One other detail that affects theories of biogenesis is the frequent number of large impacts that occurred on the earth, particularly during the Late Heavy Bombardment era about 3.85 billion years ago. The massive collisions during that period may have caused enough damage to obliterate any life near the surface. There is some evidence in early rock deposits that life had already started to develop before that era. That may indicate that life began in a more protected environment that was less affected by large collisions (for example, underground or deep in the ocean). It also may indicate that life arose quickly, during a gap between major impacts, and then spread rapidly to a large number of locations and environments. In that case, some organisms could have survived each impact event and then repopulated the world during the aftermath. Life Probabilities Even the simplest of modern life is based on hundreds or thousands of proteins, along with long chains of DNA that carry the genetic code to create them. The chances of an organism like that arising from the soup’s raw materials in one step are infinitesimally small— about akin to the chances of an automobile arising spontaneously from a random pile of scrap metal. In fact, even something much simpler-- say, a single DNA gene capable of creating a gene-duplicating protein-- could never have happened in one step from pure random combinations of molecules in the soup. Any movement towards life in the prebiotic days must have been based on very simple chemistry. It would have taken many baby steps to move from organic chemistry to biochemistry, and every step would have required a reasonable chance of happening unassisted, even in a chaotic world of many random chemicals. Fortunately for us, there were some physical and chemical processes in the early Earth that would have helped with the first steps— specifically the dehydration reactions which would have taken place in a few places, and the capacity of fairly simple molecules to act as catalysts, enabling reactions that can produce more complex compounds. We will take a closer look at those factors in the next two chapters. Previous Home Next