Puddle Evolution |
We've talked so far about a puddle where Fred, Roscoe and company started to evolve. That is very easy for us humans to visualize, since it's on a scale that we can stick our toe into, so to speak . However, on a molecular scale, even the smallest of puddles is enormous. For example, a tiny puddle one cubic centimeter in volume (about the size of a kidney bean) will contain 1 gram of water, which might include a few micrograms of organic material, if it were filled with a dilute primordial soup. A microgram is a teensy dollop of material that is nearly invisible, even if assembled into a single blob. But a microgram still contains about 50 trillion small organic molecules, and that is way more than we need for Fred and Roscoe's simple chemistry. Although it's comforting to imagine life beginning in puddles that we can actually see, the fact is that much of the early molecular evolution probably happened in smaller nooks and crannies-- like the spaces between sand grains, or in the 'micropuddles' that form in the cracks at the bottom of a puddle, when it evaporates almost completely dry. Even the microscopic spaces between silt and clay particles would be large enough for sufficiently huge numbers of molecular reactions. By providing many small, isolated pockets, micropuddles would have assisted evolutionary change, even in simple systems like the Fred/Roscoe/Sofia/Sorrel mixture. This 'chemical evolution' would not have been as quick or as efficient as later, 'real' evolution, but it still could have gradually helped to produce better versions of Fred, Sofia and friends. To understand how this 'puddle evolution' would work, let's start with the usual Fred neighborhood that is saturated with two amino acids and two chain molecules. We'll also throw in a few of the first Freds, Roscoes, Sofias and Sorrels, along with various bad copies. In a large puddle they would all mix together in a molecular mish-mash where there would be absolutely no potential for natural selection. So let's look instead at the micropuddles. There might be thousands or millions of those, within a meter or two of the main puddle where Fred and Roscoe first appeared. At each high tide there would be some wave splash, and various combinations of Freds, Roscoes, Sofias and Sorrels would move into the micropuddles, along with a dollop of raw materials. During low tides, every micro-puddle would be isolated from its neighbors, and there would be a few hours when they would act as miniature chemical laboratories. That's when Fred and Roscoe could do interesting things. Micropuddle Permutations Let's consider all the possible permutations of ingredients that might wash into a single micropuddle. Roscoe Puddles First of all, let's look at what happens in micropuddles that start out with a Roscoe and a genetic chain:
The net result is an increase in the quantity of all chains in the neighborhood, and no selection at all. The bad chains increase just as quickly as the good ones. Fred and Sofia Puddles Next, let's look at micropuddles that start out with each of the possible permutations of Fred and Sofia chains, and see what happens there:
Puddle types 5 to 8 won't affect the number of Sofias or Sorrels, but at least they keep the neighborhood stocked with top quality Freds (which are created at a faster rate than the non-functional polypeptides). Note that puddle 6 increases the percentage of better Freds-- but it has no overall selective effect, since it doesn't change the Sofia population. Natural selection only works if you select for better genes! Fred and Sorrel Puddles Now let's look at the puddle permutations with a Fred and a Sorrel chain:
The net result of these puddles is production of a moderate number of Sorrels and Roscoes (plus a smaller number of non-functional chains and non-functional proteins). Because of puddle 11, there will be an increase in the number of Sorrels that code for improved Roscoes. Fred, Sofia and Sorrel Puddles Finally, we'll look at permutations with a Fred, a Sofia and a Sorrel.
These puddles would be the dominant producers in the local puddle complex (how important they are, relative to the other puddles, depends on the quantity of raw materials present). Overall, they provide some selective pressure that would gradually increase the number of Sofias that create better Freds, and the number of Sorrels that create better Roscoes. Net Puddle Results What is the net result of all these permutations? Since each tidal cycle would rearrange the contents of each mixing pool in the Fred and Sofia neighborhood, three times a day there would be a new assortment of puddles containing all the previously listed permutations of ingredients. Thanks to the output of puddle permutations 6, 7, 10, 11 and 12-16, the neighborhood would maintain relatively high levels of Fred, Roscoe, Sofia and Sorrel. Those molecules would dominate the local puddles, and constantly wash out into neighboring pools and puddles, creating a concentration gradient that might extend for many meters. Any improved Sofia that created a more effective Fred would get a population boost from micropuddles with permutations 7, 13 and 15. It's a weak selective advantage, but one that would be gradually be effective over a large number of tidal cycles. An improved Sorrel that created a more effective Roscoe would get a similar population boost from micropuddles with permutations 11, 14 and 15. This chemical puddle-driven evolution is much slower and less effective than 'real' selective evolution, but over many tidal cycles and with many small isolated populations, it would have produced real evolutionary changes, even at this early stage of biogenesis. More Puddle Evolution In a sense, the Fred/Roscoe/Sofia/Sorrel system has a tiny bit of actual genetic transfer going on. It's not something that could ever work in the open ocean, or even in a single small puddle on the shoreline. But in a world of many small micro-puddles, each with a small dollop of chemicals that spill in for a while, a certain amount of real natural selection can occur. And that selection is just barely enough to allow the first small steps of real evolution, even in this very simple four-chemical system. Improving Roscoe Let's consider another example of how 'puddle evolution' would work. In this case, imagine the appearance of a new version of Sorrel that offers an improvement to the proteins that Fred creates from it. Thus über-Sorrel might produce a Roscoe with a few extra amino acids that just happen to hook Roscoe to the beginning of a chain more reliably. In a single pool, this change won't have any particular 'survival value' for either the improved Sorrel, or the improved Roscoe. Sure, it will create some better Roscoes, but the Roscoes old or new are just as likely to hook up with the original Sorrel as the new and improved Sorrel. Consequently there won't be any chemical selection going on. Add some barriers and some isolated populations that occasionally interact, and the situation is very different. For example, let's track some micropuddles filled with old and new Sorrels, and see what happens. Each time a micropuddle filled with a Fred and an old Sorrel, the Fred would hook up with the Sorrel and produce 5 old Roscoes. The Roscoes would also hook up with Sorrel and replicate 5 old Sorrels, plus 20 degenerate fragments because they started at the wrong place. Each time a micropuddle filled with a Fred and a new Sorrel, the Fred would produce 5 new Roscoes. Those more efficient Roscoes would produce 25 new Sorrels from the same raw materials, since they were better at starting in the right place. With many isolated pockets filling and draining and splashing during each tidal cycle, there would be a gradual net increase in new Sorrels in the neighborhood, and a gradual decrease in the older, less effective models. This is a very slow and inefficient method of genetic selection and evolution, but it has some adaptive force that would help create improvements in our self-replicating molecules, even at this early stage of chemical evolution.
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