Wednesday, September 7, 2016

Clones of Earth

If there was a planet identical to Earth somewhere out in the galaxy, how similar would life on it have to be? This is simply a thought experiment, so we can assume identical is totally identical. The match does not simply cover the conditions on the planet, but the star that it orbits and other planets as well. Let’s throw in identical experience in the galaxy, such as a lack of nearby supernovas, and the same cosmic ray field.

In this example, Earth gets the same impact from a planetesimal that formed at a Lagrangian point in its own orbit, creating the organic oceans that are necessary for the similarly-named theory of origination of life. Its atmosphere is the same after the impact. In short, everything is the same. Now, life originates here the same way as it supposedly did on our Earth, but what happens after that? This is the simplest test for interstellar convergence, which is a hypothesis that says that evolution proceeds to the optimum in most situations, resulting in similar creatures on similar planets.

Similar creatures do not mean similar in every chemical detail, but similar in function and appearance. At the very least, enantiomers could be different on two different clone Earths. It is also not at all clear how enantiomers divide themselves into groups. Clearly two different molecules which interact would have to be of the proper orientation, so a right-hand version of one could fit against a left-hand version of another, for example; enantiomers which don’t interact could be completely random and all mixed between orientations. Furthermore, the coding for different amino acids could be different, and entirely unrelated to one another. These two variations would not affect either functionality or appearance, but would prevent hybridization between the two clone Earths, no matter how similar evolution worked out.

The question to be asked is: how might evolution not result in the optimum choice for each mutation? Two effects which tend to make optimal choices happen are the large number of possible mutators, being a large number of organisms, either simple, single-celled animals or eukaryotes. If there are huge numbers of them, one of them is going to get the mutation which leads to the next optimum condition, and eventually compete its way to being the dominant and then the sole version of the gene in existence, at least in that type of organism. The other effect is that of time. So many generations of any organism exist that sooner or later the mutation leading to the next simple step up, the optimal one, will be started. Again, after enough time for competition to winnow out the winners, the organism with the best choice of that particular gene emerges.

This process does not come to fruition in the situation where there are two or more genetic changes happening nearly simultaneously. If a non-optimal, but better than the original, mutation happens and begins to multiple, displacing the original version, and then another mutation happens which provides much more improvement in overall fitness, the competition for the first mutation is overwhelmed by the competition for the second. If a third one comes by, also offering a larger improvement, it would also delay any results for the first one. When these big changes are over, if they propagate throughout the numbers of that type of organism, the weaker improvement for the first change might resume, unless for some reason the competition is no longer valid. The one or two large changes have rendered the original change moot, and it no longer has any fitness advantage. If this happened on one clone Earth, and did not happen on another clone Earth, the evolutionary sequence would be different. This change might affect some later stages, as a different mutation might be possible for one sequence and not for another. This means the two planets lifeforms, the bios as we have called it, starts to diverge in a small way. Conceivably, this could happen multiple times or the downstream effect of a few such events might have large consequences.

The mechanism by which genetic divergence is clear. One change happens and before it can become optimal, meaning a suboptimal mutation occurs and disperses, other changes happen which prevent any competition on this locked-in one from occurring. After they concluded, the optimal mutation could arise, but it would not succeed in the fitness competition. The later mutation, which would have liked to take advantage of the optimal form of the first mutation, cannot be expected to occur unless the very rare possibility of a double mutation happens. Since mutations are unlikely occurrences, having two coordinated ones is so unlikely as to never happen.

Genetic divergence can also happen in situations where the number of organisms involved is small. This might arise from a locality effect, in which the planet is divided into a huge number of locations, some small and some large, and the organisms that evolve in each are different. In some particularly small locations, a genetic mutation might be favored, and then built upon with more mutations, leading to an organism which can then break out of this small location and propagate widely. If there are only a small number of organism in the location where this is happening, it might happen on one clone Earth and not another. This is a second mechanism by which some genetic differences between the two planets can occur.

A third, related situation, can happen in large localities, but ones in which change is rapidly happening. If the location is modified somehow in a evolutionarily short period, changes might happen there on one clone Earth and not on another. Some particular mutation occurs on one of the planets, and would have eventually occurred on the other, except that the location changed. This particular mutation wins the fitness competition in the short-lasting location on the planet where it happened, but the location disappeared or changed its conditions so that if the same mutation happened on the second clone Earth, the mutation would not have been successful. Something in the environment had changed so that what was optimal for a period was no longer optimal.

There are undoubtedly many ways in which two identical planets can evolve different sets of organisms with different genetic codes. However, what should be the same is the filling of evolutionary niches. Given enough time, some organism will evolve to take advantage of an energy source or a materials source. On two clone Earths, these organisms might differ genetically either in some minor ways or even some major ways, but the functionality would be the same. If there is some location where photosynthesis would work and produce enough energy for an organism to reproduce and multiply, it ill happen eventually. If there is some organism which can be used for food, something will arise to consume it. If there is some creature which might become the prey of another, it will do so. Thus, science fiction writers who imagine worlds which appear like Earth, even ones where humans could survive environmentally, but with strange crops and strange animals, they are well within the bounds of reality. There might not be any clone Earths, or there could be many, but they would not necessarily have recognizable flora and fauna. Since human digestive systems break down organic materials into basics such as sugars and amino acids, humans might even be able to eat some of the vegetation or the animals there, barring toxic components.

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