Interstellar Convergence

In short, interstellar convergence means that aliens would look like us, and have the same chemistry as well. It might happen, and it might not.

Let’s just talk about the chemistry. There are some basic chemical choices that early cells made on Earth, and these choices have been preserved in all the species that have flowed from these cells. Take glucose for an example. It has multiple uses, including serving as the main staple of the diet of cells of any creature on Earth. Why is glucose there and not something else? Was it just random chance that a single cell specialized in using it, and then everything built up on top of that choice? Or are there intrinsic reasons why glucose is superior to everything else that could have been used in the cellular machinery? One reason might be its physical flexibility and foldability, as it can take many shapes. Another reason may be that it causes less damage to the organic chemicals within a cell. Another could be that energy transfer in the ATP-ADP cycle is more efficient than with other choices. And there may be another dozen possibilities.

In the fitness competition, in the days of primitive cells, where one cell happens to mutate to using glucose and other cells mutate to using other intermediate chemical energy molecules, does the glucose using cell always win, or statistically win often enough to drive out the competition? If this competition does indeed pick the best choice, and the selection can take place in a short enough time, then on all planets where there is some option for this competition, glucose would win and become the only energy molecule used in all organism there. This is the essence of interstellar convergence. It means that there is a competition between options, and the superior one is superior over a wide enough range of conditions that it proves itself superior on multiple planets. It means that random mutation cannot lead to a win in the competition unless the mutation actually produces the best product. Interstellar convergence means that all planets have a severe enough competition on all these molecular choices that the single optimal one wins everywhere and all the aliens have the same chemistry.

If this fitness competition depended strongly on some parameters which differed between planets, then interstellar convergence would not happen. Following our example, if glucose is a champion in oceans between 10 degrees C and 20, but something else is between 20 and 30 and yet another between 30 and 40, we would expect to see this spectrum reflected in the chemistry of alien visitors. Some would have the 10 to 20 degree selection, others the 20 to 30, and so on.

There are some reasons to suspect that basic chemistry choices like this do experience interstellar convergence. Evolution is local, meaning that the physical space that a variety of organisms compete within does not have to be the whole planet, but instead just a specialized part of it. If this energy molecule choice was strongly dependent on temperature, then we might expect to see on Earth some glucose cells in the temperate zones, and something different for organisms which live in the tropics. We do not see this. Instead we see universality of these basic molecular choices down on the cellular level.

It might be true instead that something else, like salinity for example is a sensitive parameter, and planets with much higher salinity would experience a selection of a different energy molecule. Earth doesn’t have a wide variation in salinity in its oceans, as the waters all mix. Another planet might have a different situation, and thus a different environment for its molecular selections. There does not seem to be any indication from the understanding of biochemistry that we have, but some other parameter may do the trick.

Once a very basic choice like an intermediary energy molecule is made, many other choices are affected as everything in a cell works together, so that if some planet has a different choice, they would likely have large differences in the rest of their cellular chemistry. The way to resolve the question of interstellar convergence is to create some primitive cells with different choices, and subject them to some environmental stresses, while monitoring their success is surviving and reproducing. Until that happens, we will not know the extent of interstellar convergence. Here on Earth we should be able to do these experiments within a century more of biochemical research or so.

There is some further discussion possible at this stage, related to the existence of enantiomers. There are a huge number of molecules which have left and right forms, much like the left and right hands. No amount of rotation can make a right hand look like a left hand, and the same goes for molecules. But molecules with these two forms would have the same capability and properties, and should compete equally in any fitness competition.

Molecules in the cell fit together to conduct their chemical transformations. That means, if molecule L-A, the left version of the molecule, works well with molecule L-B, then most likely molecule D-A, the right version of A would work exactly as well with molecule D-B. What this means is that whole families of interacting biochemicals would have choices of enantiomers. The labeling is a bit arbitrary, so that they would not all be what we on Earth label the L form, but there would not be mixing between the families.

This means that if interstellar convergence happens, and the aliens all look like us down to the chemistry, there could still be two separate sets of them, one set of aliens whose world started out with the L-type of some basic chemical and it now uses the enantiomers that work with it, and the other set being aliens who use the opposite enantiomers. They could look alike, but they could not breed together, or possibly even eat the same food.

This has some relevance for seeding planets. If some world, Planet X, has aliens with a stellar travel meme that has them going to other planets and replacing the biome there with things more compatible with them, with an eye toward them settling there, there could be a great mismatch if the seed target has life using the opposite enantiomer, assuming interstellar convergence is universal. It is hard to imagine what might happen, if the new predators could not consume the original creatures of the target planet without dying. How to get rid of everything and start all over with setting back the seeding process for millennia or even more?