Interstellar Hybrids

Suppose it just happened that there were two solar systems, within a hundred or a few hundred light years of one another, and on each of these there was the origination of life, and on each of them life evolved to photon-users, then to land creatures, then to intelligent tool-using creatures, and finally to the founders of an alien civilization. Suppose even further, now a quite unlikely coincidence, that these civilizations existed at the same time. And now, suppose even further that one of these civilizations decided to do some space travel and left their planet and headed for the other one.

This is also unlikely, as the principal reason that an advanced alien civilization would do space exploring would be to find a new home for their civilization, and if there was already an alien civilization on a nice planet nearby, they would not go there. They would not be interested in sharing what was left of the resources of that solar system, nor in going to long-distance war to evict the natives from it. But in the interests of covering all possibilities, they go. Perhaps they have such a large quantity of resources in their solar system that they don’t foresee scarcity, and they want to go meet the other species.

They recognize this is a thousand-year voyage, and so they expend the time needed to expand on their interplanetary mining ships to make a starship, one capable of making it to the other star system with all its parts functioning. They also figure out something we do not know, besides how to build starships, how to make sure there are living aliens on the ship when it arrives. They do hibernation of a select crew, or they make an AI capable of growing aliens from preserved DNA code, or, heavens forbid, an intergenerational ship. The details don’t matter – they just get there and the receiving civilization doesn’t blast them out of orbit into small bits of space debris, but they welcome them.

The distant aliens come down to the planet and meet the natives, and what do they find? They look alike. This is interstellar convergence at its finest. Interstellar convergence means that if evolution has enough time and area to work over, it will produce the optimal choices for all evolutionary steps, and it is no small leap to figure out that for intelligent, tool-using creatures, there is a kind of optimum. If the target planet were different from the departure planet, say, one was all water and the other all desert, these considerations are absurd and some creatures suited to the environment would have evolved there. But we’re assuming the two planets are pretty much alike. Same mass, same star type, same age, same distance from the star, same orbital parameters, same metallicity, same condensation of stabilizing outer gas giants, same moon and same history of impact, and so on and so forth. It should be obvious now how unlikely this would all be, but it is a thought experiment. You are allowed to do this in a thought experiment.

Now let’s go all Hollywood and assume that one of the arriving aliens, named H, takes a liking to one of the native aliens, named S. And H and S finally, after all kinds of social obstacles and whatnot, pair up and produce a hybrid. NOT.

Let’s get down to the basics. We know lions and tigers can produce hybrids. They are impressive huge creatures, but sterile. Lions and tigers both are members of the cat family, and haven’t had enough time for their genetic code to diverge much. So they can do this. Instead of having two animals that are very related in ancestry, the two aliens are completely different somethings. It isn’t species, it isn’t genus, it isn’t order, it isn’t phylum, it isn’t kingdom, it’s something bigger taxonomists haven’t thought about naming yet. A completely different world of life. Let’s call it bios, for the greek word for life. The two planets have two different bioses.

Let’s push interstellar convergence even farther, and look at the details. Do the two bioses each use DNA? OK, let’s assume that DNA has some combination of fortunate ability to attach to phosphate and carbon bond combinations that don’t take too much torsion to rotate for folding and unfolding, and so on, and nothing else beats it or even comes very close. The two DNA’s are true to the name and use desoxyribose sugar on the power end of their nucleotides Let’s also assume the same mechanism for reproduction of DNA, using mRNA and tRNA, works in both places, because it’s the most efficient method of gathering together amino acids and cobbling them together into a DNA strand or into a protein. Let’s assume all the internal organs are the same. Wow.

Now comes two hurdles. Mutations are random, and are selected by fitness competitions. What about chromosomes? They form randomly. Is there any reason to think that the particular combination of genes on the chromosomes one alien has wound up on the same chromosomes that the other one has? There doesn’t seem to be any fitness for most of these accidental cuttings and recombinings. They are simply genetic errors that slipped through the proof-reading process. So the genes on the incoming aliens are very likely to have the same genes in different places. This means no hybrids. Is there conceivably any way that random genetic errors could wind up identical on two planets? Not likely.

That was the easy one. Look at the coding of DNA. Triplets of selections from four bases form each codon and these are related to one of the structural amino acids used in the bios. Let’s assume that interstellar convergence picks out the same twenty or so structural amino acids in exactly the same way for both bioses and the ones used for the bases are identical also. Maybe there are slight differences in the utility of amino acids that allow the entire set of them to be identical, and interstellar convergence actually scored big again here. Hard to believe, but wait.

There is a map of these triplets, of which there are sixty four, into twenty-odd amino acids. Again, this is randomness. There is nothing in the triplet that relates to a particular amino acid. When evolution was picking the mapping out, whatever started first, stuck. How likely is it that the codings would be identical?

The coding here on Earth is somewhat blotchy. More than one of the triplets can code for a particular amino acid, and there are stop and start triplets, too. Some triplets code for nothing at all. Let’s be conservative and just figure the number of combinations of triplets that would match on a one-for-one basis with twenty amino acids and two auxiliary codes, twenty-two in all. It is obviously 64!/(64-22)!, or about 10 to the thirty eight.

So, there is a one in ten to the thirty-eight chance of this happening, and if it is necessary to match up some of the otherwise unused combinations, less. There would simply be no hybrids. The mismatch of coding would lead to failure of any hybrid cell. Even an artificially fertilized cell would die quickly. This means that any science fiction needs to not hypothesize hybrids, no matter how interesting it would make the plot, if it is to be at all realistic. Most science fiction does not have any hope or interest in being realistic, of course, but perhaps as science eduction pervades humanity, this will change.