Interstellar convergence is a term, perhaps unique to this blog, that says that evolution drives organisms to optimality, and what is optimal on one planet is close to optimal on another similar origin planet. In other words, planets hundreds of light years apart both with the same mass, stellar class, composition and other details, will have organisms that look similar, and which are similar, down to the cellular level. Interstellar convergence has limits we do not know yet. If it turns out that DNA is the optimal coding chemical for genetics, most planets will have it in their organisms’ genomes. If neural nets are indeed the optimal information processing mechanism that can be grown from genes, then all intelligent aliens will have them. If hands are optimal for tool-using, then all intelligent creatures everywhere will have them. There might be some exceptions, but we are far from being able to figure them out.
Intelligent design is used here in its essence: if genetics is understood, creatures can be designed to fill in whatever niche is desired. It means that aliens, not necessarily super-creatures, but just ordinary, hard-working, well-motivated, intelligent aliens, would be able to design a genome that would lead to a viable, living organism, able to reproduce if that was desired, able to think if that was desired, able to do whatever it was that the designers wanted them to be able to do. They could have docile personalities or be hard-as-hell to manage; this is the designer’s choice.
Here’s another word: self-speciation. This means that an intelligent alien species would have the ability to modify its own genome, and create a different species, if they chose to do so. The different species might be better than the original aliens in some aspect. Evolution is expected to do a good job of moving species to the optimal, but it can’t work after a civilization gets started, so evolution would have created a species that was good for surviving and reproducing in a pre-civilization situation. It would be up to the aliens to modify their own genome, or create a brand-new one, to match what would be optimal for living in the world of giant cities and interplanetary travel.
When alien civilizations start sending their citizens into low planetary orbit and then into interplanetary space, and finally to other planets, satellites or asteroids in their own solar system, they will soon realize that the design that evolution created for life on their home planet was far from optimal for life in other, radially different, environments. They may well embark on some self-speciation to design aliens which were at home in space or on low gravity satellites. If they happened to originate in a solar system which had two planets with similar conditions, but only one which originated life, they might migrate to the other one and modify their genome to better match it. If it had 20% higher gravity, for example, they might want to redo their skeleton to cope with that, and their musculature as well, and perhaps their circulatory system. If there was a planet with less oxygen, the lungs might be modified. If the other planet was an origin planet as well, or life had spread there from meteoritic transmission or some other way from their home planet, and had grown up differently, they might need a different digestive system to be able to consume what grew on the second habitable planet. Once the genetic grand transition had been passed, all of these would be possible. If for some reason, there was not much benefit to be gained, as for example, their naturally evolved bodies tolerated space life with few problems, they could keep their own genome; this seems to be an unlikely possibility.
This self-speciation is not something that one out of all alien civilizations might do. They all figure out the same genetics knowledge, as knowledge doesn’t depend on the planet that finds it; scientific knowledge and technology is universal and the same on all planets.
The stage of interplanetary travel and migration would almost assuredly occur before they attempted any interstellar voyages. The technology developed for going to other planets in their own solar system is an excellent beginning for the technology needed for an interstellar cruise. That technology is quite diverse, involving figuring out how to make something last for a thousand years; how to carry enough energy and propellant to get the trip and the arrival accomplished and still have enough left over for whatever they were going to the new solar system for: as a probe, for colonization, or something quite different. So, it seems likely that they would have a species, similar to their own, that is better adapted for space, and which lives on constructed space stations around some of the planets in their solar system. If there were any satellites worth building a colony on, for mining or anything else, there might well be another variant species living there.
As an aside, an alien civilization at this stage of development is likely past the times of troubles that existed on their home planets. Just because they were a different species, they would not decide they wanted to go to war with the home planet. That is a device of science fiction: taking something of present day Earth and changing the situation to a future one where different planets were inhabited. Neurology and sociology would be well-understood sciences and would have eliminated such a possibility.
Now ask yourself what they would do for colonization, should that be part of their legacy goals for their own set of species. If there was an interesting exo-planet somewhere, and they found it was an origin planet, and already had an intelligent species, would they want to go and colonize it and subjugate the original inhabitants? Not likely. They would understand that the other planet would have evolved creatures optimized for that planet, and if they went and used intelligent design for a creature like their own to live there, they would wind up with something very similar to what was already there. So why bother?
Colonization is likely to take place only on worlds which do not have intelligent species already. It would be superfluous to go there and replace something almost identical. Colonization would go to origin planets with primitive life, not solo planets with intelligent life. This is actually a very small restriction, as life exists for billions of years on an origin planet, before it can evolve intelligent creatures. Much of that time would be suitable for colonization, if it were desired. Intelligent life only lasts for a brief interval, and technological life, probably less than a million years. This would eliminate only a small fraction of planets a space-traveling alien civilization would encounter. In essence, we don’t have to worry about being displaced by aliens. There might be other reasons they would interfere with us, but it is impractical for them to bother colonizing an already inhabited planet.
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