One of the things I wanted in my fictional universe was a great plurality of more or less habitable worlds. Well beyond anything expected even by Stephen H. Dole’s Habitable Planets for Man. I also wanted a universe that made some sense within our current understanding of science. I was willing to stretch some things somewhat widely to allow for FTL and the like, but I wanted at least the mundane scale of the environment to follow known physical laws fairly closely. I won’t call this hard science fiction by any stretch, but I have my own limits to the mushy softness I’ll allow.
In order to account for the enormously common occurrence of life-bearing planets in my universe, I’ve assumed that the hypothesis of Panspermia, suggested by Fred Hoyle and others, is correct and I also posit that the history of the universe is littered with intelligent species who took it upon themselves to terraform many worlds.
Although the initial source of life in the universe may have been the same ever-present background of microscopic organisms present throughout space, this does not mean that all planetary lifeforms would be biochemically compatible(in the sense of mutual edibility). On the one hand, there may, and given the vastness of space, probably would be, many different, distinct strands of life. These would be independently evolved and would have spread into interpenetrating domains of space. These can have very different, often mutually toxic, chemical bases, not all even protein-based, much less based on compatible DNA coding. The other hand of diversity is the differences that arrive in the space-borne organisms adapting to planetary life. Even if the initially-seeded organisms on two planets are identical in nature, the process of evolution may result in very different adaptive mechanisms on a chemical level, making the descendants of these identical ancestor organisms quite toxic to each other. For example, it seems quite possible to me that two otherwise similar lifeforms may evolve very different chemical mechanisms for photosynthesis, later protection from oxygen-poisoning and still later the use of oxygen. At each of these stages, the different evolved mechanisms may produce toxic effects on consumers not evolved to handle them.
Panspermia will assure that virtually all planets in the habitable zones of the various stars will bear some form of life. The vagaries of chance and evolution means that not all of those world will evolve photosynthesis or aerobic respiration and many a world that would otherwise be considered suitable for human habitation might remain hostile with a reducing atmosphere and only a scum of monocellular organisms floating on the surface of its seas.
Later on came a number of waves of intelligent life. Many never left there homeworlds and passed on never leaving a significant trace of their existence. Others, depending on their ethical nature or carelessness, may have wiped out native life on many worlds in favor of “terraforming” those worlds to make them more suitable for their own use, or accidentally introducing invasive species which proceeded to alter the environment of those worlds, producing new mixed ecosystems, replacing the existing life, or killing off the existing life and following it to oblivion, leaving dead worlds. Some of those dead worlds might later be terraformed by later generations of sophonts or even recolonized by further waves of panspermia.
Some of these Precursor races may have been so advanced to be able to move planets. With that power, they could move bodies of appropriate size into the habitable zones of stars otherwise lacking habitable worlds.
It may be that many of the big short-lived stars in the universe may surprisingly possess clusters of garden worlds placed by Precursors eager to take advantage of the quick burst of energy those stars provide. This would serve to explain Star Trek’s perplexing worlds orbiting unlikely places like Rigel, Deneb and Antares. If I were a Precursor, I’d place big industrial Dyson Spheres around those kind of stars and keep my living space around nice sedate long-lived little stars, but if the Precursors had the kind of long-term foresight to consider things millions of years into the future they’d still likely be inconveniently present and who knows if they’d let us play with our little spaceships. Of course maybe they are still present, and our spaceships are beneath their concern as they twist the universe to their arcane ends from their thrones conveniently located just beyond the event horizons of the galactic central black hole. Heh.
A few of these races have left behind fabulous if often incomprehensible technological artifacts, while the existence of others can only be inferred from the nature of the ecosystems they changed.
At least one of those races left behind what are called seed ships, vast biological spacecraft that travel slowly between the stars and attempt to implant their ecosystems on planets they encounter. A properly-functioning seed ship, based on the Terraformer from GURPS Space Bestiary, would be about the size of a small or moderate-sized asteroid, and would have an effect similar to those seen in David Gerrold’s Chtorr books on an existing ecosystem, though less aggressive. Some of them seem to have either mutated or been altered, possibly as weapons, to be much more like the Chtorr organisms. In any case, no inhabited world would be happy to see a seed ship approaching, and would probably do anything in their power to destroy it or divert its trajectory.
The effects of panspermia and the Precursor races have left a major mark on the universe of the Solar Union and its successors.