Many sci-fi milieus are richly populated with weird and wonderful life forms.
Most of them, oddly enough, seem to be roughly humanoid and able to co-exist in a compatible environment, sharing similar gravity, air, and food.
This simplifies the challenges of storytelling, especially in the world of film where human shapes can be more readily acted, but it smacks of laziness. I will be looking at some science topics that crop up in sci-fi, and their implications for convincing worldbuilding.
The first big decision for sci-fi worldbuilding is whether or not we are alone
The origins of life are still largely mysterious, with some experimental hints and lots of fragmented and unsatisfactory hypotheses. There's plenty of guesswork but no real evidence of how easy or difficult it is for life to get started.
Spectra from interstellar gas show that very simple organic molecules are commonplace. Added to this, experiments over the decades have tried to simulate early conditions on Earth and have produced more complex molecules, including many amino acids - the building blocks of proteins. This hints that basic building blocks are in ready supply in the universe.
But that is a far cry from the bewildering chemical symphony that makes up a living cell. It's a bit like expecting a tree to spontaneously arrange itself into a Louis XV sideboard. Molecules like DNA are fragile, and it's hard to imagine how this sophisticated self-replicating machinery could have bootstrapped itself into being. This looks like an impossible challenge, yet recent experiments using more realistic "early Earth" cocktails have yielded large segments of common metabolic pathways, hinting that organic chemistry has a remarkable capacity for self-organization.
Still, just hints at possibilities, not proof. So, life arising independently may be a rare (or even one-off) event, or it could be commonplace. We just don't know.
This is frustrating for scientists, but good for writers because it gives us plenty of room for the imagination. Even so, the forms life might take will be influenced by how we envisage the history of life in our own fictional Universe.
Here is an idea of the range of possibilities.
Emergence of life is a rare (or one-off) event
We might be alone.
Or are we?
Life may have emerged billions of years ago elsewhere and spread to Earth, a concept known as panspermia. Hardy bacteria keep surprising us with their ability to survive in extreme conditions, and it's not too implausible that colonies might have survived impacts throwing them off into space to seed other worlds. The implications of this are that alien life would be expected to share some ancestral chemistry with our own.
Or life may have emerged in one place and evolved to the point where other worlds could be deliberately seeded. Again, implies that alien life will most likely share some common chemistry.
Sci-fi authors have also played with more exotic variations on this theme of life spreading from world to world.
Life has arisen independently many times
This path implies that there are deep self-organizing principles at work in the Universe that make the emergence of life probable, rather than the unlikely event that it seems at first glance. Science doesn't have much guidance to offer here, and we can't call on probability with a statistical sample of one, so we are pretty much free to invent basic principles to suit our worldbuilding needs.
Down this path, it helps to decide where those organizing principles might lead. Do they all lead to carbon-based life? Do those then converge on similar metabolisms because they are the most likely to emerge? Or do you go in the other direction and speculate that any sufficiently complex arrangement might yield self-replicating structures? In other words, might we expect to find life based on all sorts of weird substrates?
In the world of speculative fiction, all these and many possibilities in between are up for grabs. But this is just the foundation. How do you build a credible world from here? More to follow...