Diversity rules!

A while back I talked about the possibilities of alien life, and our state of scientific knowledge about the emergence of life. The fact that we have more questions than answers leaves a lot of room for sci-fi writers to play with in convincing worldbuilding.

All the same, rather than dish up a random biological smorgasbord it pays to give some thought to the universe you're building, and how it came to look the way it does. This is like developing backstory, but instead of working out the past of individuals, or of societies, you're sketching out the backstory of life itself.

Today, I'm poking a little bit at just one branch of the tree of possibilities to see where it might lead. Other branches to follow in later posts.

For today, I ask what if we assume that life has emerged independently many times, and that it is common throughout the universe? What are the possible implications for sci-fi worldbuilding?

This is a common scenario in many sci-fi worlds, and yet I feel is the easiest to deal with in an unconvincing way.

Why do I feel this way? My line of thinking can be summed up as follows:

From what we can see of the universe, it appears that extreme diversity rules. This means that, unless there are some deep organizing principles funneling life down a few narrow paths, we should expect life to be extremely diverse too.

Let's unpack the separate elements of that paragraph...

Diversity rules

All around us in nature, at all levels, we see bewildering variety stemming from simple foundations.

Life, for example, comes in all shapes and sizes even though much of it has a lot in common in terms of chemistry and cellular structure. But that is just what we can see on Earth, and life itself seems to be a pretty special case so is probably not a good example to extrapolate. The trouble is, we don't (yet) have the technology to see what's going on elsewhere in any great detail.

What we can observe of other worlds, other stars, and other planetary systems suggests that diversity is the norm everywhere we look.

Within our own solar system, no two planets or moons are remotely alike. Just looking at Jupiter's four largest moons, for example, we have: highly volcanic Io, covered in yellow sulphuric lava flows; icy smooth Europa with its crazed and cracked white surface; rocky Ganymede with a thin oxygen atmosphere and a magnetic field (the only moon to have one); and dark and cratered Callisto, speckled with frost over its highest surface features.

A similar story holds true across all the major bodies we've examined.

Given that these are all (apparently) lifeless balls of rock and gas forged from the same mix of basic ingredients, this variety is both unexpected and quite breathtaking.

So, what might other life look like?

Here we can only speculate, but here are some thoughts...

Life on Earth is based on proteins, lipid membranes, DNA, many common respiratory pathways, and all formed on backbones of carbon. Once the first precursors of life took form and started spreading, they took over the planet and gave no chance for alternatives to arise.

But what's to say that, given a fresh start, different mechanisms might not arise to do the same jobs? I bet there are other molecules that could catalyze and regulate reactions that look nothing like proteins as we know them. And why should DNA be so special? Or even if alien life evolved DNA its genetic code, and what it codes for, could be wildly different from our own.

The same thinking extends all the way up through the hierarchies that make up complex organisms. Does complex life have to be cellular, or could some more amorphous arrangement of self-replicating chemistry scale up in a workable way? What if the entire ocean of an alien planet formed a super-organism from its soup of reactions? Could deposits of silicon form complex enough interactions powered directly by photo-voltaic reactions?

And what about Douglas Adams' hyper-intelligent shade of the color blue?

Organizing principles

In the absence of any other information, my default stance would be to expect vast diversity. That means, if you paint a universe where life arose independently on many worlds and it all turns out to be based on carbon chemistry with DNA-based inheritance then you'd better give me a darned good reason!

A useful fall-back mechanism is to invoke some form of underlying principles that lead to common solutions.

Back to the example of the highly varied planets and moons. Despite the variety, they all share one obvious feature: roundness. Gravity tends to pull sufficiently large masses into a low-energy state - a sphere. In other words, despite the superficial variety of composition and features, there is an overarching principle at work that imposes some constraints. I would not expect to find a naturally-occurring planet shaped like a cube, or a teacup.

In a similar vein, it's reasonable to suppose that only carbon, unique amongst the elements, has the chemical flexibility to support the complexity needed for life. At least in the temperature range we inhabit. You might also posit that self-replication, with just the right balance between durability and instability needed to promote evolution, would always converge on DNA as the solution. But you'd have to work a lot harder to get me to accept that an alien genetic code would be in any way compatible with our own.

Of course, there are many other ways to take these speculations, and this discussion only explores the path stemming from one basic assumption. There are many other paths we could choose, with other implications. More to follow...