The Great Carbon Filter

You may be familiar with the term "great filter". The universe is billions of years old, and it took hundreds of millions of years for humans to turn up after the first vertebrates. If interstellar spaceflight is at all possible, a few million years ought to suffice to spread through a galaxy. There are also a lot of stars in the galaxy, and as we are increasingly able to tell, a lot of planets. It seems implausible and arrogant to assume that we are the first and only lifeform capable of complex thought and tool use. So where are the aliens? There must be clever tool-using lifeforms elsewhere in the galaxy, and some of them must have arrived in this state hundreds of millions if not billions of years earlier.

The "great filter" is commonly invoked as an explanation for why these assumed sentients don't make the move from tool use to spaceflight. Nuclear war used to be a common identity for the filter, and nowadays, our preoccupations have shifted to environmental issues, so pollution and resource exhaustion are often invoked. The subtext, of course, is that humanity would have a bright future among the stars if only we stopped doing the self-destructive thing du jour.

But wait a moment. Resource exhaustion? Arguably the most important resource we've built our civilization on is fossil fuels: coal and then petroleum. But the occurrence of these fuels is a happy accident! Most fossil fuel reserves were laid down during the Carboniferous period. The likely cause: the evolution of lignin, a key component of wood. Dead plant matter rots: it is consumed by fungi and bacteria, which combine its carbon with atmospheric oxygen, releasing CO2 back into the atmosphere. But lignin was indigestible by the fungi of the time, and it would take many millions of years for them to develop a way to break it down. In the meantime, atmospheric oxygen levels increased, and the semi-rotted wood piled up. It was this plant matter more than anything that eventually turned into fossil fuels.

If you think about it, fossil fuels are weird: vast stores of chemical energy that just sort of got ignored by the life of the day, left to sink and remain for millions of years until discovered by us apes. And without fossil fuels, there would have been no industrial revolution, no massive surplus of energy that freed people to do more than scrabble over food, no reason, no fuel to develop clever machines that could go fast, and fly, and eventually reach orbit.

I've remarked before that a lot of speculative fiction is really a fantasy of energy density. Jetpacks, flying cars, spaceships - all so much easier if there was a more space-efficient way to store energy. Steampunk: a fantasy about coal having enough energy density to power giant robots. But for another sentient species out there, our Earth reality would be such a fantasy: abundant chemical energy, compact enough to power cars, planes, rockets, robots. This may be what makes humanity unique: an accident of evolution gifted us with vast stores of chemical energy. The great filter is not the presence of some calamity, it's the absence of the gift of fossil fuels.