WARNING: IF YOU THOUGHT THE LAST POST WAS TOO PHILOSPOHICAL, TURN BACK NOW.
The Great Filter provides a potentially useful analysis of where the dangers might lie for the advancement of life. The flaw in the analysis is that it presumes a linear history which makes our current situation a singular data point. If one instead takes a recursive view of life's history, then our current situation can be represented by a class of similar situations that have occured throughout the evolution of life on our planet.
If that last paragraph sounded like a whole lot of gibberish, please bear with me whilst we unpack it: Recursion is the mathelogical term denoting that a set contains itself, and that set contains itself, and so on, ad infinitum. There, I'm sure that cleared it up for you.
Now it's time for a little story about some critters called the UCNs.
Over time, the ecosystem itself changed, as the arms race raged, and as strategies, species, and resource accumulations rose and fell, rose again and fell again, over and over. Occasionally, a cataclysmic energy perturbation from outside N-world would upset the fragile ecosystem within the N-world, and reset the whole chain almost back to the very beginning. Eventually, a promethean UCN looked beyond the boundaries of N-world and wondered what it would take to go beyond the edge of N-world. Many had tried and died, because they were not evolved for that hostile foreign environment. But what if they could take all the necessary dependent elements of their ecosystem and encapsulate them, then take this tiny bubble of N-world out into the great beyond to find other worlds there? So after building such a UCN+1 bubble, they did. These bubble communities were deployed. They indeed discovered many worlds similar to the N-world, as well as vast resources between worlds. This greater aggregate world, the N+1-world, became a new virgin frontier and the UCN+1 bubbles replicated exponentially as they found and converted the bountiful resources there.
And N was incremented.
The UCN had arrived in the N-world virgin frontier in search of the bountiful resources there...
To those that have been following the idea of CELSS, the theme here is probably pretty transparent: the UCN could be RNA, prokaryrotes, eukaryotes, or amniotes (which would be us). The standard ecological growth curve looks something like the beginning curve in the lower right graph:
This is normally shown for a given species. In this case it represents the base autotroph (UC) of the N ecosystem (which is the encapsulated N-1 ecosystem). Typically, the exponential growth curve for the population of a species is followed by a precipitous population collapse, naturally occuring when resources are overexploited, or the population becomes diseased (i.e., it becomes overcome by inadequate regulation of the N-1 lower organisms). These two mechanisms are the most common cause of filtering (i.e. population bottlenecks), but there are of course external factors as well: maybe the sea vent or tidal pool dries up, or an asteroid hits. These external filtering events can happen anytime, regardless of habitat limits or population size.
Certainly, some habitats are more stable than others and some populations less fragile, but ultimately these filters are all statistical. It's less clear that making the encapsulation jump to exploit the N+1 next higher level world is so statistical. Perhaps it is just not as well understood by us, because it occurs on such a short geological timescale, or because it just so happens to be the next step for Earth life. The beautiful thing about cycles is that they can be replotted from cartesian time to a polar period chart. Doing that gives us something like this:
This allows us to align the common stages in Life's progression. While it's still just a handful of datapoints, it's better than just one. It allows us to compare the differences between what we know of the different N levels, and systematically assess the odds of each filter (where the spiral crosses either a radial line or an N ring). Before moving on, it's worth noting that this formulation is a work in progress, and the left side of the chart could be done in a different order (e.g., encapsulation could occur before regulation).
What almost immediately stands out is that some steps are not quite like the others. The story of going multicellular is not quite the same as the standard UC story version above. And going from the ocean to land doesn't seem like quite a full step either. That's ok. It might be tempting to think that if these half steps were easier barriers to overcome, it makes them easy to rule out as Great Filters. But what if it's the opposite, those easy steps enabled us to surmount the odds to get to where we are (faster than other life on other planets)? After all, worlds of partial ocean (like Earth) are probably more rare than either dry worlds (like Mars) or completely wet worlds (like Europa). The barrier to go from water to air certainly seems like it would be more difficult to overcome than having an intermediate step of land.
The other thing made apparent by this analysis is that there are a lot of filters involved in evolution at every level. We like to think of evolution as leading directly and linearly toward ourselves. It didn't happen that way. There were millions of dead ends enroute, and there's no guarantee that we ourselves aren't a dead end. Yes, there was a lot of luck in getting to where we are: an ecosystem in spitting distance of having the capability of encapsulating replicas of itself and sending them into the solar system. But given a long time, statistically maybe it's not so much luck, as inevitability. After all, it did take 4 billion years for us. We should expect similar time periods for the aliens. The universe is only 13 billion years old. What if the conditions for life just weren't ripe until 5 billion years ago or so?
My guess is the aliens are out there; they just aren't much farther along than we are. Best not to drag our feet. Ultimately, we'll probably need each other, if we want to get to the N+2 ring.
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