Batches of Great Filters

We have a Great Filter concept when one of two things happens. First, we recognize that a certain event or process is statistically very rare, and it would prevent most solar systems from giving rise to space tourists. Second, we don’t understand how something happens, so we think maybe it might be one of the former type. All the Great Filter concepts are in the latter. There are none in the former. They are all due to ignorance. In a sense, the plethora of Great Filter concepts is a measure of how little we know about a lot of science.

To make things simpler, perhaps we should divide Great Filter concepts into batches, depending on when in the life cycle of the potential space tourists they fall. Note that we do not figure that any Great Filters are absolute roadblocks, as we have the presumptuous notion that someday we are going to graduate and become one. That’s why we call them Filters instead of something more definitive, like unsurmountable barriers or brick walls. The name is pretty catchy, too.

Since we have dispensed with the habitable zone exo-planets recently, we are left with the first batch being the generation of self-replicating molecules. All we need is for one self-replicating molecule to be formed and start self-replicating, and the planet is past this whole batch of Great Filters. Suppose we knew that self-replicating molecules only form in water solutions, and we knew that there were some conditions needed for the formation of the molecule and perhaps some different ones for self-replication to happen. Then we could translate these conditions into what we know about exo-planetary oceanic chemistry and decide if any of them were unlikely to happen on most exo-planets in the habitable zone of some star. What might be found? Having an ocean is one. Since we have defined the habitable zone as a zone in which water is liquid, it looks like we have used a circular definition to eliminate this one. But we haven’t. Where does the water come from? We don’t know where the water on Earth’s surface came from, and so it is hard to extrapolate to an exo-planet. We do know that there is ice on outer planets and moons in our solar system, and we think the comets have lots of it. One concept is that after the Earth formed, some comet with a lot of water bumped into it and deposited the water. If that is true, this phenomenon has Great Filter written all over it. The right kind of comet, the water bearing kind, smacks into the primordial planet and deposits a lot of H2O. The planet doesn’t disassociate the water, doesn’t absorb it into the rocks it is made of, doesn’t lose it through evaporation, and allows it to form some really big puddles. Put his in batch 1 of Great Filters.

Batch 2 of the Great Filters occur between that initial self-replicating molecule getting formed, maybe of carbon but maybe of anything else, mutating and getting large enough to form a chemical complex that is also self-replicating. It is easy to envision how it starts, by having the original molecule form a bond between itself and another self-replicating molecule, of which there may be only one kind or many. It is not so easy to envision the next steps, as the dimer turns into a chemical complex. Probably, one of the collected chemicals is a catalyst for some others, and this goes on and on, until the catalyst is a long string of catalysts, and most of the stuff in the complex gets made, not by self-replication, but by catalysis. So the second batch of Great Filters relates to the transition between a single self-replicating molecule and a chemical complex formed by catalysis mediated by one or a few specialized chemicals.

It is likely that the catalytic molecule is a carbon chain, as long chains with other backbones do not exist, as far as we know. Let the third batch be the enclosure of the chemical complex inside a membrane, also made by catalysis, or perhaps it is one of the few things that is still self-replicating. Now we have a cell, which signals the transition out of the third batch of Great Filters. The fourth batch involves much more mutation of the catalyst complex, and eventually leads to a multi-cellular organism. We are still thinking as if the oceanic hypothesis is correct, but if it is not, and life forms in some surface formation of broken up rock or something else you might find on an early planet, the batches seem to still fit.

Now that we have a multicellular organism, we are back to the ocean hypothesis. These multicellular organisms are chemotrophs, living on some source of chemical energy in the ocean. They are going to stay in the ocean until they develop a better energy source, and it is, of course, chlorophyll. The stages of the modification of the organism form the fifth batch of Great Filters. They have been discussed in another post already.

Once you have chlorophyll in a self-replicating organism, it is no longer necessary for that organism to stay around the source of chemical energy that its predecessors used, but it can go wherever there are photons. Which is everywhere near the surface of the ocean. Supposedly our atmosphere is a waste product of these organisms. When scientists talk about the first biosignatures to be searched for on other planets, they are mostly talking about chlorophyll searches, not for cells or organisms containing chlorophyll, but for their excreta. They are searching for worlds that have made it through the first five batches of Great Filter concepts.

The next step is to colonize some land, and one of the Great Filters in the sixth batch is the existence of land. Land is nice as you don’t have any water to absorb photons before they get to you. It seems ever so simple for a water organism to go to shallow water, and into periodically wet areas, and then into dry areas. There may be no other items in the sixth batch of GF’s, but that may be because we have over-simplified.

Back in the ocean there is another form of chemical energy to be extracted. That is the chemotrophs themselves. Their bodies have embedded chemical energy in them, and if one of them, a bigger than usual one, was filtering water collecting the sulfates or whatever it used for energy, and a small chemotroph drifted into the collection orifice, you have the first sign of predator-prey combinations. Mutations would make searching for and collecting smaller chemotrophs a specialization, and these predators would likely follow the new chlorophyll containing organisms to the surface of the ocean, continuing to dine on them, and then moving this process onto land. Included in this batch is the development of collection and filtering as a feeding mechanism, as if the chemotrophs are still only using sdsorption to gain nutrients from the ocean water, no predator mutation would be possible. The seventh batch of possible Great Filters involves all the mutations that a predator needs, and the eighth batch of Great Filters involves development of the attributes that an omnivore like just discussed would need to get onto land. When it is in the ocean, it can simply swim around filtering the water for chemotrophs or their successors. No swimming is possible once it gets to land, so some appendages need to be evolved. We have animals. Perhaps the only Great Filter that might exist in the eighth batch of concepts is the initial development of swimming appendages, which could evolve to something to propel the initial steps onto land.

What is needed for a simple animal to evolve to a creature on the verge of intelligence? Perhaps it is the development of additional sensors. Chemical sensors work well under water, but not so well on land. Eyes might be needed. Let this batch of possible Great Filters be numbered nine.

Then comes the great step to intelligence. Perhaps a particular type of vegetation is needed for the development of grasping and then tool-using limbs. Perhaps there needs to be a change in climate, to drive adaptation toward intelligence. Perhaps an asteroid impact to wipe out large successful species of competitors. These competitors prevent the pre-intelligent species, which are capable of evolution to intelligence, from becoming numerous. This can be batch ten, and that is all the batches there are prior to those we might find in a civilization.

As noted above, we have made a collection of our ignorance, the lack of knowledge of our history or better the potential history of any alien species that develops a civilization. With a somewhat neat division, perhaps it will be possible to start eliminating some of these, or at least making them more specific.