As everyone who eats veggies knows, green chlorophyll is what captures the sun’s photonic energy directly and converts it into useful chemical energy. There are several similar molecules, but the main photon capture mechanism is the same. It absorbs both blue light and red light, but reflects green. There are no other photon capture chemicals known that produce energy supplies. Thus, it is of critical importance for the evolution of life, at least here on Earth. If there are no other photon capture chemicals in the rest of the universe, chlorophyll may well be a Great Filter. If a planet has life, but never gets chlorophyll, it never gets complex life-forms, to say nothing of intelligent primates.
There is no solid knowledge of how chlorophyll evolved, but since the molecule has been synthesized, one hypothesis is that evolution followed this path. It has 17 steps, and there is controversy about this hypothesis. It short, it is a complex molecule, clearly appearing early in the history of life, and is critical for any forward progress to higher forms of life. These are the kinds of things that a Great Filter would have.
There are speculations that on the early Earth, there were other photon capture chemical mechanisms, but they all disappeared after chlorophyll appeared on the scene, as it is very efficient in the use of those photons it does absorb. We are not far enough along in organic chemistry that we could invent a slew of photocapture molecules and then compare them to see which might have evolved into chlorophyll or which might have independently evolved. Despite the overwhelming importance of this class of molecules to our lives, it does not have a large amount of funding and research. Some day we may know.
Backing down the evolutionary tree in a molecular sense is not something we do now. It would require an extensive amount of work to start playing with the DNA strings that code for chlorophyll and seeing just what modifications of them would produce. If we took these strings, and did an inverse of a mutation, then somehow translated that into the biochemistry of photosynthesis, it might be possible to get some good ideas on what the sequence of mutations of DNA were that led to chlorophyll. However, it is certainly not known that it was the single-gene mutational process that led to chlorophyll’s DNA. DNA also is modified by chunks being carved out or spliced in the main sequence. The spliced in chunks might come from some other section of DNA of the organism, having a quite different role. They could even come from a different organism, as bacteria are known to occasionally fuse, even bacteria of different types. So backtracking the DNA codes and the equivalent chemical structures is an extremely difficult task. That’s one reason why the picky details of evolution are so hard to tease out.
Most knowledge of evolution’s pathways come from fossil remains and similarities of shape and structure there. It also comes from the comparison of DNA between living organisms, either scans of populations of one narrow niche, or comparing across species. None of these techniques works for the chemical evolution of molecules like chlorophyll. There are plenty other chemical structures just as forbidding in complexity and just as unknown in evolutionary source. We are simply not yet at a stage of science that allows us to know these things.
Will other planets that have intelligent life have chlorophyll as well? We cannot say that there are no other photon capture chemicals that could be the basis of life on other planets. We can only say we do not know of any, and that our ability to find any is quite limited. Could we someday invent some chemical which was black, absorbing across the whole solar spectrum, and being more efficient than chlorophyll?
This raises some interesting questions. If we don’t find the spectroscopic signature of chlorophyll on a planet, after we have developed some much better astronomical instruments, do we assume it has no life? Maybe it has ‘black chlorophyll’. The hunt for life on other planets seems limited not only by our not yet having instruments to image exo-planets and get the spectrum of their reflected or transmitted light, it is also limited by our lack of knowledge about the options for some of the most basic life processes. To look for life when you don’t know what the scope of it is seems inappropriate. Our space exploration budget is billions of dollars. What is our life origin budget? What is our chemicals of life budget? What is our extreme life organisms budget? Not so much. Could more be done? Very likely. Does it take a lot of time to figure these things out? Yes. Will they be figured out before the instruments that might detect the signatures of the chemicals of life are built and used? A very big maybe, actually a pretty certain no. Not unless we get lucky soon.
Oxygen is the output of photosynthesis now, although it might not have been during the early stages of the evolution of photosynthetic single-celled organisms. Do we understand these early stages well? Not yet. So if we build instruments to detect oxygen in an atmosphere of an exo-planet, and we don’t find it, does that mean there is no life there? Does it mean there is no intelligent life there? We can’t say for some good reasons.
There is some other energy on planets that might support substantial amounts of life. On Earth, there are chemotrophs living in deep sea vents, in rock veins, in hot mineral springs, and perhaps they would be on the planet. If the chemistry and even geological structure of the planet is different, could advanced life have developed on a chemistry alone basis? The mass of life might be limited, but its variety might be more than we see, say, around a deep sea vent. Could the energy transport mechanism in exo-planet life be quite different? Evolution is a powerful force and it works on other planets, not just Earth.
If the energy source for life on another planet is photonic, it might be captured in a different way, and transported in a different way, maybe oxygen is not an output, but something else is. Perhaps nothing is, and the planet’s equivalent of herbivores gets all needed chemicals from the photosynthesizing life, instead of needing both atmospheric oxygen and chemical fuel from photosynthetic life.
Beyond those options that are blinkered by our delay in understanding enough about the possibilities of life to come up with multiple possibilities, there are the alien possibilities. If there are aliens on the planet, they could be living in closed environments, for example for the purpose of extracting resources for starships stopping over on their way to distant habitable planets. Slowing down for supplies takes a tremendous amount of energy, but perhaps the logistics works out this way. Something to be considered.
Perhaps there is a black chlorophyll, and an alien civilization has replaced all its chlorophyll plants, on the entire globe, with black chlorophyll ones. Perhaps it was just invented and let loose and it out-competed all the others. Sort of like an invasive species scenario here on Earth, but hugely more in scope. No green spectrum for us to detect. If black chlorophyll is not something that could evolve, but required synthesis in a laboratory, it would be a distinctive signature of an advanced species.
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