Updates of the Links

Some of the links on the left are to sites that are now dead or redirected. I have updated them in appropriate ways.

The Drake Equation: Lifetime

Finally, the lifetime of a communicating civilization: L.

Its value is very conjectural, because there are several factors that can limit such a civilization’s lifetime.

  1. Wars
  2. Diseases
  3. Environmental problems
  4. Resource depletion
  5. Loss of interest

(1) This was rather obvious from the Cold War. Both the United States and the Soviet Union built enough nuclear bombs to turn each other’s cities into radioactive wastelands, and other nations have tried to join in.

(2) That is a bit farfetched, but not impossible with suitable genetic engineering, like creating a time-bombed microorganism that spreads without causing symptoms, and then starts attacking its hosts.

(3) These include various ways of impairing the habitability of one’s homeworld, like ruining farmland and altering the climate.

(4) This includes running out of fossil fuels, metal ores, and the like, without developing good substitutes. I think that energy resources are especially critical, since without energy, you can’t do anything else. So it is important to learn how to use long-lived energy sources like the light of one’s homeworld’s star.

(5) There are several ways that this can happen.

  1. Reversion to a lower level of technology
  2. Turning inward
  3. Feeling threatened by the possibility of intelligent entities elsewhere in the Universe
  4. Deciding that such entities cannot exist
  5. Quitting after failing to discover such entities
  6. Considering self-advertisement too dangerous or too expensive
  7. Considering searches likewise too dangerous or too expensive

But if a civilization can overcome these challenges, then it can last as long as the Universe has usable energy to run it.

This is arguably the most conjectural of all of Frank Drake’s parameters, and a good part of the reason is our very limited experience with it, much less than for the other parameters. So L is very up in the air.


The Drake Equation: Communication

Turning to fc, the fraction of biotas with intelligent species where at least one species becomes capable of interstellar communication.

Some such species may be incapable of that, like cetaceans, because they are aquatic. Cetaceans also don’t have tentacles or pincers. So we must turn to the history of our species for clues.

The first step along the way is agriculture, since it enables larger population densities and larger-scale societies than foraging can. What is curious about humanity and agriculture is that present-day humanity did not have it for most of its some 40,000 – 100,000 years of its existence. But in the Holocene, after the end of the last ice age, humanity invented agriculture independently in several places in the world. I have seen a speculation that this curious timing is due to the climate being more stable in the Holocene than in the time since the previous interglacial about 100,000 years ago. An unstable climate makes it difficult to get started with agriculture, since the weather can turn bad for it too quickly.

In this connection, Jared Diamond has made a strong case that Eurasian people got ahead of the rest of humanity because they could exchange crop plants across the length of their continent. Also because it had some conveniently domesticatable animals. By comparison, it was difficult to get potatoes and llamas from the Andes to the North American Rockies, where they can thrive, because of the tropical-climate places in between.

Next is persistent information storage: writing. It can easily outlive its writers, and it does not need the memory cues that are convenient for memorizing large amounts of memory. Rhythmic poetry was a much-valued art, and its rhythms helped to jog the memories of its memorizers. In fact, some people apparently considered writing a bad thing because it would make people’s memories atrophy and make people only appear to be learned (The Internet Classics Archive | Phaedrus by Plato).

Writing was only invented in two or three places: Sumeria, Central America, and maybe also China. But once it was invented, it was widely copied, and many people invented writing systems out of awareness that it was possible: stimulus diffusion.

After that was development of science, but that was difficult and slow. It started in classical Greece and continued into the early Roman Empire, where it was interrupted by the “Crisis of the Third Century”: strife and civil war. It did not restart until over a millennium later in western and central Europe, and even then, it was slow going at first.

There is a further problem. The social-brain theory of intelligence suggests that sentient species may prefer to be considered with social relations and gossip and the like; that sometimes seems rather evident in our species. That extends to anthropomorphizing nonhuman species like pet species, like making LOLcat pictures.

However, some of us have Asperger’s syndrome, which may help its sufferers understand impersonal things and features and relations. Not many, but enough to be useful for the rest of us. So might other sentient species have Asperger-like variants?

Considering all these factors, fc may be much less than 1.

The Drake Equation: Intelligence

Now to fi, the fraction of planet biotas that intelligent organisms emerge from. On our planet, at least, this emergence has turned out to be a long and complicated and winding path.

The Last Common Ancestor that can be reconstructed was likely an organism much like a present-day methanogen, an organism that lives off of combining such environment substances as carbon dioxide and hydrogen, and that makes all its biological molecules.

That limits where such an organism can live, and a way out of that limit is getting energy from light: photosynthesis. It has evolved twice on our planet:

  • Bacteriorhodopsin – addition to chemiosmotic energy metabolism
  • Chlorophyll – addition to electron-transfer energy metabolism

The second type is the familiar kind, and its most familiar version uses water as an electron source, releasing oxygen. It has enabled colonization of environments that are otherwise chemically difficult. It has also made it much easier to live off of biological material, because combining it with oxygen releases much more energy than reshuffling its molecules (fermentation).

The next step is multicellularity. It has evolved numerous times (The evolutionary-developmental origins of multicellularity), mostly among eukaryotes. Most sorts of multicellularity are plantlike, funguslike, or slime-mold-like. Some prokaryotes also exemplify these types: cyanobacteria, actinobacteria, and myxobacteria. But animallike multicellularity has evolved only once, judging from present-day organisms.

Bodies of water are poor places to work with fire and electricity, so we must consider colonization of land. Animals have done so several times:

  • Arthropods: insects, pillbugs, land crabs, arachnids, myriapods (centipedes, millipedes)
  • Mollusks: land snails
  • Annelids: earthworms, leeches
  • Vertebrates: tetrapods, starting with real-life “Darwin Fish”

Plantlike organisms have done so only once, however.

Something helpful for being large on land is an internal skeleton, and that’s evolved once in animals (vertebrate skeletons), and at least once in plants (wood). Curiously, most animal skeletons are either external (shells), skin-surface (arthropod skins), or just under the skin (echinoderm skeletons). So it may be hard for an animal internal skeleton to evolve.

Grasping limbs and jaws have evolved multiple times, however.

Among land vertebrates, grasping with digits has evolved at least twice, in primates and in perching birds (Passeriformes). Arthropods have evolved pincer limbs at least twice (scorpions, various crustaceans). Tentacles have evolved at least twice, in cnidarians and in cephalopods, and arguably a third time in proboscideans (the elephant’s trunk).

Turning to jaws, vertebrate ones are modified front gill bars, while arthropod ones are modified limbs. Some polychaete worms also have jaws.

About sense organs, vertebrates and cephalopods have independently evolved high-resolution lens-camera eyes, and vertebrates and arthropods have independently evolved color vision.

We now get to intelligence proper.

It may not often seem very apparent in our species, but I think that that is because we have rather high standards for ourselves by the standards of most of the animal kingdom. Thus, when we see some shoddy reasoning, we often think that its author must be terribly dumb, but that shoddy reasoning is described using linguistic capabilities that are far in advance of what just about every other species on this planet can do. Like a creationist declaring “Evolution can’t be true because dogs don’t give birth to cats.”

By comparison, (nonhuman) great apes have much more limited capabilities. They can learn lots of sign-language signs, but the most syntax that they have used is two-sign phrases, and even that is doubtful (Great ape language). Here is a transcript of an ape sign-language session: Koko.org – Koko’s World – Talk To Koko.

Most animal language is even simpler, the equivalent of single words or canned phrases in our languages. Vervet monkeys make calls in response to seeing various predators that their fellow monkeys respond to with appropriate evasion actions. So in human terms, those calls are “Leopard!”, “Snake!”, and “Eagle!” (The Semantics of Vervet Monkey Alarm Calls: Part I « Primatology.net, The Semantics of Vervet Monkey Alarm Calls: Part II – The Experiment « Primatology.net).

So that leaves dolphins and other toothed cetaceans as our only plausible competition on this planet. Dolphin language has been difficult to decode, but some parts of it are now understood, like imitations of echolocation-system echoes of objects. In human terms, that’s like calling a dog a woof-woof or a cat a meow-meow.

I went into this digression into language because that’s necessary for describing how to build a radiotelescope for interstellar communication — and also for describing why it might be worth building.

Another criterion is self-awareness, like being able to recognize oneself in a mirror. Human children become able to do that at about 18 – 24 months old, and a few other species seem to have this ability (Mirror test): (other) great apes, dolphins and orcas, elephants, and European magpies.

Most others don’t, and to a dog or a cat, the dog or cat in the mirror is another one.

The next question is what would drive the evolution of intelligence, since big brains can cost a lot of energy to keep going.

Visual perception or echolocation interpretation can require a lot of brainpower to discover a lot of details, and the larger brains are indeed of users of these abilities (Cetacean intelligence, etc.).

Another hypothesis is Robin Dunbar’s social-brain hypothesis. According to it, large brains evolved for keeping track of other members of one’s species, and among various simian species, there is indeed a correlation between brain size and social-group size. Extrapolating to our species, RD finds Dunbar’s number, about 150.

Large social groups and full-scale language can combine for transmitting complex information down the generations.

Might that also be true of cetaceans? Or might it be a side effect of large brains elaborated for interpreting echolocation? In any case, it’s evident from a lot of research that bottlenose dolphins, the best-studied species, have very complex societies.

So we have one example of human-scale intelligence, ourselves, and some species that come close – some of the toothed cetaceans.

So while some steps involved in the evolution of intelligence have happened several times, other steps have happened only once. It’s not clear whether the latter sort of step tends to pre-empt other instances or whether it does not often happen. So fi is up in the air.

The Drake Equation: Life

This is about Drake-equation factor fl, the number of Earthlike planets where life emerges, even very simple microbes.

The origin of life continues to be an unsolved problem. But research has approached that issue from two directions:

  • Bottom-up: prebiotic synthesis
  • Top-down: looking back in biological evolution

Prebiotic-synthesis experiments have made a variety of building blocks of organisms, like amino acids, the building blocks of proteins, and nucleobases, parts of nucleic acids (DNA and RNA). Robert Freitas in his book Xenology has a big list of work on this subject up to 1975: 7.3.1 – Prebiotic Synthesis. However, it is difficult to make another part of nucleic acids: ribose (Prebiotic ribose synthesis: A critical analysis – Springer). Gunter Wächtershäuser has proposed that a sort of prebiotic metabolic network had emerged in hydrothermal vents and similar places, but that notion has only limited experimental support (Iron–sulfur world hypothesis, Stirring The Volcanic Pot For A Hydrothermal Origin Of Life).

Going in the other direction, we seem to have a difficulty. The last universal ancestor of all well-studied present-day organisms turns out to be rather complicated. We can plausibly identify these features of it:

  • DNA genome
  • DNA-to-RNA transcription and RNA-to-protein translation
  • The 20 canonical protein-forming amino acids
  • ATP used as an energy intermediate
  • Lipid-bilayer cell membrane
  • Chemiosmotic energy metabolism: pumping hydrogen ions across the cell membrane out of the cell, and making them assemble ATP when they return
  • Electron-transfer energy metabolism, ending in nitrogen oxides instead of in oxygen (Heme-copper oxidase superfamily)
  • Carbon fixation, incorporating the carbon from CO2 into its molecules
  • Combining CO2 and H2 for energy, giving methane or acetic acid
  • Numerous protein enzymes, including a complete set of biosynthesis enzymes, likely making the organism autotrophic, able to make all its biological molecules

Fermentation was likely not ancestral, because of its complexity (How did LUCA make a living? Chemiosmosis in the origin of life).

But one can reconstruct some of the evolution of its ancestors, and the earliest sort of organisms that can be reconstructed with any confidence is the RNA world where RNA served as both information storage and as enzymes. Proteins were later developed to serve as enzymes, and DNA was later developed as a modification of RNA for master-copy duty. There are even bits of RNA in various coenzymes, relatively small molecules that work with enzymes.

The main criticism I’ve seen of the RNA world is the origin of the RNA. It’s difficult to make ribose prebiotically, and I’ve seen speculations about alternatives, like amino acids (peptide nucleic acids) and polycyclic aromatic hydrocarbons (PAH nucleic acids).

Finally, I must note that all well-studied organisms have several shared molecular-biological features, something that suggests that only one origin of life on our planet has had present-day descendants. It also suggests a lack of contamination by organisms from elsewhere in the Universe.

So the two research frontiers have yet to meet, and fl is still up in the air.