As some of you may be aware, over the last few years I have been modifying and adjusting my position on the occurrence on intelligent life in the Universe (what we normally think of when we invoke the acronym SETI). I have come to the conclusion that although the circumstantial evidence
that has so far accumulated favors that life is common throughout the cosmos, intelligent life is very rare, perhaps non-existent. I base this speculation on, admittedly, only one datapoint. We know that the chemical precursors to life are common throughout the universe, and that so-called primitive organisms can thrive in what we might consider extremely hostile environments. We also know from the fossil record that life arose on Earth almost as soon as the planet formed from the primordial solar nebula over four billion years ago, but that multicellular life did not appear until about five hundred million years ago. Intelligence as we generally define it required a very long time to appear.
This suggests single-celled life such as bacteria and algae are extremely common in the Universe, but that multi-cellular organisms of sufficient complexity to allow intelligence to evolve are pretty rare. Perhaps single-celled communities can somehow evolve a form of collective intelligence, and heaven knows there may be numerous alternative possibilities inherent in commensalism (such as stromatolites, sponges and slime molds, perhaps sentient ecosystems can evolve, remember the Gaia hypothesis?
Using our own Earth as the (only) example, it appears that a planet/sun must be relatively hospitable to life for several billion years before the metazoans make an appearance, but that intelligence can then arise relatively quickly, in less than billion years (still a very long time, astronomically speaking). This kind of stability over astronomically and geologically significant time periods is fairly rare; there are a lot of violent events occurring in the galactic environment, and stellar systems constantly evolve without any consideration to our comfort or benefit.
Although I cannot justify these speculations quantitatively, I suspect that my general assessment is fairly accurate, and that most scientists involved in astrobiology (with the exception of the SETI community itself!) would tend to agree with me. Extra-terrestrial intelligence (perhaps defined as the ability to conduct interstellar travel or communication) is not impossible, but it is extremely rare. Most planetary systems simply do not remain hospitable to life for astronomically significant periods, and those that do do not generally remain stable long enough for technical civilizations to evolve. In addition, we also have plenty of circumstantial evidence to suggest that perhaps technical intelligence itself is not necessarily a survival trait over the long term.
Consequently, I have concluded that technologically capable civilizations are highly separated in both space AND time. The amount of time it took Earth to build space ships and radio telescopes is about one third the age of the universe. There is no fundamental reason ETI may not be out there, but they are either very far away, very long ago, or very far in the future. Even removing the speed-of-light barrier does not mean we’re very likely to ever make contact. For that matter, it may be that no two extraterresrial species will ever actually make contact, anywhere or any time.
Now, it can certainly be argued that once a species develops the capability of interstellar travel (or colonizing hard vacuum, or terraforming lifeless worlds) its life span could be extended indefinitely, avoiding planetary, stellar or even galactic catastrophes. The same could be said if the civilization develops genetic prostheses, or is superseded by its own artifacts (robots or AI). Once any race has expanded or evolved to a point where no single astronomical event could wipe out all of it, it could be fairly said to be culturally immortal, although it could be safely assumed different parts of it might evolve in many different ways. There is a lot of interest now among SETI theorists in seeking out these “super brains”, or “megacivilizations” although so far there has been little astronomical evidence of their activities or artifacts. And that assumes we would even recognize them as such! Astro-engineering projects might be easily mistaken for natural phenomena, or the acts of the gods.
In the science-fiction series “The Expanse”, a super civilization (one perhaps involved in a conflict with yet another super civilization) has used a biological method to cross space and colonize other worlds: the Protomolecule. The Protomolecule is a weaponized microbial or virus-like artificial organism capable of invading alien biologies, taking over their genetic machinery, and diverting their evolution. And it is not targeted against any one type of DNA, it is a generalized parasite, a cross-species (indeed, cross-biology) infection. This is not an absurd suggestion, after all, viruses do exactly that: they invade the genetic machinery of other cells and commandeer it to stop reproduction and instead make copies of the virus. Viruses are incapable of life, they can’t eat, move, metabolize, grow or reproduce, but they can invade other cells and take them over. All they bring with them is information, coded in their own structure and DNA. And they can evolve! Could viruses be alien visitors, blown here by the stellar winds from long-dead civilizations. I don’t know enough biology to be able to answer that question. But if they are not visitors, then they must have evolved AFTER the microorganisms they attack evolved. And we know cells (or at least, multi-cellular organisms), have evolved elaborate immunological defenses against them. At any rate, this is why we need science-fiction; its unlikely any discipline but speculative fiction would have come up with the Protomolecule. The Protomolecule isn’t just a bioweapon, its designed to create or enslave life-forms to create more bioweapons.
Perhaps multi-cellular life, with specialized systems to fight viral infection, was the biosphere’s evolutionary response an interstellar viral invasion. The first viral invaders arrived a half billion years ago and metazoans evolved as a defense. Or maybe, multi-cellular life was created and initiated by the viral invaders, in order to carry out some deep purpose of their own, a purpose designed and programmed into them and implemented billions of years ago, on another galaxy altogether, in order to carry out another mission billions of years from now.. Maybe SETI researchers should be relying more on microscopes than telescopes. Maybe the aliens are already here.