The Silurian Hypothesis: It was the Cephalopods
Text, footage and photographs by Dr. Klaus M. Stiefel
A long time ago, as an enthusiastic young, naive graduate student I went to a conference, which was just a few years away from its inevitable transition from a fun get-together of math nerds who loved to exchange ideas and crack jokes, into an academic business meeting where lab leaders advertised their near-completed projects to the competition.
Back then the conference organizers gave away funny, tongue-in-cheek awards (this would be inconceivable by now – humor!), and one of these awards was for the “most interesting hypothesis most likely to be false”. I thought this was a great award, honoring science which was daring, and which had just missed the mark by a bit. The winner that year was a sympathetic Italian scientist who had, with great enthusiasm and outstanding rhetorical fanfare, presented fancy computer-simulations of language comprehension in a brain region known to be unrelated to language processing. The researcher had missed the actual award ceremony, and I was by chance having a coffee near him and his students when someone broke the news of his newly won award to him. The otherwise so cheerful face of the bearded, portly professore dropped upon hearing about the “honor”. He was visibly distraught by his win! I assume that at home, where he ruled over his lab with charisma and reputation, none of his loyal students had dared to tell him that, based on all that’s known about the human brain, language processing is not happening in the Hippocampus. I remember the young me thinking that this is what scientific progress looks like: hypothesis falsification.
The Silurian Hypothesis
A hypothesis called “The Silurian hypothesis” wins the title of “most interesting hypothesis most likely to be false” for all of science. In brief, the hypothesis postulates that previously a species different from ours had achieved high intelligence and technological civilization on this planet. The Silurian hypothesis (named after “Silurian” aliens in the brainy British TV series “Doctor Who”) was initially proposed by two astronomers, Gavin A. Schmidt and Adam Frank, as a thought experiment, to see if it would even be feasible to detect the traces of such a hypothetical civilization which had existed many millions of years ago. Would there still be detectable changes in the sedimentation patterns if someone (not human) had built cities and military bases a hundred million years ago? Would ancient trash dumps be conserved somewhere, somehow? Would there be changes in the patterns of radioisotopes in the rocks as a result of an ancient nuclear war?
So, in their original paper, Schmidt & Frank didn’t actually voice belief in an ancient civilization, but pondered the question if and how it would be detectable. They conclude that no ruins of ancient football stadiums, highways or housing projects would survive geological time. In contrast, unusual episodes of global warming and the presence of certain artificial radioisotopes (Plutonium-244 and Curium-247) would give an ancient civilization away. Mass extinctions could be a sign of an ancient smart, technological, fast-expanding species.
Actual artefacts might not persist on Earth with its plate tectonics and active atmosphere, but spacefaring ancients might have built big bases on the Moon or on Mars; there, they would have persisted much longer.
Can you see me?
Geological Thought Experiment versus Actual Ancient Civilizations
But besides being an interesting nudge for geological thinking, the question if such an ancient civilization actually happened, and who would have been the species to sustain it is extremely intriguing. The proposition that this was the case, is what I consider the actual most interesting hypothesis most likely to be false, ever, by a wide margin. While Schmidt & Frank didn’t consider this question, there is some really well-written science fiction discussing the question, but it hasn’t been tackled in a biologically well-informed way. This is what I’ll try to do here.
Probably the esteemed reader has noticed by now that I am no true believer in the Silurian hypothesis, but I like to entertain it; and while much has been written about hypothetical humanoid descendants of carnivorous dinosaurs as the bearers of the ancient civilization, I believe that it’s the cephalopods – the octopi, cuttlefish and squid- who are the most likely candidates to have reached at least some level of civilization.
Two somewhat different levels of ancient cephalopod civilization are worth considering. The levels are quite distinct in how realistic (or: how more or less extremely unrealistic) they are.
The first level is the Neolithic stage, reached by a species of cephalopod eons ago. Reaching “only” the Neolithic stage could be described as a “Silurian hypothesis light”; it’s not highly significant achievement for a species, and even such a species can significantly turn over the planet’s fauna. Even our hunter-gathering (Paleolithic) ancestors hunted a number of large animal species to extinction (you don’t have to kill every last mammoth or giant bird to do that), and our farming ancestors (before the advent of even the simple most metal tools) caused massive modifications of the fauna and florae of extensive landscapes. Some of these faunal changes might be detectable millions of years into the future.
Reaching the industrial or space age is much harder than reaching a stage of organized, wide-spread farming. Many of the ways of how Schmidt & Frank propose an ancient civilization which was burning fossil fuels, building nuclear reactors and spaceships. This level is nothing which can easily imagined following from what’s known about cephalopod behavior, which I will rave about below. This second level is the science fiction of the science fiction; it’s in the heavyweight division of the make-believe, and in this essay, I don’t want to go there.
Cephalopods
I came to the conclusion that only the cephalopods are viable candidates for purveyors for an ancient civilization after plenty of reading of the scholarly literature on cephalopods[1], and also via my personal experience – I scuba dive a lot, and I do some in one of the best diving locations in the world, the central Philippines. I divide humanity into those who have seen a living cephalopod with their own eyes, and those who haven’t. Knowing about a squid from anatomical drawings in a book or from squid rings as a starter is like knowing sex only from cheap erotic novels.
As a scuba diver, I observe corals, sponges, sea stars, and most fishes. Squid, cuttlefish and octopi, I interact with. There is a definite two-way component in my encounters with these animals. I look at them, and then I come closer to them. They look back at me, clearly with a theory of my mind, and without any doubt gauging what I intend to do in the next seconds, and then they react: sometimes they decide to flee or hide, and in other cases they keep doing what they were up to before my arrival, or even inspect me in return. Octopi seem to be particularly curious animals.
These encounters are more like those of tourists from far-flung countries interacting with Austrians in the touristy spots of my home country. The tourists might not completely understand what the Austrians are thinking, when they perform their alpine folk dances, but it’s an encounter eye-to-eye, and neither tourists nor folk dancers doubt that the other side is sentient and conscious.
Across a vast species barrier, the situation feels very similar when interacting with a cephalopod.
A flamboyant cuttlefish, a small cephalopod found in the tropics and subtropics of the Pacific Ocean. This animal is particularly impressive with its fast skin color and pattern changes.
The scholarly literature is well in sync with my personal observations of cephalopod behavior being outstandingly complex (there is a great monograph with that title by Roger Hanlon and John Messenger). Cephalopods have been called “honorary vertebrates” for their complex nervous systems and sophisticated behaviors.
Cephalopods are among the few animals which mate in different positions (some squid do), and in some species smaller males pretend to be females, and then sneak by large, territory-owning dominant male. These “sneaker males” are often surprisingly successful in siring offspring, courtesy of their Machiavellian cleverness and deceptive behavior.
Cuttlefish hunt by seemingly hypnotizing crabs by generating highly psychedelic moving stripe patterns on their bodies; other cuttlefish camouflage themselves as harmless, herbivorous parrotfish to sneak up on their prey. And the master of behavioral camouflage, the mimic octopus, pretends to be alternatively a flounder (spiny), a sea snake or a lionfish (both venomous) to deter would-be predators. These animals are clever beyond belief, for animals grouped by biological taxonomists with such passive or slow creatures as clams and garden slugs.
An octopus using the discarded lid of a can as a shield to cover the entrance of its lair. Seen near Sydney, Australia.
There is even an octopus named after its tool use, the coconut octopus (Amphioctopus marginatus). And finally, there is a very sophisticated communication system between cephalopods, based on color and pattern changes in their skins. A large array of tiny, circular muscles contracts or relaxes, and hence shows the skin color above or below the skin musculature. This allows many species of cephalopods to change their color, and the pattern on their skins within seconds. The skin color changes are used for a variety of purposes, including camouflage and communication with conspecifics. Many species have a multitude of different patterns which they can project onto their backs, and which signal messages like “I am angry” or “I am ready to mate”.
Two male cuttlefish fight over a female (white color, on the right). These are three animals of the same species, the different patterns on their backs are instances of communication. The tiger-pattern says “aggression”.
With all of these complex behaviors in place, couldn’t it all have come together sometime, just like it did with humans fifteen thousand years ago, when ecological conditions and human ingenuity came together to spawn the Neolithic revolution. We moved from being very smart creatures who were hunting and gathering in smallish tribal groups to beings involved in organized farming, organized in much larger and more complex groups. I don’t think that taking this step is completely out of question for a particularly large-brained ancient squid. Since almost all cephalopods are carnivores, they would have been some kind of pastoralists, raising snails or clams for their consumption. This level of technological achievement is not likely, and certainly not supported by any actual evidence; however, it’s mildly realistic. The step from an octopus bringing along a coconut shell for protection to an octopus picking up snails and placing them near his lair for farming is at least feasible. The same octopus using stone tools to smash these snails is conceivable. Cephalopods living in structured societies? Squids already live in schools with strict hierarchies, with the bigger animals frequently cannibalizing the smaller ones. Imagine a time-traveling biologist, visiting the proto-human Lucy three million years ago, and the cephalopods of the Jurassic. Whom would she put her bet on to produce a civilization? The primate or the cephalopod? I am not convinced that she’s go with our ancestors.
In contrast, the next level, that of industrialized technology, chemistry, nuclear physics and space travel seems much more unrealistic to achieve by any squid or octopus; science fiction is rich in space-faring cephalopods or cephalopod-inspired aliens, most notably Admiral Gial Ackbar of the original Star Wars movies. However, this is a level which came late, and which does not look inevitable in hindsight, by any means, in human history. Who would have guessed when looking at European warfare in the 1066 (Battle of Hastings: large axes! A brawl with home-improvement equipment) that less than a thousand years later the same people would kill each other using satellite-guided missiles?
A factor which makes reaching this level even more unlikely is that experimental chemistry and physics would be harder to pull off underwater. Air is an excellent insulator of electricity, and all the hours I spent in chemistry labs as an undergraduate makes me doubt that you can pull of a successful chemical synthesis underwater. Reagents would inevitably mix with seawater; this might be a narrow-minded terrestrial-centric view of chemistry, but I suspect that more fundamental physics is at play here, making it easier to combine liquid solutions when you live in the atmosphere.
Hence, octopus Neolithic: maaaaybe. The initial conditions are present in squid and octopi. No proof that it ever happened, but the idea does not smell like looniness, at least to some biologists (well, me). Squid space travel: almost certainly absolutely no.
So why are we not seeing octopus or squid societies today? Fish would eat them. Anyone who observes cephalopods underwater will notice a few striking aspects of their ecology: there are almost always fewer of them around than there are fishes. During an hour’s worth of diving on an average southeast Asian tropical coral reef, you will see hundreds to thousands of fish, but only an octopus or two, or a school of a few dozens of squid – and maybe not even that. If you don’t see an octopus, it might not be there, or it might be so well hidden to escape the eyes of hungry predatory fishes. Somehow, in most habitats, fishes massively win out against cephalopods, with many more individual fishes and many more fish species present.
Consequentially, to understand then when an ancient cephalopod civilization could have happened, we have to look at the evolution of cephalopods, and of that of fishes, and see how they overlap:
So, When Do You Say Did That Happen?
The story of cephalopod evolution is an absolutely fascinating chapter in the development of Planet Earth’s fauna. It’s also murky, and some of the fossils supposedly representing the earliest cephalopods are not easy to interpret, even by expert paleontologists who interpret ancient fossils all day.
Sometime in the late Cambrian, the first geological epoch with diverse animals with complex bodies, the Nautiluses evolved, primitive cephalopods with less well-developed nervous systems and larger numbers of simpler tentacles instead of the eight or ten sophisticated arms of modern octopi or squid. A few species of Nautilus still exist today as “living fossils”, living in deep water where they scavenge.
From these still somewhat primitive animals evolved the giant shelled Ammonites, and the modern cephalopods (the “Coleoids”), which include the cuttlefish, octopi, squid, and some animals closely related to squid, the belemnites, which we will encounter again. These are the animals with fast propulsion via contraction of a water-filled body cavity, fine motor control of their arms and hunting tentacles, keen senses, good memory, skin-pattern=based communication, social lives and sophisticated mating rituals.
A Nautilus, a cephalopod which remained from an ancient lineage. The few species of Nautilus which still exist have more bodies, with simpler eyes, and less developed nervous systems, and generally live as scavengers in deep waters. I took this picture in Palau, where the crew of the dive boat had baited it with a piece of chicken at depth, and hoisted the animal up to shallow waters, where divers could photograph them.
The Cephalopod Civilization Window
A crucial window where cephalopod civilization could have occurred is the time between when mentally high-performing cephalopods came to their own, and the time when aquatic vertebrates really took over. Modern (Teleost) fishes exploded, evolutionarily speaking, only in the late Cretaceous, the last of the three ages of the dinosaurs. These are the snappers, groupers, large wrasses and other predatory fishes which keep the octopus numbers on reefs low in this day and age.
Cephalopods got into their evolutionary gears earlier – the number of genera increased substantially, since the beginning of the Triassic, about 55 million years earlier than the diversification of bony fishes. That’s a long time for evolution to push a species over that last hump of intelligence and cooperation needed to attain simple farming civilization. If there ever was an ancient cephalopod civilization, my bet would be that it happened in the Jurassic.
Again, I think that the likelihood of squid kings ruling the Jurassic seas or of octopus knights jousting for Mesozoic reef dominance is not high, and there is no support yet in favor of it. As usual, unusual claims need especially strong support, and this support doesn’t exist yet. But the ancient cephalopod civilization is one of the poetic believes I keep for myself to remind myself of my pre-grad student, sci-fi devouring teenage self. And should modern geology proceed to univocally falsify even the light version of the cephalopod Silurian hypothesis, I’ll take it easier than the Italian professor.
About the author:
Dr. Klaus M. Stiefel is a biologist, science writer, underwater videographer and scuba instructor originally from Austria, now based in the Philippines. His latest popular science book “Your Brain on Diving” was published by Hübner, Hamburg in 2023. You can find Klaus’ videos and images on social media with the handle Pacificklaus.
Some papers worth reading if you’d like to dig deeper:
Schmidt, G. A., & Frank, A. (2019). The Silurian hypothesis: would it be possible to detect an industrial civilization in the geological record?. International Journal of Astrobiology, 18(2), 142-150.
Klug, C., Landman, N. H., Fuchs, D., Mapes, R. H., Pohle, A., Guériau, P., … & Hoffmann, R. (2019). Anatomy and evolution of the first Coleoidea in the Carboniferous. Communications Biology, 2(1), 280.
Tanner, A. R., Fuchs, D., Winkelmann, I. E., Gilbert, M. T. P., Pankey, M. S., Ribeiro, Â. M., … & Vinther, J. (2017). Molecular clocks indicate turnover and diversification of modern coleoid cephalopods during the Mesozoic Marine Revolution. Proceedings of the Royal Society B: Biological Sciences, 284(1850), 20162818.
