An old pal from high school mentioned in an email the other day that he had an interest in Adam Frank’s work, which we’ve looked at in these pages a number of times. Although my most recent post on Frank involves a 2022 paper on technosignatures written with Penn State’s Jason Wright, my friend was most intrigued by a fascinating 2018 paper Frank wrote for the International Journal of Astrobiology (citation below). The correspondence triggered thoughts of other, much earlier scientists, particularly of Charles Lyell’s Principles of Geology (1830-1833), which did so much to introduce the concept of ‘deep time’ to Europe and played a role in Darwin’s work. Let’s look at both authors, with a nod as well to James Hutton, who largely originated the concept of deep time in the 18th Century.
Adam Frank is an astrophysicist at the University of Rochester, and one of those indispensable figures who meshes his scientific specialization (stellar evolution) with a broader view that encompasses physics, cultural change and their interplay in scientific discourse. He fits into a niche of what I think of as ‘big picture’ thinkers,’ scientists who draw out speculation to the largest scales and ponder the implications of what we do and do not know about astrophysics for a species that may spread into the cosmos.
Now in the case of my friend’s interest, the picture is indeed big. Frank’s 2018 paper asked whether our civilization is the first to emerge on Earth. Thus the ‘Silurian’ hypothesis, explored on TV’s Doctor Who in reference to a race of intelligent reptiles by that name who are accidentally awakened. The theme pops up occasionally in science fiction, though perhaps less often that one might expect. James Hogan’s 1977 novel Inherit the Stars, for example, posits evidence for unknown technologies discovered on the Moon that apparently have their origin in an earlier geological era.
Image: Astrophysicist Adam Frank. Credit: University of Rochester.
I won’t go through this paper closely because I’ve written it up before (see Civilization before Homo Sapiens?), but this morning I want to reflect on the implications of the question. For it turns out that if, say, a species of dinosaur had evolved to the point of creating technologies and an industrial civilization, finding evidence of it would be an extremely difficult thing. So much so that I find myself reflecting on deep time in much the same way that I reflect on the physical cosmos and its seemingly endless reach.
Consider that we can trace our species back in the Quaternary (covering the last 2.6 million years or so) and find evidence of non-Homo Sapiens cultures, among which the Neanderthals are the most famous, along with the Denisovians. Bipedal hominids show up at least as far back as the Laetoli footprints in Tanzania, which date to 3.7 million years ago and were apparently produced by Australopithecus afarensis. Frank and co-author Gavin Schmidt also note that the largest ancient surface still available for study on our planet is in the Negev Desert, dating back about 1.8 million years.
These are impressive numbers until we put them into context. The Earth is some 4.5 billion years old, and complex life on land has existed for about 400 million of those years. Let’s also keep in mind that agriculture emerged perhaps 12,000 years ago in the Fertile Crescent, and in terms of industrial technologies, we’ve only been active for about 300 years (the authors date this from the beginning of mass production methods). Tiny slivers of time, in other words, amidst immense timeframes.
So as Frank and Schmidt point out, we’re talking about fractions of fractions here. There is a fraction of life that gets fossilized, which in all cases is tiny and also varies according to tissue, bone structure, shells and so forth, and also varies from an extremely low rate in tropical environments to a higher rate in dry conditions or river systems. The dinosaurs were active on Earth for an enormous period of time, from the Triassic to the end-Cretaceous extinction event, something in the range of 165 million years. Yet only a few thousand near-complete dinosaur specimens exist for this entire time period. Would homo sapiens even show up in the future fossil record?
From the paper:
The likelihood of objects surviving and being discovered is similarly unlikely. Zalasiewicz (2009) speculates about preservation of objects or their forms, but the current area of urbanization is <1% of the Earth’s surface (Schneider et al., 2009), and exposed sections and drilling sites for pre-Quaternary surfaces are orders of magnitude less as fractions of the original surface. Note that even for early human technology, complex objects are very rarely found. For instance, the Antikythera Mechanism (ca. 205 BCE) is a unique object until the Renaissance. Despite impressive recent gains in the ability to detect the wider impacts of civilization on landscapes and ecosystems (Kidwell, 2015), we conclude that for potential civilizations older than about 4 Ma, the chances of finding direct evidence of their existence via objects or fossilized examples of their population is small.
Image: The Cretaceous-aged rocks of the continental interior of the United States–from Texas to Montana–record a long geological history of this region being covered by a relatively shallow body of marine water called the Western Interior Seaway (WIS). The WIS divided North America in two during the end of the age of dinosaurs and connected the ancient Gulf of Mexico with the Arctic Ocean. Geologists have assigned the names “Laramidia” to western North America and “Appalachia” to eastern North America during this period of Earth’s history. If a species produced a civilization in this era, would we be able to find evidence of it? Credit; National Science Foundation (DBI 1645520). The Cretaceous Atlas of Ancient Life is one component of the overarching Digital Atlas of Ancient Life project. CC BY-NC-SA 4.0 DEED.
Intriguing stuff. The authors advocate exploring the persistence of industrial byproducts in ocean sediment environments, asking whether byproducts of common plastics or organic long-chain synthetics will be detectable on million-year timescales. They also propose a deeper dive into anomalies in current studies of sediments, the same sort of analysis that has been done, for example, in exploring the K-T boundary event but broadened to include the possibility of an earlier civilization. I send you to the paper, available in full text, for discussion of such testable hypotheses.
Back to deep time, though, and the analogy of looking ever deeper into the night sky. In asking how long a civilization can survive (Drake’s L term in the famous equation), we ask whether we are likely to find other civilizations given that over billion year periods, they may last only as a brief flicker in the night. We have no good idea of what the term L should be because we are the only civilization we know about. But if civilizations can emerge more than once on the same world, the numbers get a little more favorable, though still daunting. A given star may be circled by a planet which has seen several manifestations of technology, a greater chance for our detection.
A cycle of civilization growth and collapse might be mediated by fossil fuel availability and resulting climate change, which in turn could feed changes in ocean oxygen levels. Frank has speculated that such changes could trigger the conditions for creating more fossil fuels, so that the demise of one culture actually feeds the energy possibilities of the next after many a geological era. How biospheres evolve – how indeed they have evolved on our own world – is a question that exoplanet research may help to answer, for we have no shortage of available worlds to examine as our biosignature technologies develop.
Culturally, we must come to grips with these things. In an essay for The Geological Society, British paleontologist Richard Fortey discusses the seminal work of James Hutton and Charles Lyell in the 18th and 19th Centuries in developing the concept of geological time, which John McPhee would present wonderfully in his 1981 book Basin and Range (I remember reading excerpts in The New Yorker). The Scot James Hutton had literary ambitions, publishing his Theory of the Earth in 1795 and changing our conception of time forever. Hutton knew Adam Smith and spent time with David Hume; he would also have been aware of French antecedents to his ideas. But despite its importance, even Lyell would admit that he found Hutton’s book all but unreadable.
It took a friend named John Playfair to turn Hutton’s somnolent prose into the simplified but clear Illustrations of the Huttonian Theory of the Earth in 1802, making the idea of deep time available to a large audience and leading to Lyell. Which goes to show that sometimes it takes a careful popularizer to gain for a scientist the traction his or her work deserves. The emphasis there is on ‘careful.’
Lyell’s Principles of Geology, published in three volumes between 1830 and 1833, famously traveled with Darwin on the Beagle and, as Fortey says, “donated the time frame in which evolution could operate.” He goes on:
“…once the time barrier had been breached, it was only a question of how much time. The stratigraphical divisions of the geological column, the periods such as Devonian or Cambrian, with which we are now so familiar, were themselves being refined and put into the right sequence through the same historical period. Just to have a sequence of labels helped geologists grapple with time, and, in a strange way, labels domesticate time.
But domestication co-exists with wonder. I imagine the most hardened geologist of our day occasionally quakes at the realization of what all those sedimentary layers point to, a chronological architecture — time’s edifice — in which our entire history as a species is but a glinting mote on a rockface of the future. Our brief window today is reminiscent of Hutton and Lyell’s. Like them, we are compelled to adjust to a cosmos that seems to somehow enlarge every time we probe it, inspired by new technologies that give birth to entire schools of philosophy.
John Playfair would write upon visiting Siccar Point, the promontory in Berwickshire that inspired Hutton’s ideas, that “The mind seemed to grow giddy looking so far into the abyss of time.” We are similarly dwarfed by the vistas of the Hubble Ultra Deep Field and the exquisite imagery from JWST. Who knows what we have yet to discover in Earth’s deep past?
The paper is Schmidt and Frank, “The Silurian Hypothesis: Would it be possible to detect an industrial civilization in the geological record?” published online by the International Journal of Astrobiology 16 April 2018 (full text). Gregory Benford’s Deep Time: How Humanity Communicates Across Millennia (Bard, 2001) is a valuable addition to this discourse. For a deeper dive, Fortey mentions Martin Rudwick’s Bursting the Limits of Time: The Reconstruction of Geohistory in the Age of Revolution ( University of Chicago Press, 2007). Fortey’s own Life: A Natural History of the First Four Billion Years of Life on Earth (Knopf, Doubleday 1999) is brilliant and seductively readable.
Both this introduction and the background works are quite interesting essays.
And when the subject has come up previously, I have found it intriguing.
With so many possible lead ins on this subject, the Seaway map reminds me of a Discovery Channel (?) series of programs describing with animations many marine dinosaurs that inhabited the region around Dallas, Texas. But evidently, they could have done much the same with Denver – or else illustrate it as a marsh.
Looking at the terrains where brontosaurs are recovered, one would be led to believe that they were cliff dwellers.
From decades back I do vaguely remember a story or two about sentient dinosaurs in s/f. One or the other might have even suggested that these predecessors fled the comet impact – or else the Decca volcanic eruptions associated with the Indian subcontinent’s collision with Eurasia of 65 million years ago. One paleontologist s/f writer was more inclined to the latter explanation. A terrible era no matter how you re-construct it.
But either way you slice the end, under the dust there was an underlying notion that before the age of mammals there was an extraordinarily long period where there could have been stone age activities including tools of some sort, if not industrial development that could have been erased by surface dynamics of extreme nature.
Just to check my bearings, I found a definition of the Silurian period as
” a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago (Mya).”
Now that might be a favored temporal period for some Dr. Who explorations, but the Western Interior Seaway illustrates an era closer to The End of the Cretaceous around 65 million years back – currently occupied or flanked by the Rocky Mountains. The illustration brings home that the Appalachians are relatively ancient compared to the western contemporary counterparts. Though this assessment might need some adjustment too.
In addition, while we have to wonder how completely a civilization or its archeological remains can be erased from the earth after a lapse of tens of millions of years, we are also discovering intact dinosaur eggs throughout the world. This is not to say that they are ready to hatch, but scans can be done to detect the fossilized embryos inside. Some eggs are hard to crack, of course, but they have to be fragile enough for a dinosaur to get out too. Now, if there were a large organized society of some forerunner to “civilization”, shouldn’t we be able to detect an ax or two?
The Drake equation for detecting civilizations among the stars started with rather small number of probabilistic factors to assess the chances of communication with other intelligent species. The more the equation is examined against what this civilization had to overcome before it could construct a radio antenna at Greenbank or elsewhere, we could come up with many more than the prototype provided. One to add in the context of ancient species. Search for the substitute or probability of an opposing thumb.
“shouldn’t we be able to detect an ax or two?”
Indeed we should. If of course the wielders had three axes of movement in their shoulders, prehensile hands to hold the ax, opposable thumbs with a strong pinch to hold and fashion tools, stereognosis to aid in this, binocular vision with sufficient overlap of visual fields to give stereoscopic vision with depth perception so that the ax could be wielded with a modicum of accuracy.
Fortunately for us, our primate forebears brachiated aplenty in their arboreal lifestyle in a continuous canopy of tropical rainforest.
The illustration shown of “Principles of Geology” is of the Roman markets in the city of Pozzuoli near Naples in Italy. Often identified (as in the picture) as the temple of Serapis (I believe erroneously).
Pozzuoli is more or less at the center of a large caldera (Campi Flegrei) that is currently in a state of unrest with the ground that has been pushed up by a few meters since the 70s.
In fact I remember it in the early 70s with some water between the columns, a bit like in the illustration, with mullets swimming. Marks made by sea shells up in the columns indicate that terrain was even lower in the past.
I’ve been last year and it’s completely dry and this soil uplifting is causing small earthquakes continuously.
Experiencing an uplift of various meters in only 50 years feels a bit like experiencing that deep time we can only imagine.
Regarding the lack of evidence of civilization in Deep Time…It is possible that few of our artifacts would survive over geological time spans. No doubt we could build things that would last forever, but unless we deliberately chose to do so, we have no reason to design them for immortality. Only our spacecraft, still orbiting the sun, or creeping out into the galaxy, would remain. Or would they?
When the railroads were being built in Florida in the 20th century, there was no suitable stone in this sedimentary state to use as a foundation for the tracks.
Consequently, granite from Georgia (the same stuff Stone Mountain is made of), was imported, broken up into fist-sized chunks, and the rails were laid on top of that.
I’ve often wondered if our own civilization were to fully collapse, and after millions of years were replaced by another, what would the geologists of that distant future make of the legacy of our culture. All of our technical and architectural achievements will have crumbled into dust. Perhaps even our plastics would have finally broken down, or become unrecognizable by their very ubiquity.
But the land of Florida would still have evidence of its ancient railways. The steel rails and spikes would have rusted away, and the wooden ties rotted to dust. But those granite rail beds would still survive, an igneous anomaly, a geological impossibility, criss-crossing the local limestone and silica sand. Even if the state were to be submerged under the sea, and then thrust up again, under layers of marine sediments long ribbons of granite would still remain, to baffle the geologists of that future time.
Under the right conditions, even the wooden ties could be fossilized, just like the fossil trees we find.
Just as soft bodied animals have left impressions and traces in marine sediments, so might some of our artifacts rot away. Imagine metal screws and bolts that rust away yet leave a strange threaded trace in the rocks.
While it is true we have fairly few complete dinosaur skeletons, we do have many more fragments, and new species are discovered every year. We also tend to explore where the appropriate rock formations are exposed. Many of the British reptiles and dinosaurs were uncovered in the coastal limestone cliffs of Southern Britain where erosion exposed the fossils. We have barely scratched the surface of our planet.
It still amazes me that despite the well-trodden landscape of Europe, new archaeological finds are discovered every year, sometimes on sites that have been worked over for construction. London is full of Roman building remains that are unearthed as buildings are replaced. Sometimes it is about using methods to locate new remains, whether archival or technological. Many of these remains are just less than a meter below the surface.
Sometimes discovery is serendipitous. Who but spelunkers would have thought to enter that deep, Rising Star cave in South Africa to find the remains of Homo naledi?
How long could artifacts survive? If there were the equivalent of Roman roads in the Cretaceous, they would likely have been preserved. Whether they could be found is another issue. Under the right conditions, metals might survive, even as patterned traces in the rocks, indicating technology rather than natural processes. In the less than a million years old category, ice cores may reveal our civilization to a future one from industrial pollutants trapped in the core whether in air bubbles or washed out from the atmosphere and captured. There may even be large discontinuities in the age of the materials through the core, indicating a period of catastrophic warming. If the organic detritus can still be detected in Devonian shales, then I have to think that the petroleum-based surfaces of roads may be detectable as a thin layer in very ancient rocks. If Silurians used that technology to build roads, then they should be detectable if we look in the right places. The rubber particles that are washed off from road surfaces might be another more global, detectable layer, found in sedimentary rocks,
The deepest time is being revealed by the JWST, with stars and galaxies that were formed a scant few hundred million years after the “Big Bang”. Is it possible that we might eventually be able to peer into a time before that event?
Good point Alex Tolley. The concrete and tar streets would be hard to hide and the iron age and today’s steel alloys would last like skyscraper beams, etc. There would be no catastrophic period of warming because none would be necessary. Recall the entire Cretaceous period has an atmospheric concentration which had a low of 400 parts per million with the high at nearly one thousand parts per million. Consequently, the sea way in the map on this paper is the result of the complete melt of the polar ice caps everywhere during the entire Cretaceous period, the sea level was over 210 feet higher than today. This should be of serious concern considering we are at 420 ppm. We may already have passed the tipping point of the complete future melting of our polar ice caps which is why it is important to not just greatly reduce our greenhouse gas emissions, but to also remove the excess carbon dioxide from our atmosphere and bring it down to pre-industrial levels of 280ppm before the year 1850. We have to use carbon capture and carbon scrubbers to do that which is expensive but can be done in several decades. Nature takes 50,000 years to remove or add 100ppm according to the Milankovitch cycles. The carbon cycles takes longer or tens of millions of years which is deep time. The carbon dioxide levels were much higher in the past and have been reduced by the carbon cycle which is slow. Photo synthesis, and the oceans have also helped bring lower it. Continental drift plays a role because when the continents spread far apart there is more coast line, and more basalt wind weathering to remove carbon dioxide which coincide with cooler periods and polar ice caps. The super continents like Pangaea and Rodinia, etc., coincide with warm periods because there was less polar land and much more sea area which absorbs more sunlight than land and sea water keeps the heat longer than land making the planet warmer. Wikipedia source.
This is relevant to this paper because an unknown past civilization would have to reach today’s level of population and today’s amount of greenhouse gas industry to cause any warming. It’s hard to think we would not find any trace of that. In other words, such a civilization could not be small and isolated like the lost continent of Atlantis in myth which was 10,000 years ago. Recall that agriculture was necessary first before any kind written language was developed and without schools, math and language it is hard to believe a technological civilization could have formed before the rest of the world discovered written language. Words and language are necessary for the development of consciousness, at least a modern, abstract consciousness. There is also no fossil evidence other than primitive man.
There is always the possibility that our Lamda cold dark matter big bang theory is wrong or our universe is much larger than predicted and we have limited it’s size to what we see only locally. We are at the limits of our light cone and light gathering power of telescopes so it is difficult to tell. With a much larger space telescope we could look much further back in time if our universe was much larger than predicted. Consequently we might still see galaxies at 15 and 20 billion light years or more. This idea has already been mentioned by NASA’s astrophysicist John Mather’s prediction on what we potentially might see from views from the JWST. He predicted we would see the big bang or just more distant galaxies. It looks more like that latter prediction coming true. We did not know what Quasars were because they were too distant and we had not thought of the right comparison. Now we know they are galaxies with active nucleuses.
Two points on the surface of an inflating balloon move apart faster and traverse a greater distarce if their initial positions were further apart. On account of this (and the accelerating expansion of the universe) the farthest visible galaxies are now moving away faster than lightspeed; in due course only the galaxies in our local cluster held together by gravity will be visible and the rest of the universe will appear empty: and that is the concept to be formed by civilizations of that time which lack a connection to our history.
Absolutely…if we are stuck looking at physical objects in the universe. But the CMB is an example of seeing further back in time because of how we see it today. Some years ago there was an idea that the structure of the visible universe might indicate other features of reality, like the shape of the multiverse. There seems to be a renewed idea that there was something before the “Big Bang” and that maybe we can infer what that was. I am open to thinking that we have not really understood the universe/multiverse yet, and that just possibly we are at a stage of understanding the true picture of reality as the astronomers were at the beginning of the C20th when they thought our galaxy was the extent of the universe.
Yes there seems to have been some hasty redefinition of what ‘big bang’ means owing to the deep past observations made in recent years. Rather than the origin of time and space, ‘big bang’ is now being used to describe an event that precipitated the cosmic background throughout an earlier, smaller, and heterogeneous but non-singular early universe. Considering the gargantuan black holes currently being discovered at the centre of early galaxies, we might find the observable CMB is more of a corner we can’t easily see around rather than an impenetrable wall to deeper observation.
If we wanted to look for earlier civilizations, we should look for equipment on the Moon or dead satellites in very high orbits. Plate tectonics would have crushed any clear signs of ancient technology into oblivion (or into the mantle).
The concept reminds me of the pastoralist Thomas Cole and his series, “The Course of Empire ” https://en.wikipedia.org/wiki/The_Course_of_Empire_(paintings)
I’m sure he intended for the series to be viewed from The Pastoral State through Desolation, but it can also be viewed as a cycle.
It may be difficult to exclude the existence of an intelligent and technological species on Earth an eon ago. However, if it did exist it was almost certainly not native. That is, a temporary ET colony is the only likely possibility. The reason is that a native species of this type does not arise and fall in isolation.
There would be a long evolutionary tree of many branches with markers for increasing demonstration or the potential for intelligence leading up to that specific outcome. After the fall of the technological civilization and species, those adjacent species and their biological traits would not also vanish. I am not aware of evidence that demonstrates the existence of that.
The 1% urbanization number is misleading. If you stumble across the remains of a city, you can’t help but see a civilization. But finding a single manufactured object in undisturbed ancient strata would be enough. Any paved road would do, if it was buried instead of eroded away—not only a Roman road or a modern highway, an asphalt surface would remain chemically distinct from its surroundings, likely for as long as it remained mechanically undisturbed. Or a bit of industrial debris. Trash is everywhere humans go, and it includes glass, ceramic, aluminum, and stainless steel that stand a chance of remaining recognizable over geological time. One worked stone, if provably old enough, would be a sensational find. The hulk of an abandoned car or fridge or whatever, no matter how decayed, should at least be detectable as artificial.
Here is something that is a bit spooky. Oldest wooden structure found, it is about 476,000 years old. Not mentioned is how complicated a society would need to be to produce such structures. Well before Homo Sapiens were around.
https://news.liverpool.ac.uk/2023/09/20/archaeologists-discover-worlds-oldest-wooden-structure/
This article reminds me of the thought that I’ve often had when people maintain that the Earth has been visited by ETs.
Yes ET has visited the Earth, they arrived 100 Myears ago and stayed for 300,000 years. Their technology was closer to biology than ours and they could completely break down and recycle materials to the molecular level.
Their recycling technology and the 100 Myear gap between their time and ours is why we don’t see any evidence of their presence.
In terms of traces that we might leave behind to be discovered by far future archaeologists. I remember seeing a doco that mentioned that gold bullion stored in underground vaults could last for millions of years and might be the longest lasting trace of our presence.
Or their visit was to leave a legacy in a different way – “uplifting” species like Clarke’s aliens using the monolith (2001: ASO), or Kneale’s arthropod Martians breeding big-brained hominids (Quatermass and the Pit). Or even earlier like Scott’s “engineers” (Prometheus) or the Martians seeding life on Earth (Mission to Mars).
Thomas Gold’s “space garbage” theory of the biological wastes from visiting ETs fits this category.
I’m reminded about a comment made by J B S Haldane when asked what might disprove evolution and he said: ” Fossil rabbits in the Precambrian.”. Finding an artifact or other evidence of technological life on Earth would certainly upset our view of the evolution of terrestrial life (see comment by Ron S.). Alien visitors would bypass that argument, although I have to wonder if they left other artifacts of their visit – in space (satellites? discarded objects in orbit, or on the Moon?), but without contaminating the Earth with any of their biology that might be detectable even today. Immaculate visitations by aliens are a bit like “perfect murders” – there is always some evidence left however careful the agent is. Robot visitors would have a better chance of leaving no trace, but why would they want to colonize a living world?
Hi Paul
A very interesting read along with everyone’s comments
Thanks Edwin
Two thoughts.
First, if a species of dinosaur had reached a Middle Ages level of civilization, using wood, stone and minor bits of metal, we’d probably never know.
Second, the Silurian Hypothesis probably applies to any inhabitable worlds we may find/visit in the future.
Look for artifacts on the Moon. If there was a previous technological civilization, is it not likely they would have gone to the Moon?
I had the same thought. It’s interesting to think that we might confuse a technology found somewhere in our Solar System for an ETI ‘lurker’ probe, when in fact it could have been created in our own planet’s past.
Here’s a silly thought. If our larger-brained Neanderthal cousins had got their act together and left for the stars 100,000 years ago, even at just barely over solar system escape velocity, they’d be 14 light years away by now… and could have reached more than 30 nearby stars. But those ‘Silurian’ dinosaurs who left 65 million years ago would be nearly 10,000 light years away… I suppose that is not quite as far as Star Trek Voyager’s Voth had traveled, but then they had transwarp ;)
A clue for Earth’s past would be the widespread occurrence of only one, or very few, fossilized animal species. Humans wiped out, or contributed to wiping out, contemporary megafauna. I have not read of a period in which only one species of dinosaur dominated the fossil record.
Under the right conditions, even the most delicate things can be preserved as fossils (such as invertebrates, or leaves) so it is not unreasonable to expect a prior civilization may have left evidence of its existence. But lets not forget plate tectonics does not only bury things deep underground, it dissolves them back to their component atoms. Is there any part of the earth’s crust that has not been subducted and erased?
I know there are places on earth where rocks billions of years old are lying on the surface, but I don’t think that record is complete all the way back to the planet’s formation, and those sites become increasingly rarer (and smaller) the older they are. Others may exist intact, but lie covered by miles of sediment or cooled lava, or in the deep ocean basins.
Its possible other civilizations, either native or immigrant, may have existed on earth in the distant past, but we have no guarantee any evidence of them has survived. And if any does survive, it may be in places we may never look, or cannot look at all. And the further back we go, the longer-lived, the more widespread, and the more technologically advanced they must have been for them to have left traces of their existence.
It seems ironic, but some of the most primitive of our artifacts (graves, ancient campfires and stone tools) seem to be those who can survive the longest.
What if Chicxulub was a dark forest strike on a Silurian predecessor?
Tool manipulation and usage to modify the environment, and speech with its transcription to longer lasting symbols to transmit and preserve conceptual thought might both be regarded as hallmarks of intelligence.
How far away in time and space would they be detectabre? Efficiency may work both ways. by permitting more modified matter and energy to be used for longer and more widespread to enhance detectability. Or efficiency may reduce the disruptions of matter and energy thereby reducing their detecctability.
The search for alien intelligence should take both aspects into consideration.,
LLMs like ChatGPT suggest otherwise. They are artifacts of intelligence, but not indicative that the LLM itself is intelligent. A few years ago there was a controversial claim that Facebook’s[?] learning algorithm was creating its own language.
Suppose an organism that demonstrably used language, used some other means to translate its speech to some sort of non-symbolic storable form that could be interpreted by other members of the species. Would that keep the species outside the hallmark of intelligence or not?
OTOH if you trained a parrot to not just repeat human phrases but scratch some sort of symbol into the sand that corresponded to each word or phrase, is the parrot intelligent or not?
Chimps taught sign language can teach it to their offspring. That this does not involve storable symbols, does that eliminate them as intelligent?
I would prefer to modify your statement to “hallmarks of human intelligence.” It would recognize that our intelligence may be just one form that could be recognizable but that other forms may be recognizable.
Apart from the decoding issue, it is one reason that I think interstellar beacons should use recognizable images such as a picture of their home world, themselves, or anything that might be universally recognized by species using light to see.