How Long Will Humanity Last?

An upper bound can be generated somewhat easily. The sun continues to grow larger and larger, and one day it will become so large, in what is called the red giant stage, that it will consume the Earth. The Earth will have become thermally uninhabitable long before then. This means that humanity, if it wishes to continue to survive, must move somewhere away from Earth before Earth's thermal death. This event is billions of years in the future, and it is much more likely that some event will happen before then to threaten the existence of humanity.

The title question might have several answers, depending on the lifestyle situation of the people. Since it began, human society has been progressing through different forms, which were controlled by the development of technology. The earliest days of human society were back in the Stone Age, many millennia ago. Then someone invented a process for producing copper and then bronze, which allowed some changes to society. Following these developments, iron was discovered, probably simultaneously with charcoal, which permits smelting at a higher temperature. Agriculture was started somewhere in these early ages, and animals became domesticated, both as food sources and as work animals. Wind began to be used, with sails for water transportation and mills for grinding grain. Flowing water was also harnessed for mills.

All this set the stage for the discovery and utilization of fossil fuel energy, first coal, then oil and finally natural gas. These fuels led to a large increase in excess energy, and science and technology began a very serious period of development. Technology led to the utilization of steam power, and later electricity was invented. This led to a plethora of inventions, including electronics and microelectronics, which led to computers. The stream of inventions continues to flow strongly and constantly. This involves the mining of a great many different ores around the world and their transportation.

We might ask: How long can humanity survive at the agricultural level, where more concentrated forms of energy have disappeared? We might also ask: How long can humanity continue to exist at the high standard of living we now possess? But this ignores the obvious fact that society will continue to develop new scientific breakthroughs and technological discoveries. The rate of change is astounding, compared to earlier eras; it is unlikely that society only one century from now will be very similar to what it is now. This problem may seem unsolvable, but it is not. It is fortunate for our discussion that science is not infinite, but instead there are only a finite amount of discoveries that can be made. As more are completed, diminishing returns sets in, until some asymptotic value is reached. Science doesn't stop entirely, but since almost everything will have already become known, there are only some smaller details that need to be figured out. Thus, we can also ask, how long can humanity survive at the era of asymptotic technology?

Society might be capable of destroying itself, so we need to ask these three questions under the presumption that humanity has started thinking about its long term future and can make the choices necessary to avoid damaging its future prospects or even committing social suicide, as with a nuclear war or horribly malevolent virus. Now the questions appear to be more nicely framed: given that humanity adopts the goal of lasting as long as possible, how long would this be for us, at an agricultural level, at the current industrial and electronic level, and at the asymptotic level?

What exactly does it mean to make choices to promote the long-term survival of humanity? It is necessary to come up with a list of events that might put an end to us and then see what mankind might do to avoid them. At the agricultural level, having the soil become depleted and gradually produce less and less food every year is one possible problem. This problem is quite noticeable and could be coped with, over the course of many decades, by the means already devised for preventing soil death. Growing fewer crops is the simplest solution, and growing non-edible restorative crops certain years is another. But there must be enough food to feed the whole population, so the essential answer is not to allow the population to exceed the number that can be fed using sustainable practices, in the worst years of agricutural productivity. It would also be worthwhile to develop storage for a few years of food to allow humanity to get through the very worst years, which might be years in which volcanic dust so fills the upper atmosphere that sunlight is blocked and crops fail all over.

It would be most interesting if a few groups of people who like accounting and agriculture would make the calculations necessary to give us an indication of what the population and storage numbers might be. Their results would depend on some assumptions about transportation. If one region of Earth suffered from, say, serious floods, could other regions supply them for a few years by sailboat shipment of food and other recovery supplies? This might affect the calculations. As a wild guess, based on historical numbers, a half billion might be the maximum headcount for our planet in these restricted circumstances. There would also be a minimum number of people necessary to maintain all the operations of this future agricultural society.

This level 1 future society would be vulnerable to catastrophes. There are many, both geological and astronomical. The one most familiar is that of a large asteroid that somehow manages to score a direct hit on Earth. If it were large enough, all large lifeforms would die out. The asteroid might rupture the crust, leading to what is called a basalt flood, where lava from the mantle flows to the surface for thousands of years, polluting the atmosphere and killing off much plant life. Basalt floods can also occur without the intrusion of any asteroids, and there is evidence of them in many parts of the world. Another possible catastrophe is a nearby supernova, sending enough gamma rays to the surface of the Earth to sterilize everything. Less well known would be the passage near our solar system of a black hole or star, which might disrupt the stable orbit of Earth, causing it to move to a different radius or change its eccentricity, rendering it uninhabitable. A massive ice age could also lead to an end to human society.

A level 2 society depends on having sufficient energy to run much machinery and computation, enabling a higher standard of living than can be accomplished with an agricultural-only society that lives on sunlight alone. Fossil fuels are what have brought us to this standard of living today, but they will be depleted soon enough, probably a few centuries more at the most. Nuclear power does seem sufficient to replace fossil fuels, perhaps with some optional additional contribution from what is called renewable power. This includes hydropower, wind farms and solar panel acres. It is not clear what will be the constraining resource in such a society. Perhaps it will be thorium, perhaps lithium, perhaps rare earths, perhaps something entirely different. These calculations cannot be made at the present, as we do not know exactly how such a society would be organized. The profligate use of resources would certainly have come to a halt, but there would necessarily be some resources used every year, and some one of them would run out sooner or later.

Such a level 2 society would also be subject to extermination by the same list of catastrophes that the level 1 society would, but there is a saving grace. It may be possible to establish self-sustaining colonies on other planets or satellites, either within or outside of our solar system. This project is even more hazy than the project of revising society to conserve resources and be efficient at energy use. It might be possible, but there could also be some unforeseen barrier to such colonization. We are too immature scientifically to make a good call on this question. However, the length of time that humanity can survive is very dependent on the sustainability of such an insurance colony. If the mean time between life-destroying catastrophes is a few million years, having a colony somewhere would multiply that many times over. It is not even necessary that the colony prepare to reconstitute life on Earth after the catastrophe, as long as it can continue to survive and someday establish an insurance colony of its own. A recent book, “Looking Back from Luna”, is built on the hypothetical possibility that a sustainable colony can be established on the moon, and one theme of the book is the discussion of these colonists about what they might be able to do were a catastrophe to strike Earth, but not them.

The level 3 society takes some real imagination to contemplate. This society would have gone through several future technological revolutions. These would likely include a genetics revolution, after which genetics would be as controllable as software and would become industrialized like software. It would also likely include a neurological revolution, where we learn how to maximize both the non-verbal and verbal learning of humans, and thereby increase their resulting intellectual capability. This would have education and psychological developments as fundamental components. There would also likely be an organizational revolution, where politics is replaced by some form of governance that looks forward to the long-term prospects of human society.

Even though there are many, many options for organizing such a society, and few of them could be determined by us now, they would all still be subject to the similar limitations on energy and mineral resource use, just as a level 2 society would be. This means they would have similar expected endurance as a very-high-tech society as level 2 would have, within a factor of ten most probably, and would require the same option of a self-sustaining colony as insurance against catastrophes.

So the answer to the title question is much the same for all levels of society, and it is the mean time before a catastrophe hits. Is this a million years? Can Earth's resources be stretched to last this long? We are not even capable of making this estimate except as a simple guess. It is clear, however, that for levels 2 and 3, if the population becomes serious about having humanity last for a tremendously long time, measured in millions of years, figuring out how to establish colonies on other planets, which would be self-sustaining for long periods, is perhaps the most important project we can undertake.

Ancient Civilizations with Different Cultural Bases

When we think about the existence, ten or twenty thousand years ago, of an ancient civilization and ask what signs might be left behind, we imagine our own civilization. But there is no reason that an ancient civilization could not have been built around a very different cultural basis. In other words, in the pathway from hunter-gatherer tribes to city-dwelling, tool-using, more civilized ancestors, there may have been a fork in the road. We went one way to get to where we are now, and they went another way. The possibilities need to be explored, or else we will just be out looking for signs of our own type of civilization ten thousand years before but extinct, and miss the signs of a different type of civilization, with knowledge and capabilities similar to our own, but with a different set of foundational rules.

One thing that must be the same is the science. Once a civilization has realized the periodic chart of elements, there are no more elements to be found. The exact same periodic chart will appear, given time, in all civilizations. The same holds for the laws of nature. Newton's laws would not have been named the same in ancient civilizations, but the mathematics which describes them would be the same. Telescopes are the tool of choice for observing the planets and galaxy, and optics is optics in any civilization. Combustion makes heat, which can be turned into motive power, in any civilization. So, the science and the engineering it enables would be the same. A society might be rated as to how far along the line of scientific progress they have travelled. There is some variation possible here, as biology might go a bit slower and astronomy a bit faster in one society compared to another, but there is only so much science, and a civilization which lasts long enough gets it all figured out. We are a couple of centuries from 'asymptotic technology', the point were there are only some small details left unknown, but an ancient civilization might have had cities and science study for more centuries that we have had, and have reached the culmination.

If science and engineering are the same, what can be different? Perhaps nothing is different in civilizations which reach and pass 'asymptotic technology', and the main difference is where a civilization is on that route. This concept is called 'technological determinism' and says that technology is the driver for cultural changes, and when a society picks up some new chunk of scientific knowledge, it will inevitably be changed by it. That means that societies would converge to some final state, not too distinct.

So, we might rephrase the question and ask, where will technology take us, and assume the ancient civilization was there already. We don't know where technology will take us, but it is possible to make some guesses in this regard, as much of technology has been worked out already, and more is being done every year, showing us a direction.

Figuring out where our own society is headed is a fascinating activity, and has been done by many fiction writers, who usually don't have 'technological determinism' and 'asymptotic technology' in their vocabularies. This means they would be very lucky if they are correct, as the fundamental rules by which society develops, at least after the dawn of the scientific method, provide a great deal of information that makes a great deal of difference.

Perhaps we might just ask a more pertinent and relevant question: what would an ancient civilization be like if it left almost no records behind. The obvious answer is 'small.' If an ancient civilization existed, say fifteen thousand years ago, but had only, for example, a half million total population, it would be much more likely that there would be no evidence left behind that indicated they were here on Earth before we were. That might mean three small cities, which could easily have been wiped away by environmental factors.

Does advanced civilization mean giant population? In most science fiction, populations are large. Population has been growing exponentially for hundreds of years, so why would it not continue? The reason might be that an ancient civilization asked itself what population it wanted to have, and the population of that civilization was wise enough to adhere to the answer, and limit or reduce its population to whatever choice was made.

What possible basis could they have had for making some numerical choice as to their own population? Why not ten billion or ten thousand? What kind of reasoning could they have used to fix this target? In our society, population is determined by billions of choices, as each couple decides how many children to have. In an ancient civilization, the same might have been true, but the mode of decision-making could have been different. One factor that could have been in play was the resources on planet Earth. Ten billion people use them up a million times faster than ten thousand. In our society, few people talk about the concept of resource exhaustion or anything else in this subject area, and those who do mention it note that resources are finite and there must be some stopping point in growth, perhaps followed by decline.

One difference between our society and the ancient one could be that they had this discussion much earlier in the population growth curve, and never got up to the hundred million point. This would explain why there were so many easily accessible resources left here for us to find and exploit. If their population had maxed out at a quarter million, there could not have used up much resources by their demise, and therefore we do not see some ancient quarries or remnants of mine openings or spills of petroleum or anything else that would be a signature of a huge population, with a high standard of living but little care for resource conservation.

The next step is to ask, what would a small population do, if it anticipated some catastrophe in the near future, and wanted to leave something behind? Another question is, suppose they didn't care about leaving behind some monument, what might have lasted ten or twenty thousand years which they created and used, not for the purpose of communicating with some new civilization in the future, but just useful for their living or important for their art or whatever else they valued? Since the non-exhaustion of resources is an important clue as to the nature of their society, we can ask about what other features there would be that might have the same origin as this decision.

Of course, another question is, were they from Earth, but that one needs to be put off until later, although an alien colony might have the characteristics we have found to be likely, low population and minimal resource usage. That would mean a modification of the original premise of this blog, which was why we haven't seen any aliens to a whole host of other ones, such as why would an alien civilization form a colony on a planet that was prone to catastrophes?

What Lasts Ten Thousand Years?

You can walk around in monuments two thousand years old in Egypt. Archaeologists dig up mounds with relics from five thousand years ago. If we wanted to know if there was an advanced civilization somewhere on Earth ten or twenty thousand years ago, what might be found that could tell us at least where one of its cities were and even better, tell us a little about it?

If the ancients wanted to just let everybody in the future know they had been here on Earth, they might just make a huge block of tungsten, and set it up somewhere so that any following civilization that could identify metal would know that this was not a natural object, and could not have been made by some set of hunter-gatherers. Finding and refining out a cubic meter or ten of tungsten requires some serious metallurgy.

If we assume that the ancient civilization had some smart people, and for whatever reason, they wanted to leave a mark on the planet that they had been there, they would certainly have tried to think through all the possible events that might happen between the time they build their marker, and the time that another civilization would get to be sufficiently advanced to know what it was they were looking at. They wouldn't have thought of building the marker, or cared about it at all, if they didn't have the premonition, or more than a premonition, that their civilization wasn't going to survive much longer. They would have recognized the threat and deduced it was unavoidable. It is hard to imagine an entire civilization disappearing, and we don't have any clue as to what might do that, just some concepts that are perhaps possible. The ancients would be thinking of how to build a marker which wouldn't be engulfed by whatever was going to do their civilization in, and they also had to think about other possible catastrophes that could occur in the inter-civilization period.

Perhaps thinking of an example would make things clearer. Suppose the ancients were good at astronomy, and had noticed that there was an asteroid, far out in space when first discovered, that was going to make a direct hit on Earth, and it was big enough to annihilate almost everything. They had the telescopes to detect these, and had been detecting them for long enough to predict orbits very exactly. This means they might have been at least a couple of hundred years older, as measured from the advent to telescopic astronomy, that we are now on Earth.

Asteroid strikes have happened many times before to Earth and a prominent theory of why the dinosaurs stopped ruling the Earth and gave way to mammals was that a large asteroid hit the planet, landing in the Yucutan or just offshore, producing such a chaos of heat and dust and shock waves and tsunamis and earthquakes and vulcanism and lots more that dinosaurs couldn't survive. Some tiny mammals figured out how to, and they led to us, after another 66 million years of evolution. The asteroid for our example couldn't be this big, as there would be a geological record, but it couldn't be too small either. If it hit the deep ocean, there might not be any crater to find and no clues like the iridium layer that Alvarez found as a signature of an impact event for the Yucutan strike. An ocean impact would flood all coastal terrain, and create a huge amount of hot water vapor in the atmosphere, which would probably mean rain for a long, extended period, almost everywhere. The temperature would rise and stay up for a long time, as the Earth slowly returned to its pre-impact situation, less most life.

Orbits of asteroids vary in their periods, and some of the larger ones go far beyond the gas giants, and take a hundred years or so to cycle back to the near planets. So the ancients might have one or more centuries to plan how to make their marker, and could think long and deep about its preservation for millennia.

One initial question would be: where to put the marker? It couldn't be anywhere near the coast, or inland as far as the tsumanis would reach. It couldn't be inland anywhere that would be washed away by huge rains, which could flow in existing rivers but also might find other paths to the ocean. It couldn't be anywhere where the crust was thin, as there could be volcanos caused by a rupture in the crust from the impact.

Then there is the problem of ten thousand years of dust falling down on it, perhaps burying it. If it was put on a pedestal, that couldn't be too high and thin, as it might be tipped over. Maybe it should be huge, so the erosion of time would still leave something recognizable. If it was huge, it couldn't be made out of a single metal, like tungsten. Maybe there could be a cap of tungsten at the highest point. Rock easily stays around for ten thousand years, but it can't be too ordinary or the follow-on civilization might think it was from some so-far-unexplained natural phenomena. Before they became sophisticated enough to appreciate what the marker was, they might just think it was another opportunity for quarrying. Then the marker would wind up in parts in places that needed defensive walls, or temples of rock, or anything else extremely solid. Many archeological sites have been victimized by humans, in recent centuries, who had no interest whatsoever in preserving the past but a great interest in finding things that could be sold or used for their own purposes. Incan sites have been especially victimized by those bent on re-use of good materials.

One way to prevent re-use of the marker monument rocks would be to make them too big for a second civilization to use in its earlier period, before they became sophisticated enough to appreciate the preservation of ancient structures. They could also make them into something that could be re-used itself, or added to for re=use, rather than disassembled and carted off. Perhaps they would try to make their monument impressive, with giant statues of solid rock, so that the new civilization would think twice about abusing it. A new civilization might then try to make use of the monument for some purpose, like a temple or a royal palace or something else, and the new monarchs might even claim they had produced it, rather than found it after millennia of being ignored and abandoned. Perhaps a block of tungsten or titanium is the wrong approach, and something that caters to the likely situation in the new civilization's early years would be better. Writing on the monument might be fruitless, as the new civilization will need a long period of development before writing is established and they recognize what those markings are. So, a simple monument, made from large whole rocks that were of the hardest kind that could be used, perhaps with some statues, might be the final choice of the ancient civilization as they faced their doom.

Maybe there might be some remnant of their cities, or vacation spots, or ports or something else which survived the catastrophe and the following millennia, while the few remaining humans went through a return to prehistoric living conditions and gradually re-invented civiization. Sounds like some excellent archaeology needs to be done, and some careful scrutiny to make sure a misclassification does not occur.

Why Now?

Asking the question about whether there could have been a more advanced civilization of humans that was eliminated in a catastrophe, or related questions, leads to some deeper ones. Why did intelligent humans evolve at the time they did? Why didn't we evolve into city-living, culture-appreciating, educated, adept, clever humans two hundred thousand years ago? What delayed our approach? Why weren't we delayed another hundred thousand or two years? Why now?

One way of looking at this is to examine the preconditions for the final leap of evolution, to thinking brains and everything they required, and see when they arose for the first time. The simplistic solution is to just make a list, accurate as possible, of what steps led to humans and see why one of them couldn't have happened earlier.

One detail needed to follow this approach is to decide just where on the taxonomy of animals intelligence could have arisen. The pat answer is that we needed thermally regulating bodies so our brains didn't turn off in the winter. Why is this true? What about evolving in a region with fairly constant temperatures over the year? Perhaps it is a day/night temperature difference that excluded reptiles from becoming intelligent. Suppose some lizard had a complex brain, but could only think during the day when temperatures were warmer; why is this an impossibility? During colder temperatures only the lower brain stem, which is what today's reptiles have, was working. That part would allow the reptile to live like other dumber reptiles, except when temperatures got warmer, and then it could think great thoughts.

If we cannot determine some incontrovertible reason why reptiles couldn't have become more intelligent, the boundary of time when intelligence could have started is pushed back, hundreds of millions of years, when, supposedly, reptile species ruled the entire planet. We should ask: what good would being able to think more complex thoughts do for a reptile? Their ability to survive and reproduce depends on their visual skills, their speed, their ability to recognize hiding places, their ability to capture prey using body and head muscle linkages with eye coordination, and perhaps a few other things. Nowhere in this list is anything that a complex thought might help. Compare that with chimpanzee-like species which could begin to use found objects and then shaped objects as tools. Tool-using elevated species from chimpanzee level to human level. Current eptiles don't have the physiology for that.

So, could we have reptiles of millions of years ago, those who were living in forests, take an evolutionary jump to climbing trees and developing opposing thumbs and dextrous hands? If evolution could do this, why not, over another million years of evolution, could they not develop thermal regulation to some extent? Thermal regulation requires energy, and could reptiles become better hunters or more complete omnivores, and simply follow the pathway to intelligence that proto-chimpanzees would follow millions of years later? Why weren't the steps needed for intelligence, whatever they might have included, completed long ago, in the millions of years scale.

Perhaps evolution couldn't make the total number of jumps needed for this, simultaneously. Was the jungle many millions of years ago more hostile to the growth of intelligence that the forests of a few hundred thousand years ago? What about hands? Some animals climb trees using their claws, which penetrate into the bark or catch on irregularities in the bark of trees, and evolve so that this method improves, as opposed to developing grasping hands, which is a totally different evolutionary path.

Without grasping hands, evolution couldn't take one of its sideways steps. A sideways step in evolution is when a species either mutates its genome by moving one section to another place, perhaps copying it there, which then allows the species access to some new capability, not related to the one for which the genes had evolved for. We can think of the software side of evolution, which is what happens when one generation imparts some wisdom to the next one, which allows the newer generation to use its mental and physical capabilities in a task that it wouldn't have, without the training.

What else in evolutionary pressure serves to force hands to develop? If the species lives on fruits and other pickable objects, hands might be useful here. Alternately, if the animal simply eats leaves and flowers for nourishment, then hands don't play much of a role and wouldn't be selected for in the evolutionary process. Fruit provides more concentrated nourishment that leaves, as do seeds and some roots. Was food selection the problem that kept reptiles from becoming intelligent millions of years ago?

This doesn't sound correct. Why couldn't reptiles evolve to eat fruit and seeds, if primates could? Were there fruits around millions of years ago in the equivalent of forests?

Perhaps the question should be asked in a completely different way. How do we know that some lizard species did not develop intelligence of some sort two hundred million years ago? Would there be anything detectable this many years after they became extinct? Perhaps the intelligent lizards lasted a million years and build cities. What kind of rubble lasts two hundred million years? Do we know how to do excavations to figure out the answer to this question?

One thing we do have is fossils. Fossils occur when some animal does some stupid thing and gets caught in some mud and dies and then the mud turns to stone. Because of some perversity of nature, braincases are not often found in fossils. But recently some have.

To be intelligent, one needs a large brain, measured in terms of brainweight to bodyweight. Some recent finds of reptiles raises the possibility that some of them may have larger brains that has been expected by the earlier-discovered fossils. If we assume that civilized reptiles two hundred million years ago managed to largely avoid getting stuck in mud pits and turned into fossils, then their absence in our dinosaur skeletal displays in the different natural history museums around the world is understandable.

What else might be left behind from a civilizatin of intelligent creatures that lived for a million years and died out two hundred million years ago? What might get buried and refound that would last two hundred million years? For early human civilizations, we look at burial mounds. These are put together in the first few thousand years of civilization, and then everybody stops doing it. Inside these burial mounds there are gold ornaments and jewels, which might be contenders for enduring the forces of nature for millions of years. Would a civilization that lasted much longer not simply collect these things from their own archaic burial mounds and put them in a museum? And since the Earth changes its profile in much shorter times that two hundred million years, moving dirt and rock and lava and water and any materials around on the planetary surface, how could we expect anything from an ancient city to survive. Maybe they invented materials that were more durable than concrete? Concrete might be good for tens of thousands of years, if no earthquake or flood gets to it. What is left after a short time such as a hundred thousand years? Rubble. Maybe there might be some chemical test to see if some rubble we find has some unique features? Rubble near the surface probably wouldn't stay in one place, however.

One thing we can detect for long periods, in very unique situations, is the materials embedded in layers of rock. That is how we suspect a large asteroid hit the planet some 65 million years ago, from the thin layer of iridium-rich deposits all around the world. Would the lizard civilization have put something into their air which would be detectable? It is very hard to think of any possibilities in this area.

The conclusion is beginning to look inescapable. There is no way to tell if we are the first intelligent species to emerge on Earth. All the hubbub that goes on about aliens on other planets coming to visit us might be expanded to ask if there were some 'aliens', of the homegrown variety, right here already. If it could have happened once, maybe it could have happened twice or more times. All of these things would leave no evidence. One result of realizing we might be the tenth intelligent species on Earth rather than the first is that we really don't have a good understanding of evolution yet. Maybe there are clues buried in the genomes of the organisms of Earth that indicate something intelligent was around a very long time before us. It is certainly not clear how this might happen, but we need to grasp at straws to answer this question.

Ancient Civilizations

Common belief here on Earth is that our civilization has been continuously improving since the human species came into existence. It has been a steady sequence of more population, more technology, more areas inhabited, more organization, more culture and so on. With that as our history, it is easy to project onto possible alien civilizations on exoplanets that they too had a uniformly improving history. Then some comparisons can be made, some calculations, and some predictions. That has been the basis of this blog.

What if this is all wrong? What if there have been one or more civilizations on Earth which were wiped out by one or more catastrophes? There are at least two questions that immediately spring up. One is about the evidence that might indicate this is at least possible and not ruled out by everything archeologists, geologists, and other scientists have collected and interpreted. The other is, provided the answer to the first is that the evidence for the simple single rise of civilization is not wholly compelling, what does this mean about potential alien civilizations? If our planet had one or even a series of catastrophes, wiping out mankind down to the hunter-gatherer level, and then mankind built up a following civilization virtually from scratch, maybe this happened on exoplanets as well. 

Why even consider this? There are some scientists and others who see something in the evidence available to us, overlooked to date, which indicates the progression of society has not been wholly linear. They noted that the level of the oldest stonework at a few sites appears to be significantly more capable than later stonework. They raised the possibility that there was a retrogression of technology at least once in the history of humankind. Instead of a fruitless discussion of the arguments involved, just suppose that it is a possibility. We might first ask what kind of catastrophe might destroy civilization but not lead to species extinction, not for humans or for any other noticeable species. Is there even a possible phenomena which could wipe out civilization without ending the human species? 

To be able to completely collapse after such an event, totally but not permanently, means that civilization is much more fragile that has been appreciated before. This fragility needs to be understood in terms of the civilization that existed at the time of the catastrophe. That civilization might have taken some different paths that made it more vulnerable that ours is. Or perhaps we underestimate the fragility of our own civilization. What could wipe out our civilization so that only hunter-gatherer tribes were left? Are there any unique events that could do this? 

There are natural catastrophes, like volcanoes, and human catastrophes, like a biowar which targeted food crops. The list of natural catastrophes is quite well-known, as only a few things could affect mankind world-wide. Ice ages could top the list, as there have been several major ice ages, and many more mini ice ages during the non-ice-age intervals. The causes of ice ages are not conclusively determined, but that is of no consequence to the determination of their effects on a civilization. Perhaps one of the most interesting factors is the albedo of ice. Since it is higher that that of uncovered dirt, vegetation, or ocean water, that means that if something happened to increase the fraction of Earth covered by ice, then the amount of heat received by Earth, in total, would decrease, and it would cool down more. This is a positive feedback loop, and could go either way. Orbital variations might be the trigger for this rapid change. The large gas giants affect the orbit of Earth, as an example, and change its eccentricity, and perhaps other parameters. If we look at the collection of possible Earth orbits over the last billion years, we would see there is a distribution, perhaps a bell curve, of the insolation averaged over each year. If the orbit of the Earth was at one end of the distribution of solar energy intercepted, the end where insolation was largest, the climate could snap from ice age to minimal ice in a short time. At the other end of the distribution, it could snap the other way. 'Snap' might mean less than a thousand years or even less than a few hundred. 

Ice melting on such a vast scale changes the sea level depth by something of the order of a hundred meters. If civilization had adopted mostly coastal cities, they would be wiped out in short order. Perhaps this is one candidate for a civilization-terminating catastrophe. Even a lesser amount of melting might drown most cities. Would this end civilization? 

What would happen to a civilization similar to ours if such an event were to happen? Perhaps over a few centuries, most cities would be inundated. People would have to move to higher ground. There is no question, at least on Earth, that there is bare land available for cities, but the benefits of the locations of the previous, now flooded, cities would not be available. These might be ports. A tremendous amount of our trade is by ocean transport. New ports might be available when the seas stop rising, but during the period of continuous rise: no one would be able to build a port which might be flooded in a few more years. So transportation would be seriously affected. 

The costs of building new cities would be very large, and perhaps enough to overwhelm the economy of Earth, or of any comparable civilization of aliens on an exo-planet with a similar ice age phenomena. Would the economy crash, or just degrade enough so that science and technology would be preserved, and the living standard would only decline a moderate amount, not entirely down to hunter-gatherer level. 

Some of this land would have been agricultural, so some food production would be lost. Many countries would be little affected, and others very seriously affected. Would this mean massive migration? Would it mean wars fought over who would control the remaining good land? On top of the possibility of the economy crashing or at least declining significantly, there is the possibility of war, where the holders of good land attempt to stop huge populations of those from inundated lands entering and taking it over. War might not be local, but in many different places, almost at once. If a faltering economy did not cause enough damage, widescale war on top of it might, and here is a possible scenario for a collapse of civilization. 

There are many other questions related to this issue, but they deserve a separate post.