If We Were to Vanish...

Several recent books and TV series have speculated on what the world would be like if humans were to disappear (now or in the near future). For the moment, let us not dwell on why we might disappear. Instead, consider that remains we are creating that will be present in the future fossil and geologic record. (Who might be finding that record? Alien explorers? The intelligent descendants of raccoons? It doesn't matter for now.)

The Body Fossil Record of the Anthropocene

Human population is vast: nearly 7.8 billion and counting, far higher than most large-bodied mammals. With greater population size, statistically there is a better chance that at least some humans will wind up buried in sediment, preserved by various diagenetic pathways, to be discovered later in future sedimentary rock.

There are some particular aspects of our habits that can potentially increase our entering the fossil record. In particular, many cultures ritually bury our dead, often with preservatives (embalming fluids) to reduce the rate of decay. Should the cemetery itself be buried, that means there can be dozens, or hundreds, or tens of thousands of bodies as potential fossils. The shared pattern of the position of the bodies, the presence of the remains of clothing and other possessions, the collapsed remains of the caskets, and so forth would indicate some form of intentional burial rather than a natural happenstance.

That said, natural burial does happen. Various types of natural disasters--flooding rivers, surges from hurricanes and tsunamis, debris flows from mountains, ashfalls from volcanoes--all can and do bury people and communities from time to time. If not excavated, these bodies have the potential to be future fossils. Similarly, but rarer, the small number of individuals who might be swept offshore or drown at sea (or in lakes) might wind up as fossils in marine sediments, so long as all or part of them is buried before scavenged or decayed.

An even greater number of future body fossils will come from the remains of our domestic animals. The situation for pets is somewhat similar to human body fossils (including the existence of pet cemeteries and of unfortunate pets buried in natural circumstances. Much like humans, most pet body fossils have the potential to be complete skeletons.

But the most common body fossils of our society will not be complete. These are the remains of the vast numbers of domestic food animals: chickens, cattle, sheep, pigs, and so on. Their bodies are definitely dismembered, but their bones are often disposed of as waste (and so potentially preserved.) (Should a farm be flooded, though, there is the potential for the complete sheep and cow and chicken skeleton.)

One important aspect of burial to remember is that sea level is quite low at present compared to the majority of Earth's history. (It is lower, of course, at a glacial maximum.) As the Greenland and Antarctic ice sheets melt in the future, the lowlands around the world will be covered in layers of sediment. It is unlikely that all the old cemeteries will be moved, so all of these in coastal regions (up to 80 m, when all the ice melts!) will be blanketed in sediment. Along with whatever else of your society's products were present there.

The Human Trace Fossil Record: Our Objects

Most animals produce more potential trace fossils (walking traces, feeding marks, coprolites, etc.) than body fossils. And this is true of humans, too: our footprints on the lake shore are just as likely to become a trace fossil as a dinosaur's was in the Mesozoic.

But we have one HUGE class of potential trace fossils that is guaranteed to leave a distinctive record: our artifacts. We make a lot of physical objects (without which archaeologists would be out of a job...) And we make far more per capita now than at any point in our history. Some of this material is biodegradable, and thus will only be fossilized in the same circumstances as body fossils (rapid burial prior to decay.) But other made objects are made of more durable substances--metals, ceramics, plastics, and so on--which might have a far better chance of surviving burial and recovery over geologic time.

Of important note is the fact we produce anthropogenic "reefs" of material in the form of landfills. Even a modest solid-waste landfill is much bigger (and has a greater density) than any individual organism, so they would be conspicuous features when buried. And, after all, we go out of our way to bury them (often under additional layers of sold waste, but also under soil. These will be distinctive deposits where at least some of the objects will retain a (probably quite compresssed) sense of the original shape.

In contrast, an aspect of our built physical environment that we might think of as quite distinctive and enduring won't have much record: our buildings. Some of the material we build with (such as wood) will decay quickly. Reinforced concrete (one of the main structural materials of the 20th Century) is already falling apart, and will break down into sand, lime, and rusted metal. Metal itself rusts, tarnishes, and falls apart when exposed to the elements. And stone buildings get weathered away by rain and ground down by wind and grit over time. The only buildings that will survive on geologic timescales are those which are buried by floods, rising sea levels, and the like.

At least with regards to the parts of the building that stick up, that is. Foundations are, by their very nature, dug into the Earth and thus are already buried. The same is true for mine shafts and subway tunnels. Such excavated features have a much longer term potential for preservation.

Any individual stretch of roadway is unlikely to survive for long without repair (as we all know after a bad winter, with the asphalt fractured with potholes.) But there are so many vast kilometers of roadway around the world, some of which is in regions with sandstorms or flooding from ocean surges or river floods, that patches of roadway will inevitably make it into the geologic record.

Another surface feature that we produce that might survive are open pit (i.e., "strip") mines and other anthropogenic erosional surfaces. These are often in areas of non-deposition (like mountains), but these landforms might be recognizable if re-excavated after landslides and other deposits have filled them.

Our most widespread physical evidence will be physically quite small: plastic and microplastics. Plastic tends to break down into smaller and smaller chunks, which get spread into ocean systems. Already layers of microplastics have been identified accumulating in sediments in the sea floor. Since this material is ubiquitous, it will be an important future marker to correlate deposits in the terrestrial realm, the near shore, and even the deep sea. There are already specimens discovered where chunks of plastic have been cemented together by lava or by natural carbonate cementation, producing plastiglomerate. This (along with concrete and asphalt) represents a type of anthropogenic rock.

Archaeologists have long used the appearance, spread, and disappearance of distinctive tools, pottery styles, and other artifacts for correlation. Future stratigraphers might be able to do so with the wide variety of made objects that we produce, many of which will have distinctive chemical signatures.

One particularly distinctive chemical signature will be found in nuclear waste. We already bury this material in vaults to keep it from interacting with the surface and with ground water, and thus it will last for a long time. On geological timescales its radiation will be greatly decreased, but its unusual chemical composition will point to a process unlike those in natural systems.

Should there eventually be a full-scale thermonuclear war, these kinds of materials will not be limited to nuclear dumps. They will be concentrated at the sites of destroyed cities and military and industrial facilities. But the uncontrolled fires from these places will spread nuclear fallout around the world, leaving as distinct a layer as the iridium spike at the end of the Cretaceous.

However, future stratigraphers do not need us to destroy ourselves with nuclear obliteration for there to be an anthropogenic event horizon. We actually have already produced several such geochemical indicators, guaranteeing that distant geologists will be able to pinpoint in the rock record when our society existed.

The Human Geochemical Record

MODERN stratigraphers can already see the presence of anthropogenic effects in the youngest strata. For instance, industrial smokestacks release fine particles that contain various metals (lead, vanadium, copper, zinc, etc.). Although these are in trace levels, they can be picked up by analysis of sediments being formed at various parts of the world. Similarly, different forms of industrial activity also produces and releases various complex organic compounds that similarly are traceable in the youngest strata worldwide.

The burning of fossil fuel (which is derived from organic material, and thus lighter in ¹³C than is non-organic carbon) has already caused a negative carbon isotope excursion nearly as big as that at the Paleocene-Eocene Thermal Maximum. This will be recorded in carbonate sediments, mammal teeth, and other geologic materials. Similarly, the oxygen isotopes of materials being deposited now and throughout the Anthropocene Event will track the rapid increase in global average temperature.

And just as the PETM was discovered by the disappearance of foraminiferal ooze in deep sea cores due to ocean acidification, so to the modern ocean acidification will have a similar presence in future deep sea cores.

We've already looked at the Sixth Extinction. Consider it running its full course. The survivors will on average be smaller than today's wildlife, and obviously smaller than those of the Pleistocene. This is due to the fact that nearly all the larger animals on land and sea are endangered or threatened.

So a post-human world will (at first) lack most or all of today's animals greater than a ton or so: the elephants, rhinos, hippos, giraffes, large whales, large sharks, etc. All of our closest kin (the other surviving hominids: orangutans, gorillas, chimps, and bonobos) are endangered, and are unlikely to survive without specific concerted and sustained support on our part. Similarly, nearly all the big wild carnivores (bears, dogs, big cats, and hyenas) will die without human aid. And the combination of rapid global warming and ocean acidification will destroy most (and very possibly all) the modern coral reef communities.

Not all organisms are suffering, though. Domestic animals are doing well, and a portion of these escape into the wild and survive as feral populations. Invasive species in general have a proven record of survival in a new habitat, and are likely to persist once present. And there are some species with very broad tolerances, which can even thrive in anthropogenic habitats: coyotes, raccoons, opossums, grey squirrels, various parrots, pigeons, Eurasian starlings, the Australian white ibis, cattle egrets, to name just a few.

A result of the spread of intentionally introduced and accidental invasive species is a globalization (or homogenization) of the world's flora and fauna. This would be recognizable in the future, as the fossils of the same suite of animals and plants will "instantaneously" appear around the world.

What will life after us look like? People have speculated about this. Perhaps the most famous of these is the 1981 book After Man: A Zoology of the Future by paleontologist Dougal Dixon. Written to teach basic concepts of evolutionary biology in an entertaining way, it gives a sense at what we would expect in the Earth's biota after the devastation of the Sixth Extinction. As with previous mass extinctions, the foundations of the post-human ecosystem will be the survivors of this one. They won't be the biggest, strongest, and fastest of the older forms, as most of those specialists will have gone extinct. Instead, the smaller, generalist, faster-breeding taxa will be the ones to more likely survive and radiate into new forms.

It wouldn't take particularly long for species to evolve larger body size. For comparison, the largest mammals after the K/Pg extinction were around 2 kg; within a million years there were ones that were 50 kg, and another million they were 100 kg.

Thus, there would indeed be a big turnover-pulse centered on the Anthropocene. The extinction of today's biota, followed by a globalized diversity of generalists, and with sufficient time a radiation of new forms to fill the old niches.

Many of the changes above will happen whether humanity survives or not. But would humanity be likely to go extinct?

On the one hand, there is a lot of gloom and doom that focuses on the detrimental effects of our industrial technology. And it is notable that civilizations (that is, specifically, a culture that builds cities) are highly specialized systems requiring complex economic and resource links (stable food and energy supplies; raw materials for construction; efficient waste management; public health systems; transportation systems; etc.). Exhausting any of these could bring a civilization to an end.

And sadly throughout history Civilization has been essentially a purely-extractive enterprise. Regardless of the particular cultures involved, the city-builders have focused on the extraction and exploitation of new resources. But ALL natural resources are ultimately limited in supply and have finite rates of replenishment. (Heck, there is a growing supply crisis of industrial grade SAND, of all things!) Most of the geological resources replenish only at geological (or even astronomical) rates. For extractive societies to survive, they would eventually have to drain the Earth of all resources then move on to other worlds in the Solar System. In contrast, regenerative civilization--one that emulates ecosystem processes--might indeed survive for much longer durations, since it would be sustainable.

Something very important to remember, though, is that "Humanity" is not the same thing as "Civilization". We've only had cities for around 6000 years; our species is over 300,000 years old. Some forager and agrarian societies can persist for millennia; by definition, they have become sustainable, at least for the conditions they are in. Maybe Humanity's best bet for a longterm survival isn't as Civilization, but as low-impact post-civilization cultures.

Since H.G. Well's classic science fiction novel The Time Machine there has been speculation about future speciation of humans. However, there are many reasons to suspect that is not particularly likely, barring some rare circumstance. Homo sapiens owes our success in a great part to our broad environmental tolerance and our great capacity to disperse. (Keep in mind, we colonized nearly the entire planet with stone age technologies.) While a post-civilization diversity of humans would adapt to local conditions (just as historic ones did), it is unlikely they would be separated long enough to truly speciate. Our capacity and desire to travel to new lands would almost certainly bring disparate peoples back together within a matter of millennia. Perhaps the best prospect for speciation in our lineage is if there ever are interstellar colonies: since faster-than-light travel seems to be impossible, colonies of people in different solar systems would be isolated for tremendously long intervals.

In the end, though, humanity on Earth would eventually come to an end. There are devastating, K/Pg and P/Tr (or worse!) catastrophes somewhere in the millions or tens or hundreds of millions of years to come. Events on this level might take out even low-impact populations.

More significantly, life on Earth will come to an end, as will the Earth itself. A recent study suggests that the general decrease in the levels of CO₂ over geologic time will mean that the C3 plants might not be able to grow on Earth after 200 million years or so into the future, and that C4 plants (which do better in low carbon dioxide) might persist until 800 million years from now. But after that, plant life might die out (barring the evolution of new pathways.)

More importantly, the slow growth of solar intensity means that in about 1600 million years the surface of the Earth will be above the boiling point of water, and thus the biosphere (barring microbes living inside the crust) will die off. And after that, things will continue to grow more intense. When the Sun finally converts into a red giant (in about 5 billion years), it will engulf the Earth entirely.

Earth is a special planet, but it is likely not unique. Although alien life would never exactly duplicate the same evolutionary events that we had on our world, it is possible that animal-grade life exists on many worlds. (However, keep in mind that for nearly all of Life's 3.8 billion year history, it was microbial forms alone that were present; many worlds may be slime planets.) And a tiny fraction of the inhabited worlds may have technological civilizations.

But as exciting as it is to think that we might eventually meet other civilizations, the chances are against it. On geologic timescales, it is not very likely that two worlds would be at similar points in their history at the same time. So it might be that the "first contact" that we have with an alien culture is the discovery of remains in the crust of their planet.

Or that the first alien to land on Earth will only know Humanity from our fossil record.

Some of the most influential workers in terms of the scientific thinking about alien life make the tacit assumption that technologically advanced civilizations are a likely, even expected, outcome of evolution. Physicist Enrico Fermi was troubled by the fact that there did not seem to be evidence of alien contact, which formed his paradox.

Astronomer Frank Drake formulated an equation to predict the number of civilizations in the Milky Way able to communicate with us by radio. This equation starts with the number of stars in the galaxy, and then predicts in turn the number of habitable worlds, the number of those with life, the number of those with intelligence, and so on. Here is a version of Drake's Equation:

where:

• N = number of civilizations in the Galaxy with which we might be able to communicate
• N_* = number of stars in the Galaxy (estimated at around 100 billion)
• f_p = fraction of those stars with planets
• n_e = number of planets in those star systems within a habitable zone for life
• f_l = fraction of those worlds in which life evolves
• f_i = fraction of those worlds in which life evolves intelligence
• f_c = fraction of those worlds in which a civilization able and willing to communicate develops
• f_L = fraction of the lifespan of that world during which that civilization endures

(NOTE: traditionally, the first value is R_*, the rate of star formation per year, and the last value is L, the lifespan of a communicating civilization in years. I have shown the version of Drake's Equation used by Carl Sagan in his book and TV show Cosmos.)

The number of stars in the Galaxy can be fairly well estimated, and discoveries of recent years have given us a good idea of the frequency of stars that have planets; the rest of the factors, however, are very poorly constrained. Let's look at two alternative scenarios:

• A scenario in which life evolves in any world that it can, where intelligence evolves in about 1% of all living worlds, where communicating civilizations evolve about 1% of those times, and where civilizations typically endure for billions of years (i.e., 0.333 a planetary duration of about 10 billion years):
  • N_* = 100 billion
  • f_p = 0.5
  • n_e = 2
  • f_l = 1
  • f_i = 0.01
  • f_c = 0.01
  • f_L = 0.333
  • Then N = 3,330,000 communicating civilizations in the Galaxy. Drake favored results in the N=10⁴ to 10⁶ range. To which the Fermi paradox would then ask "Where are they?"
• A second scenario where all factors are the same, except that civilizations typically endure for centuries only (due to the probability of nuclear war, exhaustion of their resources, etc.):
  • N_* = 100 billion
  • f_p = 0.5
  • n_e = 2
  • f_l = 1
  • f_i = 0.01
  • f_c = 0.01
  • f_L = 0.00000001
  • Then N = 0.1 communicating civilizations in the Galaxy. In other words, we'd expect only one communicating civilization per 10 galaxies of the Milky Way's size.

What insights can paleontology and the other natural historical sciences (geology, evolutionary biology, anthropology, etc.) add to these values?

Life needs a place to originate, evolve, and survive. Life is highly entropic (it uses a lot of energy), so it almost certainly needs a world in which renewal of resources is common. This helps narrow things down a bit. For instance, we now expect that worlds towards the Galactic Core are unlikely to survive undamaged for a long enough time to develop and sustain life: too many supernovae and collisions with matter from those crowded parts of space! On the other hand, the worlds towards the Rim seem to be too metal-poor, and thus would typically lack the abundant radioactive materials which help start and drive plate tectonics. So probably only the middle third or so of the star systems are likely to have planets on which life can evolve.

The big problem we face is that we only have one instance of life known: all Terran life is descended from a common ancestor. So we have to be very theoretical for the frequency of worlds in which life evolves. Terran life is all carbon-based, but there is more complexity to our biochemistry than a macroscopic view would suggest! In terms of big things (animals, plants, fungi, protists), we have photosynthesizers that eat light, get carbon from carbon dioxide, and oxidize it with oxygen, and consumers that eat organic material, get carbon from organic material, and oxidize them with oxygen. However, there is a huge diversity of prokaryotes, some of which get their energy from hydrogen, ammonia, nitrogen dioxide, hydrogen sulfide, sulfur, and iron, get their carbon from carbon dioxide, and oxidize with nitrates or sulfates. And in principle there are several other alternative biochemistries not found on Earth that could work in principle. So lots of worlds might harbor life.

On Earth, at least, life seems to have evolved very early. Problematic traces of life go back to 3.8 billion years ago, and definite traces back to 3.4 billion years. So Earth has harbored life for about 75% of its history minimally, possibly much longer. This suggests that life is easy to evolve in conditions favorable to it. Or, to put it another way, life will be likely on worlds that could support the origination of life.

On the other hand, terrestrial life was single-celled for the vast majority of its history (87%). Animals only show up around 600 million years ago, and then only as very simple jellyfish, sponges, and worms. Animals first appear during a time of major environmental changes. It might be that life is easy to evolve, but that animal-grade life is far more difficult.

How often does intelligence evolve? As we have seen, tool-use, complex language ability, social structures, and the other hallmarks of technological human societies do not necessarily all show up in the same species. There's lots of potential out there, but even our close relatives among the homininans seemed to have been poor candidates for communicating societies: their technologies remained unchanged for tens of thousands of years, for instance. So Homo sapiens seems to have been the sole species to evolve on Earth capable of becoming a communicating civilization, and only during the last 50,000 years or so (i.e., the Great Leap Forward).

A common myth of our culture is that we (agricultural, technological societies) represent the ends to which other human societies were evolving. Instead, the evidence suggests that most human societies were relatively stable at the band-to-tribal level, but that it is the development of agriculture that allowed explosive growth of a small subset of societies. These societies have swamped the non-agricultural peoples around the world very rapidly from a geological standpoint. But only these sorts of societies would be capable of supporting the specialists required to develop a scientific, technological civilization.

What is the long-term prospect for such civilizations? (In other words, what are likely values for f_L?) In the short term, civilizations face the exhausting resources (energy, food, space, etc.), catastrophic alteration of their environment, nuclear (or other apocalyptic) war, etc. So a couple of centuries at communicating-level technologies might be a realistic maximum. On the other hand, should sustainable technologies develop, civilizations that last tens of thousands to millions of years would face the same sorts of perils that all ecosystems due: the potential for massive ecological change and mass extinctions driven by volcanism, asteroid/cometary impacts, etc. On the upper boundary, a super-civilization that lasts hundreds of millions of years or longer will face the end of its planet's tectonics and the death of the ecosphere as its homestar continues to grow hotter (as Earth faces in about 1 billion years time). For civilizations to survive for very long times, therefore, it is best to move on to other (preferably multiple) worlds!

How does Drake's Equation look now? We'll add a new value, f_h, to reflect the fraction of stars that exist in the habitable zone of the Galaxy. Here is a possible scenario:

• N_* = 100 billion
• f_h = 0.333
• f_p = 0.5
• n_e = 2
• f_l = 0.8 (Life is likely to evolve)
• f_i = 0.000001 (reflecting that intelligent life has only appeared in the last few tens of thousands of years)
• f_c = 0.01 (still a total guess as far as anyone's concerned!!)
• f_L = 0.000001 (civilizations lasting for tens of thousands of years)
• Then N = 0.0002667 communicating civilizations in the Galaxy. In other words, only about 0.03% of Milky Way-type galaxies would be likely to harbor a communicating civilization at any time!! (Even raising the duration of civilizations to the billions of years range only gets us to N = 2.667).

But that is only for communicating civilizations. We would expect about 2.667 low-technological civilizations/galaxy (assuming hunter-gatherer societies can persist for millions of years: multiply by 10 if they might be sustainable for tens of millions, and by 100 for hundreds of millions of years).

And why this obsession with communicating civilizations? (Okay, that's because they are the only ones that we'd likely know anything about in a reasonable amount of time!) The Earth did perfectly fine without behaviorally modern Homo sapiens for all but that last 0.000011% of its history. It is likely that there might be many worlds in which "slime" (i.e., microbial organisms equivalent to prokaryotes or protists) are the most complex organisms.

Ignoring intelligence, civilizations, and the like, what about the possibility of animal-grade organisms? Introducing f_a (the fraction of living worlds in which animal-grade organisms evolve), and f_La (the fraction of the planet's lifespan in which animal-grade organisms are supported), we find:

• N_* = 100 billion
• f_h = 0.333
• f_p = 0.5
• n_e = 2
• f_l = 0.8 (Life is likely to evolve)
• f_a = 0.2 (reflecting that animal life appeared during the last fifth of Earth history)
• f_La = 0.2 (reflecting Earth's potential lifespan of 10 billion years from formation until it is destroyed by the Sun's final expansion, and that animal life has been around for 750 million years or so and will likely persist until things get too hot for liquid water about 1 billion years from now)
• Then N_animal = over 1 billion!

Last modified: 19 January 2023