The Earth After Humans
All the worst things we can imagine doing to Earth — nuclear war, climate change, massive pollution, habitat loss and all the rest — may affect multicellular species like us but are unlikely to lead to the extinction of most major lineages on the evolutionary tree. Much of the biological world actually prefers conditions that are more extreme than the conditions we prefer or even those we can tolerate.
Nature — the existence of life on Earth, the diversity of ancient lineages and the ability of life to continue to evolve — is not going anywhere any time soon (which is to say, not in the next few hundred million years). What is under threat are the lifeforms that we most relate to and that are most integral to our own survival — the species we love and the species we need.
One Species’ Horror Is Another Species’ Ideal
When our own family on the tree of life, the hominids, evolved roughly 17 million years ago, average conditions were relatively hostile for many lineages, but not for our own ancestors. And by the time Homo erectus evolved about 1.9 million years ago, concentrations of oxygen and carbon dioxide were essentially what we experience today, as were temperatures — if anything, it was a little cooler. It isn’t chance that we would now perceive these conditions as relatively pleasant. Most of the features of our bodies related to our ability to withstand heat, our ability to sweat and even the details of our respiration evolved during this period. Our lineage, in other words, is fine-tuned for the conditions of the last 1.9 million years, conditions that have been rare for nearly the entire history of Earth.
Our bodies evolved to take advantage of a relatively unusual set of conditions that we think of as normal. It is easy to take those conditions for granted, but the truth is that the more we warm Earth, the less our bodies are suited to the world around us. The more we change the world, the more we increase the disconnect between the conditions we need to thrive and the world we live in. On the other hand, species that hold on by finding small pockets of hospitable conditions have the potential to persist — and even thrive — as we make Earth warmer.
Many ancient lineages of life preferred conditions that, from our own perspective, seem likely to be lifeless. Microbes live at extraordinarily high pressures in volcanic vents at the bottom of the ocean and harvest energy from the core’s hot exhaust. They have lived there for billions of years. One of those ancient microbes, Pyrolobus fumarri, is the most thermally tolerant species on Earth. It can withstand temperatures of up to 113°C (235°F). Such deep sea microbes die if brought to the surface, unable to deal with our pressures, with sunlight, oxygen and the cold.
“Species that hold on by finding small pockets of hospitable conditions have the potential to persist — and even thrive — as we make Earth warmer.”
Elsewhere, bacteria live inside salt crystals, in clouds or a mile underground, growing on oil. The bacteria species Deinococcus radiodurans lives through radiation intense enough to weaken glass. The atomic bombs dropped on Hiroshima and Nagasaki in World War II contained about one thousand rads of radiation. One thousand rads kills humans. Deinococcus radiodurans can withstand nearly two million rads. Nearly all (and perhaps all) of the extremes we are engendering on Earth correspond to at least some set of conditions from the past and to some set of species capable of thriving. Any horror of the future is, to some species, a description of ideal conditions, especially if that future horror matches some period in the distant past.
However, we know very little about most of the species that will thrive in these new old conditions. Ecologists have been overly focused on species like us: large-bodied, big-eyed mammal and bird species, many of which are very threatened by the changes we are causing. Ecologists love to go study rain forests, ancient grasslands and islands. They hate to work in toxic dumps and nuclear sites — and who can blame them? Meanwhile, the most extreme deserts on Earth are both remote and inhospitable, the kinds of places one is exiled to rather than the kinds of places to which one flocks when classes are done. They too are rarely studied. As a result, we lack awareness of the ecology of some of the most rapidly growing ecosystems, which represent the future’s extremes.
After The Last Cow Falls
In the near future, parts of Earth will be much more pleasant for extremophilic life-forms but much less suitable for humans. We can find ways to survive such change — just not forever. Eventually, all species go extinct. This reality has been called the first law of paleontology. The average longevity of animal species appears to be around two million years, at least for the taxonomic groups for which the phenomenon has been well studied. If we consider just our species, Homo sapiens, that means we may still have some time. Homo sapiens evolved roughly 300,000 years ago, suggesting that if we last an average amount of time, our road is still long. On the other hand, it is the youngest species that are prone to fatal mistakes.
The only species that tend to survive much longer than a few million years are microbes, some of which can go into long dormancy. Recently a research team in Japan gathered bacteria from deep beneath the sea that were estimated to be more than a hundred million years old. The team gave the bacteria oxygen and food and then watched. After a few weeks, the dormant bacteria, which hadn’t properly respired since the dawn of mammals, began to feast and multiply.
After we go extinct — and after the last cow falls — life will be reborn from what is left. The species that remain may, as Alan Weisman put it in “The World Without Us,” heave “a huge biological sigh of relief.” The life that remains will be reshaped by natural selection into a diversity of new and wondrous forms. On some level, the details of those forms are unknowable, yet we do know that they will still obey life’s laws.
“Familiar themes can recur after mass extinctions, revisited by evolution the way one jazz musician might echo another jazz musician’s riff.”
If we consider the last half billion years of evolution, one of the clearest conclusions is that what comes after a mass extinction does not necessarily match up with what came before. The trilobites were not followed by more trilobites, nor were the largest herbivorous dinosaurs succeeded by more enormous dinosaurs, or even similarly sized mammalian herbivores (a cow is no brontosaurus). The details of the past do not necessarily predict those of the future (or vice versa). A version of this sentiment has been called the fifth law of paleontology.
But familiar themes can recur after mass extinctions, revisited by evolution the way one jazz musician might echo another jazz musician’s riff. Evolutionary biologists call such themes convergent. They are cases in which two lineages, separated by space, history or time, evolve similar features in similar conditions. Sometimes convergent themes are subtle and idiosyncratic. The horns of the rhinoceros evoke the horns of Triceratops. In other cases, they are more obvious and grounded in the reality that often there are relatively few ways to live a particular lifestyle. Desert-dwelling lizards have evolved lacy toes, with which they can more easily run over the sand, many times. Ancient marine predators had sharklike shapes. Modern marine predators, including sharks but also dolphins and tuna, have nearly identical shapes. They also tend to have similar ways of moving (both mako sharks and tuna only move the last third of their body to swim).
Rats The Size Of Elephants
My informal survey of my colleagues suggests they agree that the way the evolution of new species proceeds in our absence depends on how much is lost. In general, though, they would also agree that life tends to become more diverse, varied and complex over time, a sentiment that is also sometimes considered a law of paleontology.
So if there is a species of a lineage left and it survives, it will become more than one species. If there are still representatives of the major groups of mammals, they might evolve anew in the ways they evolved in the past. If there are half a dozen species of wild cats left, each might, depending on its location and details, evolve into a dozen different new cat species, some bigger, some smaller. The same with canids: from one species of wolf or fox, many new species. Some species might be remarkably similar to those we are familiar with today; others will be unpredictably different.
The colleagues I surveyed agreed about one other predictable feature of the rediversification of any group of mammals. In general, when conditions are colder, warm-blooded animals tend to evolve larger body sizes. Larger-bodied animals have proportionally less surface area over which to lose heat. If humans go extinct far in the future during a glacial cycle, larger-bodied individuals may be more likely to survive, and hence larger bodies may evolve in many lineages.
“In the contest between the clever crow and the fecund pigeon, sometimes the pigeon wins.”
Conversely, if we disappear during warmer times, many species, particularly mammal species, may evolve smaller body sizes. The evolution of small-bodied mammals is well documented during the last period in which Earth was extraordinarily hot. Tiny horses evolved. Natural selection has no sense of whimsy — it has no sense of anything — and yet the reality that tiny horses once existed, prancing about in the ancient warmth, is as whimsical a thing as I can imagine. The effects of heat on body size can also be seen in the recent past by considering individual species. Over the last 25,000 years, the body size of woodrats in the desert southwest has tracked changes in climate. When it was hot, their bodies shrank. When it was cooler, they became larger.
If we leave in our wake a wave of more extreme extinctions, natural selection might more actively reinvent the world, futzing with the leftover pieces and bits at its disposal. Imagining a scenario in which most mammal species have gone extinct, the authors of “The Earth After Us,” Jan Zalasiewicz and Kim Freedman, posited a whole suite of new kinds of mammals that might evolve. They started with the assumption that the organisms most likely to diversify would be those that are already widespread, could live without humans and would be isolated by our absence (which would also mean the absence of boats, planes, cars and other sources of transport).
They thought rats meet these criteria — rats would be the future. Some rat species and populations are very dependent on humans (and hence our existence). However, there are many rat species and even some populations of human-associated rat species that are not, and these might then beget the future mammal fauna. If they do, one might, Zalasiewicz and Freedman wrote,
imagine, perhaps, a diversity of rodents derived from our present-day rats. … Their descendants may be of various shapes and sizes: some smaller than shrews, and others the size of elephants, roaming the grasslands; yet others are swift and strong and deadly as leopards. We might include among them — for curiosity’s sake and to keep our options open — a species or two of large naked rodent, living in caves, shaping rocks as primitive tools and wearing the skins of other mammals that they have killed and eaten. In the oceans, we might envisage seal-like rodents and, hunting them, larger, ferocious killer rodents, sleek and streamlined as the dolphins of today and the ichthyosaurs of yesteryear.
In addition to the evolutionary scenarios we can imagine, whether in light of life’s convergent tendencies or other processes, it is tempting to ponder those that might be so different that they are not anticipated anywhere in the life we know. Could we really imagine elephants if they did not exist? Or woodpeckers? Their unique lifestyles and features (trunk and wood-pecking beak, respectively) have evolved just once. But I suspect we are not creative enough to imagine the species that both might be favored by evolution and are truly different from those we know.
When painters try to imagine such species, they often give animals extra heads (Alexis Rockman) or legs (Rockman again, though also Hieronymus Bosch). Or they combine traits of different organisms into one (saber teeth, deer antlers, rabbit ears and cloven hooves). The results tend to seem either too much a hodgepodge to be viable (the multiple heads) or too unusual to be probable. Yet if we are honest, so too do some of the species we find around us on Earth. The platypus, for instance, has a duck’s beak, webbed feet, poisonous spurs and an assortment of other oddities. Could we imagine a platypus if we didn’t know it to exist?
It is common, in pondering the unusual features of the far future, to consider whether any of these species that succeed us might evolve the sort of intelligence that we find impressive, which is to say intelligence like our own (the kind that leads a species to warm its planet to its own detriment). Could the future after us be one of ever-cleverer crows or, say, city-building dolphins? The unambiguous answer is maybe.
Evolutionary biologist Jonathan Losos thinks that, given enough time, some other primate might evolve humanlike intelligence. Maybe. But if we extinguish primates, he’s less sure. And anyway, the sort of intelligence we know so far on Earth is only helpful in a subset of situations. It is useful when conditions are uncertain from year to year, yet there is some level of uncertainty beyond which a big brain is no longer helpful. Sometimes conditions can be so challenging that the species that survive are not the smart ones but, instead, the lucky ones and the fecund ones. In the contest between the clever crow and the fecund pigeon, sometimes the pigeon wins.
Then again, maybe a different sort of inventive intelligence will thrive anew in the future. A number of recent books have reconsidered, with some urgency, whether some sort of artificial intelligence, distributed among different machines, might take over Earth. These machines would be able to learn and would replicate, somewhere, out in the wild. They would need to find energy. They would need to be able to repair themselves. I’ll leave it to those books to ponder whether the computers — roving, thinking, mating, self-sustaining computers — take over. Meanwhile, it is interesting that in some ways we find it easier to posit that we can invent another entity that can live sustainably than to imagine that we can do so ourselves.
“The oldest termite mound may well have been inhabited for longer than the oldest human city.”
But there is also another kind of intelligence: distributed intelligence, the sort found in honey bees, termites and, especially, ants. Ants are not inventively intelligent, at least not individually. Instead, their intelligence stems from their ability to apply rules about how to deal with new circumstances. Those fixed rules allow creativity to emerge in the form of collective behaviors. Looked at this way, ants and other societies of insects were computers before computers. Their intelligence is different from our own. They are not self-aware. They don’t anticipate the future. They don’t mourn the loss of other species, or even their own dead. Yet they can build structures that last. The oldest termite mound may well have been inhabited for longer than the oldest human city.
Social insects can farm sustainably. Leaf-cutter ants farm fungi on fresh leaves, fungi that they then feed to their babies. Leaf-cutting termites do the same on dead leaves. They can make bridges of their bodies. They are everything that one imagines self-teaching robots might someday be, with the additional features that they are alive, they already exist and they already influence a large proportion of Earth’s biomass, whether by farming fungus, herding aphids, gathering prey, moving soil or even producing antibiotics. They run their worlds more quietly than we run our own, and yet, collectively, they run them all the same. In our absence, they would thrive as rulers, at least for a while, until they too went extinct.
After the societies of insects, the world is likely to be microbial, as it was for so long in the beginning and, if we are honest, as it has always been. As the paleontologist Stephen Jay Gould put it in his book “Full House,” “our planet has always been in the ‘Age of Bacteria,’ ever since the first fossils — bacteria, of course — were entombed in rocks.” Once the ants are gone, it will remain the age of bacteria — or more generally microbial life — at least until conditions eventually become, for any of a variety of cosmic reasons, too extreme for microbes, too. Then it will be quiet, a planet once more moved by physics and chemistry alone, a planet on which the innumerable rules of life no longer apply.
This is a modified excerpt from “A Natural History of the Future: What the Laws of Biology Tell Us About the Destiny of the Human Species” by Rob Dunn (Basic Books, Nov. 9, 2021).