A burst of atmospheric methane would disturb more than the climate. Methane also destroys oxygen and would have contributed to the anoxic (oxygen-depleted) conditions in the ocean that, along with acidification, contributed to the Great Dying. Less oxygen in surface waters would have meant more sulfide, possibly releasing toxic amounts of hydrogen sulfide (the poisonous gas that gives rotten eggs their awful smell), helping to doom land animals. Organics from dying organisms may have fed further growth of methanogenic bacteria, producing an accelerating feedback cycle of animal death, bacterial growth, methane emission, climate change, and extinction.
So, the Great Dying may well have been biologically induced, or at least amplified, with massive volcanism and microscopic bacteria conspiring to cause the greatest calamity in the history of life. It’s a crazy story, and it just might be true. I don’t know if this wild new idea will stand the test of time, but it serves to illustrate the kinds of feedbacks that can plausibly occur between self-multiplying biological and random geological elements within the complex Earth system. Many of the biggest extinctions in Earth history probably resulted from intertwined changes of the first and second kind.
Now, traveling about twice as far back again in Earth time, back to 542 million years ago, we encounter another sudden massive wave of extinction instigated by runaway biology: The Cambrian substrate revolution, when the texture and chemistry of the seafloor (at that time, the entire habitable surface of the planet) was rapidly remade by a spurt of biological innovation that caused both mass death and fantastic new evolutionary opportunity.
This happened nearly simultaneously with the onset of the Cambrian explosion, one of the most profound and portentous transitions in the history of Earth, when life suddenly became complex. Prior to 542 million years ago, almost all life consisted of microbes in the ocean. Cells had already experimented with many colonial, communal living arrangements, and a few multicellular forms that can be considered early animals had appeared. Yet, by and large up to this point, life was simple and single-celled. Then a catastrophe occurred (in the sense of sudden change), a dramatic increase in biodiversity, an anti-extinction, if you will. In a flash, there were not just some animal forms, but all of them. In the fossil record, all modern body types appear together at this moment. We don’t know why this happened when it did. It’s likely that the explosion had to wait until oxygen levels rose high enough to support the greater energy needs of larger bodies, and it came right on the heels of a “snowball Earth” episode, when Earth’s climate temporarily lost its balance and the whole planet froze over. It’s almost as if Earth’s biosphere was so relieved finally to melt its way out from under all that ice that it celebrated with an exuberant burst of animal evolution. Then, almost as abruptly as they appeared, many of these earliest animals suddenly went extinct. The exact cause is not known, but one suspect is the Cambrian substrate revolution.
Up to this point the seafloor was covered with microbial mats, soft, slimy, layered carpets of bacteria that lay upon the ground like a thick coating of living Jell-O. Many of the earliest animals were well adapted to resting upon this mat-covered seafloor, and grazing on its surface. The soil beneath the mats was devoid of oxygen and inhabited by bacteria that emitted hydrogen sulfide, poisonous to animal life. Yet among the multiple animal inventions at the dawn of the Cambrian were new forms of mobility. A number of critters began burrowing into these mats and braving the toxic soils beneath them. These digging, tunneling creatures are known as “bioturbators,” basically life that stirs up the ground. In fact, the fossils that officially define the beginning of the Cambrian in the geological record, Trichophycus pedum, are identified by the looping, burrowing patterns they left in the seafloor. Once this bioturbating disturbance of the Cambrian seafloor got started, it began changing the texture and chemistry of the dirt in ways that made it more inviting for other burrowing creatures. Oxygen began penetrating the loosened soil beneath the mats, neutralizing the lethal hydrogen sulfide. This started a positive feedback. The more oxygenated the soils were, the less dangerous they were for animal life, allowing more animals to evolve to burrow underneath. This additionally broke up and oxygenated the ground, inviting subsequent waves of animal invasion. The era of a seafloor covered in unbroken microbial mats, which had lasted for hundreds of millions of years, was soon over. This may explain the sudden extinction of many of the early Cambrian animal forms, as the environment into which they had evolved, a world of endless soft microbial mats upon which to sit and feed, rapidly disappeared, shrinking to a few isolated rocky coastal areas. The evolution of the new burrowing lifestyle greatly changed the world in which early animal life had appeared, determining the winners and losers during that time of rapid evolutionary change.
Now if we travel still farther back in time, about four times again as far, to over 2 billion years ago (halfway back from now to the beginning of the world), we come to the most dramatic example of a biologically induced catastrophe: the radical and abrupt chemical change that swept Earth’s surface and atmosphere after life perfected the use of solar energy. Around 2.5 billion years ago the oceans of the world were suddenly filled with efficient, self-multiplying solar cells. These cyanobacteria, sometimes called blue-green algae, proliferated around the planet and collectively transformed it in the most promising and devastating way. Life had flirted with solar energy earlier, but the cyanobacteria evolved much more efficient chemical pathways to exploit photons of sunlight, using their energy to build useful organic chemicals. This development was both wonderful and horrible.
Wonderful, because life gained the ability to harvest sunlight for energy. What a fantastic breakthrough! Earth’s biosphere learned to plug into the best power source the universe has to offer, tapping a nearby star to manufacture food from light. This liberated huge amounts of free energy for the biosphere, enabling the profuse diversification of life that has come in its wake, turning the planet green with phytoplankton, ferns, forests, and flowers, and the fish, flamingos, ferrets, and frogs that feed on them, furthering the journey from light to flesh.
But first it wrecked the world. It filled the atmosphere with a dangerous, corrosive, poisonous gas that brought about horrible worldwide death and destruction. What was that terrible pollutant? It was O2, oxygen!
With oxygenic photosynthesis, life harnessed the Sun, using the energy to split up H2O molecules, keeping the H to react with CO2 and make organic biomolecules (i.e., food) and discarding the O. Even after this started, it took a huge amount of time for oxygen to build up in the atmosphere, because it reacts with nearly everything. For hundreds of millions of years, the cyanobacteria kept spitting out oxygen, but all the excess was hungrily snapped up by the abundant iron in Earth’s crust and interior. Yet eventually, around 2.4 billion years ago, the crust was thoroughly oxidized and there was no more available iron lying around. The oxygen was finally free to accumulate, and the amount of O2 in the air shot up—which sounds great to us, but believe it or not, oxygen is an awful poison that reacts violently with and destroys the molecules of life. When life made oxygen, it was literally playing with fire. When we burn wood, coal, or oil, we are taking advantage of this tendency for oxygen to combust organics. Burning is the reversal of photosynthesis. In a flash of flame, energy once gained from the Sun is released again. When you bask in the glow of a wood fire, you are enjoying, years later, the heat and warmth from all the sunny days when that tree was growing. For all those years, the energy was stored in the organic matter of wood. Similarly, when we burn fossil fuels, we are releasing solar energy extracted and stored by plants millions of years ago. Burning snaps up oxygen, returns organic carbon to CO2, and throws the energy back out.
Oxygen and organics do not play well together unsupervised. Exploiting that imbalance, our animal ancestors evolved respiration. Respiration is controlled combustion that uses enzymes to carefully marshal that powerful energy release, harvesting it so that instead of dissipating in a wasteful flame, it is in
stead converted into phosphate bonds. These are the little chemical batteries that store energy within each of our cells and release it only when we need it. Before we evolved that ability, however, the buildup of corrosive oxygen in the atmosphere was massively fatal for most of the species that existed on Earth at the time. For much of the biosphere, the rapid oxygenation of Earth 2.1 billion years ago meant game over.
This Great Oxygenation Event was the most extreme chemical transformation Earth ever experienced—and it gets worse. The consequences for life were even more dire because not only was oxygen poisonous to organic life, but to add grave insult to fatal injury, its sudden rise seems also to have caused one of the worst climate disasters Earth has suffered.
The Great Oxygenation Event was contemporaneous with one of the most severe ice ages this world has ever known, an event known to geo-nerds as the Paleoproterozoic Snowball Earth episode.* This was probably no coincidence. At the time, Earth’s climate was likely being kept above freezing by a methane greenhouse. Methane is such a powerful infrared absorber that a very small amount of it can significantly warm a planet. It is also, however, an organic molecule that is easily and eagerly consumed by oxygen. So when all that oxygen released by the cyanobacteria built up in the atmosphere, it quickly destroyed the methane greenhouse, the atmosphere suddenly became more transparent to infrared radiation, and the temperature plummeted, plunging our planet into a complete global freeze. Such a deeply frozen condition could even potentially become a permanent dead-end state for a planet like Earth. Under what circumstances such a perma-freeze planet might emerge is a classic problem I had to solve in grad school, and later made my students solve. Simple climate models show that a frozen-over world is stable because ice has a very high albedo, meaning it reflects most of the sunlight striking it, which keeps the planet cold and maintains the reflective ice. Fortunately, the real world, more complex than a simple climate model, is full of competing feedbacks and holes in the ice. Ocean currents thin the ice in places, creating patches of open water that, along with areas of dirty ice, absorb more sunlight. Then what really springs the trap is that volcanoes keep pumping out CO2 and other greenhouse gases, oblivious to the freeze-out. Along with the slowly warming Sun, that was enough to melt the world. Life persevered, and eventually thrived in the newly oxygenated world. Evolution, the ultimate opportunist, turned destruction into creation. Some organisms developed the chemical machinery to derive energy from the extreme, deadly reactivity of oxygen with organic matter and to funnel it into useful cellular work. That’s called respiration, and it allowed the evolution of new animal life. This led, among many other things, to us.
For a planet like ours, it turns out, it’s not so easy being blue-green. The wild global success of the cyanobacteria, enabled by their use of solar energy, caused one of the closest calls to total extinction our planet has ever faced.
The Anthropocene Dilemma
Those irresponsible cyanobacteria. They not only caused a mass extinction, but they nearly wiped out all life with their careless climate interference. They discovered a powerful new energy source and recklessly exploited it, bringing ruin to all species that couldn’t get with their program, taking the world to the brink of ecocide. Talk about unintended consequences! Unwittingly, they changed the world and made it unlivable for many, perhaps most, of their fellow travelers.
Of course we don’t really think of them as irresponsible. They’re just bacteria. Yet today we see ourselves behaving in a similar way, and we look on with horror* because our actions do seem deeply irresponsible. So what have we got that the cyanobacteria didn’t have?
Just as the collective action of billions of cyanobacteria, each responding to its own survival needs, once led to a catastrophic transformation in the chemical state of Earth’s atmosphere and oceans, so today the collective action of a new species is causing sudden global transformation. Yet there are novel aspects of this transition that signal a new kind of global change, neither random nor biological.
In a sense, of course, this new catastrophe is biological. It is brought about by living organisms: human beings. Yet to call it merely another biological catastrophe would be to ignore mechanisms at work that are unprecedented in the history of the planet. Sure, you could make a good argument that this current change, like the oxygen catastrophe, is brought to you by bacteria. Our multicellular ancestors evolved from collectives of single-celled organisms. Our individual cells still resemble them to the point where you can reasonably view yourself as a kibbutz of bacterial cells that act out the coherent pattern of activity that defines your self. That’s true even for the parts of you (the minority, it turns out) that are actually “you” in the sense of being coded in the chromosomes you got from your parents. Now we’ve discovered that this parentally inherited part is but a portion of you that forms a habitat for the rest, for your microbiota, the shifting, porous microbial community that makes up the rest of you. So, certainly it can be said that whatever humans do is being done by bacteria, but not just by bacteria. We are also obviously something completely new that other bacteria, not organized in this way, are not; and we are something that other animals are clearly not, either. Something that can make art; invent, reinvent, and remember new tools; create, remember, and pass down stories, songs, dances, and hypotheses; record the past; imagine the future; plan and reflect; figure out laws of nature; walk on the Moon; and wreck the world.
From the planet’s point of view, what is happening now is as much a departure from the biological change it has experienced for most of its lifetime as that biological change was a departure from the brief earlier time of naked physical changes. This new form of global change comes from biology, but it is not described by biology any more than biology is fully described by physics and chemistry.
Yes, I realize that such an assertion reeks of human exceptionalism, the hubristic claim that humans are not just another species but that we represent an entirely new phase in evolution. Yes, this is problematical, but also unavoidable. From a planetary evolution perspective, our significance as a new kind of change agent is at least as obvious as it is disturbing, and it’s interesting to try to pinpoint what exactly is so new and different about us. My approach to this question is an astrobiological one. Imagine for the sake of argument that the evolutionary developments that led to this change represent a phenomenon that may occur throughout the universe, on certain lucky (or cursed?) planets. Whether, given what we know, this is a good assumption, and what implications that has for life and intelligence throughout the universe is a subject I’ll return to. Just as we can speculate about the universal qualities of global biospheres, we can ask what emerging global technospheres should have in common. What might their essential characteristics be?
It would be wrong to hang it simply on the evolution of intelligence, or the development of language or technology. On Earth, none of these is unique to humanity, and other species possessing these qualities are not using them in ways that cause catastrophic global changes. Pods of whales have sophisticated language and engage in clever group planning and coordination when hunting. Many nonhuman primates creatively use sticks and stones to catch termites, reach honey, and open shells. Elephants modify branches into back scratchers and fly swatters. Numerous species of birds have been observed using twigs as simple tools, or adapting human artifacts for nest building or various other clever purposes. A popular online video shows a playful daredevil crow using a plastic lid repeatedly to sled down a steep snowy roof. After watching this a few times, it is not too hard to imagine, if human technical civilization does not persist on Earth, that some other species will come along in a few tens of million years and give it a go.
Yet, in other species, these activities have not led to an acceleration of technological innovation with widening planetary consequences. Something else is going on here. One species, through some combination of language, cognitive skills, social intelligence, and technical inventiveness, has become successful
and powerful enough to start rapidly changing the world while, at first, having absolutely no awareness that it was doing so.
Such inadvertent global change, at least as a phase, seems inevitable for a young, expanding technological species. If members of a species successfully develop technology to effectively solve their local survival problems (such as how to grow and transport more crops or survive in the cold), but are operating under the reasonable assumption that the world is infinite, and still behaving according to the biological imperative to go forth and multiply, they’ll increase their numbers and impact to the point where they start to have a global influence. It seems likely that this activity would usually precede any knowledge of its world-changing nature. Why should they imagine they could change the world?
There was always a seemingly infinite, immutable world beyond our sphere of influence, and we never perceived of our own mortal actions as altering the very world itself. The World—that is, all of nature, all that there is. Who could tinker with existence? Maybe gods, but not us. This illusion would be shared by any young species first wielding the power of world-changing technology. They would not know their own strength.
Such clever creatures will have global influence but likely will lack both awareness of this and self-control (or even a self?). Even if or when they did realize what they were doing, could they act on this knowledge, even in the interest of their own survival, if they were not really set up for globally coherent action? In order to have self-control, one needs a sense of self. Do we have this, on the global level at which intentional action is now required? Recall the Asimov essay I describe in the first chapter. We are at a stage between the individual and the multiorganismic. Our planetary-scale challenges require us to perceive ourselves and act as the latter.
Earth in Human Hands Page 12
