2

EARTH ENLIVENED

Time and Space

Some years ago I went to a university seminar about the evolutionary history of animals, and heard a talk by a dinosaur scientist. He started by setting up the big picture, in a familiar sort of way. Human life is a tiny sliver, a scrap within a larger span. If the history of the Earth were compressed down to a year, our species would arise in the last thirty minutes or so of the final hour.

My mind wandered a little. Versions of this thought experiment can be done on different scales: we can think about the Earth, or about the universe. And the “we” might be humans, Homo sapiens, or might be something larger than that—animals, perhaps, or all of life.

If we stick to our species, then we do rush on in a half-dressed flurry at the very end. That is true on the time line of the Earth, and even more so for the universe. But suppose we think of “us” as life as a whole—living beings. Life does not look like an insignificant scrap in the history of Earth, and not even in the history of the universe. The age of the universe, as far as we can tell, is a bit under 14 billion years. Life has existed on Earth for something like 3.7 billion. That is over a quarter of the total span.

We, in this very broad sense of “we,” have been around a while. Living organization is one of the longer-running features of the universe. It has existed only in a tiny spatial region, as far as we know, and even if there are other inhabited planets, the whole of life’s extent will probably remain tiny in spatial terms. But within that small region, life is not something that scrambles on at the end of a vast dead stretch. Instead, life is a long-term tenant.

Perhaps this broad conception of our place in things, using a sense of “we” and “our” that includes bacteria, won’t resonate with everyone. We might think instead about animals. Even animal life is not too fleeting a part of history. Animals might be 650 million years old or so, which gives us about 5 percent of known time. But I do want to think in those broader terms, about life in its entirety.

Given that living beings are long-term tenants within the total span of the universe, we take up a much larger fraction of the history of the Earth. The Earth is about 4.5 billion years old. So life has been around for most of the time the Earth has existed, a good majority of the time.

In this sense, life is a significant part of the Earth’s story. And it is even more so in other senses—when we think of what life does, what sort of engine it is, what sort of factor in the whole.

Origins

How life arose is still uncertain. There’s not much progress or consensus, as far as I can tell, about the likely details, the where and how. But we know roughly what sort of things had to happen, and that gives an overall shape to the story.

A living organism is a pocket of order, a cluster of chemical processes that maintains itself, self-perpetuates, keeps recreating its otherwise improbable organization. Energy and other resources are required for such a thing to arise. The processes also have to be confined, so they don’t diffuse away and become lost into their surrounds.

One setting in which this might get started is around ocean vents, where a natural flow of energy and materials comes up from beneath the Earth, and porous rocks provide compartments in which reactions can be partly confined. From there, some of these cycling tangles of activity could start to produce boundaries—rough and imperfect—that are self-made rather than externally provided. The result is something increasingly cell-like.

The more stable of these cell-like pockets of order will persist, and might bud off new ones, like daughters, containing small samples of these interacting chemicals. The budded-off daughter systems themselves might drift away, or accumulate into clumps. Each one is self-perpetuating, maintaining itself through cycles of chemical reactions, and occasionally creating new systems of the same kind.

Perhaps the story does not involve deep-sea vents. Darwin imagined a warm pond as the site; Graham Cairns-Smith suggested an environment of damp clay. All these scenarios do include a role for water—it can’t happen on dry land. A similar general picture can be seen across these hypotheses: pockets of organization, making use of a source of energy and some initial way for reactions to compartmentalize, leading to cell-like self-maintaining beings that proliferate within a more chaotic and disorderly sea around them.

Not everyone thinks the sequence of events has this shape. In those sketches over the last page or so, I took sides on a divide between two camps. I treated the “metabolic” side of life as basic and original—chemical reactions forming cycles, using energy, becoming marked off into cells. Another framework sets out from a different view of what is fundamental. In Richard Dawkins’s 1976 book The Selfish Gene, life begins with replication, with some molecule arising that produces copies of itself. These copying molecules spread, become numerous, and can also have effects on what’s around them that influence their chances of being copied. Some are better at this than others, and through this initial Darwinian competition they become able to form cells and control metabolic processes, slowly bringing more of the world under their control. One form of this scenario is the “RNA world” hypothesis. RNA, a molecule found in all cells today, might be the original molecule of replication and control, with DNA and all the rest coming later.

The choice between the origin stories I compared just now can be described as one between “metabolism first” and “replicator first” scenarios. Eventually, we need both—molecules that are copied, and the energy-using processes of metabolism. The details can go in different ways. I have always had a nonexpert attraction to the metabolism-first approach, and a suspicion that replicator-first views will come to seem traceable to the preoccupations of the twentieth century—the “century of the gene,” as the historian of science Evelyn Fox Keller has called it. But even if the replicator-based view is a reflection of a genecentered mindset, that does not preclude it from being right. Metabolism, those energy-using chemical cycles, might have grown up around replication, rather than metabolizing systems arising first and then making themselves a molecule, RNA or DNA, that works like a cellular “memory,” enabling these systems to push their organization onward through time.

Alternatively, perhaps the two activities were bound together from the start. Somehow we get both, metabolism and reproduction, a package of features tied together. What we call “life” usually involves both of these phenomena—the use of energy to maintain order, and the production of new living beings from old. These two activities can still come apart; viruses are packets of genetic material that use the metabolisms of others to reproduce, and have no metabolic activity themselves. Are viruses alive? They have a part of the usual combination, and whether this is enough for them to be alive is not something to argue about. The idea of life as a sharp and definite category has been replaced by a view that accepts gradations and gray-area cases.

Another feature that seems to run deep through the history of life is sensing, and responding to what is sensed. This is not known for sure, but sensing is so common across widely divergent forms of life now, including relatively simple ones, that it is probably very old. Recall the start of this chapter, with the dinosaur talk and the idea of life as a long-term feature of the world. If you think of early life as a sort of inert mold or slime, then perhaps its duration is less notable. But when sensing and responding are also very old, that changes the feel of things. Someone has been looking out, just a little, for all that time.

A further feature is also ubiquitous, perhaps more inevitably so. This is having effects, changing one’s surroundings. Initially this might involve no more than the consumption of nutrients and the emitting of waste, but life, from the start, does not leave things as they were.

If early life involved the formation of pockets of order, pockets of improbable patterning, with this comes the formation of selves—the marking-off of self and other. The origin of life is the origin of new divisions in nature. With these divisions comes a kind of complementarity—the creation of complementary roles. Here is an organism, a self-maintaining pocket of order, and here is the environment it depends upon and transforms.[1]

When I talk about “selves” originating with life, the simplest picture is one where those pockets of order have clear boundaries. There has to be traffic across those borders—hence gates or portals—but we might still picture the divide as pretty sharp. But often—perhaps always—an image with sharply drawn lines is not right and the border is vaguer.

One place to see this is a coral reef. A coral is an animal, a relative of jellyfish and anemones. Huge numbers of these animals live in colonies within a reef. Those coral animals, the polyps, often have symbionts living within them that absorb energy from the sun. The polyps themselves also build a rock-like external body or support, which is responsible for the hardness of the reef. Doing this—building rock, essentially—requires a delicate chemical balance. Some of this takes place within the cells of the coral, while other parts of the process take place just outside, in a kind of controlled space just beyond what would usually be seen as the border of the animal. Living activity often extends some way into a semi-transformed or controlled zone of this kind.

Cycles and Burial

I am sitting out in the garden, being cooked by the sun in a black shirt. Thinking, as best I can, about energy.

Large amounts of energy come in from the sun to the Earth. Much is radiated back, but not all. Some is absorbed and stays around. This energy can do different things according to the form it is in. A simple wash of heat can’t do much. How does energy make its way into more useful forms, the forms that can power our lives?

In photosynthesis, the sun’s radiation is transformed into chemical energy. This can be done in a few different ways. In all of them, light is absorbed by a molecule of some kind that uses the light’s energy to excite its electrons (the charged particles that orbit the nucleus in atoms). If sufficient light comes in, this initiates a cascade by which electrons move from molecule to molecule. By means of these cascades, various kinds of processing and pumping can be achieved.

Crucially, the electrons that are sent off onto an “electron transport chain” by the incoming light need to be replaced. In the main kind of photosynthesis, the kind seen in plants today, this is accomplished by splitting water into its constituents, hydrogen and oxygen, and extracting electrons from the hydrogen atoms as this is done. You start from light, carbon dioxide, and water, and at the end you have chemical energy, in all its useful, portable controllability. You also end up with oxygen gas, which so far in the story is a mere byproduct.

To invent the water-splitting, oxygen-producing form of photosynthesis, two molecular machines from other simple organisms were brought together into a combination. (These are called photosystems 1 and 2. They did originate in a single invention, further back, then diverged and reunited.) This trick seems to have evolved just once.

Not all photosynthesis is like this. Scattered over Earth now, in twilight zones and marginal places, are single-celled organisms that do it differently, organisms with names befitting their exotic lifestyles—the green gliding bacteria, the purple sulfur bacteria. These use other substances as sources of electrons, instead of water. That means that not all photosynthesis is “oxygenic”—not all gives rise to oxygen gas. The special kind was invented by cyanobacteria (or their near ancestors), in the momentous event commemorated in the previous chapter at Shark Bay.

As time passes, over decades of research, this particular reaction has come to seem more and more important. Its place in Earth’s history becomes clearer and the language used about it gets stronger. Andrew Knoll, a Harvard biologist who works on early life, says that the bringing together of the devices needed to make oxygen-producing photosynthesis might be regarded, from an ecological perspective, as “the central event in the history of life.” Without it, life would have remained more a fringe dweller than an Earth changer, confined to chemically special environments like those deep-sea vents. James Barber, who admittedly worked for much of his career on photosynthesis itself, says that the splitting of water molecules, that difficult step, is simply “the most fundamental reaction on Earth.”

Writing this book made it necessary to read and learn in areas I’d never looked at closely before, and this is one of them. By the end, I found myself picking up some of the awe seen in those who spend their lives unraveling the tiny conveyor belts and turbines of photosynthesis, a multitude in every leaf. This is awe at what life came up with, and awe at its consequences. On a lifeless planet, or even one without photosynthesis, energy pours in continually but radiates away as heat because there is no way to convert it into chemical energy. The light-harvesting molecules in bacteria and plants take the whole system into a new place because they absorb and accumulate energy from several incoming photons, and make the transition to chemical energy possible. The sheer amount of energy present on a living planet is greater than on a dead one, as energy has been converted and held. This feeds not only living activity, but geological cycles and processes as well. Life starts storing the sun, and everything is affected.

In photosynthesis itself, oxygen gas has no role; it is left behind when the more “useful” parts of the water molecule have been taken away and put to work. Oxygen is used in other reactions within organisms, including organisms that photosynthesize, but its creation is as a byproduct.

The history of oxygen on Earth, from this point onward, has several stages. Cyanobacteria began to quietly produce the gas. For a while, not much happened. The oxygen was absorbed by rocks, as seen in those red deserts of the first chapter, and taken up in various other ways. But it slowly began to accumulate. Later still, cyanobacteria were incorporated as symbiotic partners inside other organisms, and eventually made their way into plants. This ancient engulfing is similar to an event in which some bacteria were swallowed up by other cells and became mitochondria. These are the powerhouses that lie within us, and within plants, and aid in respiration, which is the biological flip side of photosynthesis; it burns fuels with the aid of oxygen. So animals (and also fungi, and some others) are carrying around one kind of bacterial remnant, mitochondria, and plants and algae are carrying around two.

Back during the early stages, oxygen levels were only a few percent of the atmosphere (at least most of the time—they may have jumped around). That is nowhere near what animals like us, and perhaps just about any animals, need. But this change was still important enough to be called “The Great Oxygenation,” beginning roughly 2.4 billion years ago. The levels rose again much later, perhaps near the start of the Cambrian period about 540 million years ago. Fitful increases continued, until we reached the sort of chemical environment in which lives like ours, with muscles and brains, are possible. This enveloping of the Earth in oxygen also had other consequences, on a geological scale, changing the chemistry and geology of the planet. New kinds of minerals—new “mineral species”—came to exist, including semiprecious stones like azurite and malachite, formed through processes featuring the reactivity of oxygen, or through the chemical action of life itself.

With the early history of oxygen in place, it’s time to bring carbon to the fore. As the ongoing dance between oxygen and carbon is so important, and as it has taken me a long time to understand it even imperfectly, I will take a pretty slow walk through it.

We have two main players at the level of chemical elements: carbon, with its versatility as a building block, able to form all sorts of complicated molecules; and oxygen, with its reactivity. We also have two pivotal gases they make up: oxygen as a gas, O², and CO², carbon dioxide.

All through here, we are dealing with a combination of processes working at different scales—fast and slow, biological and geological. What new factors like human behaviors do is nudge, perturb, push, against a background of many others.

The picture we are sometimes given is that plants breathe in carbon dioxide and breathe out oxygen, and we depend on this, day to day. The rainforests are the lungs of the Earth, it is said. Or, the Earth’s lungs are the forests plus the oceans. But plants “breathe” in two ways. They take in carbon dioxide when growing, but they also respire, as animals do, to keep the processes of life going. When plants do this, they use oxygen. As long as a plant is growing, it produces more oxygen than it uses. The growth of plants is a process gently out of balance, in a way that adds oxygen to the atmosphere and stores carbon in living matter. But a forest as a whole, when mature and in a steady state, includes not only the growth of plants but also their ongoing existence, their breakdown by microbes and fungi, and also all that animals within a forest do. These are oxygen-consuming processes. A forest is a great producer of oxygen, and also a great consumer. That to-and-fro in a mature forest is pretty much in balance—except when the breakdown of plants does not happen. If trees, once built, are buried and squashed before they can break down, then some of their carbon is interred in the Earth, and oxygen that might otherwise combine with this carbon remains above.

These processes act against a background of others. Carbon and oxygen are also tied together in a to-and-fro involving the weathering of rocks, the laying down of sediments in the sea, and the action of volcanoes. In this slower cycle, carbon dioxide in the atmosphere is absorbed into rain to make a weak acid, rainfall over land runs into the sea, weathering rocks as it goes, and this flow contributes to a mix of chemicals that is used by marine organisms to make carbon-rich shells. That carbon is laid down into rock, eventually pulled deeper into the Earth, and released back into the air by volcanoes. This slower “inorganic” carbon cycle involves life, not just because the sediments that store carbon in rock are full of seashells, but because life on land has a big effect on the weathering stage. Plants and fungi can increase the rate of erosion and weathering considerably, by holding water against rocks, making stronger acids, and slowly fragmenting rocks with roots and fungal strands.

We breathe oxygen made by plants and plankton. But if you instantly took all the plants and plankton away, we’d be able to keep breathing for a long time (millennia at least). This is possible because there’s now a huge reservoir of oxygen in the atmosphere. That reservoir came from life, from a slight imbalance between oxygen-making and oxygen-using processes over long periods. It is being cycled, used and replaced; the breath you take now could contain oxygen that has not been in the atmosphere for long at all. But the reservoir is huge.

If we suddenly burned all the known accessible stores of fossil fuels on Earth, this would also have little effect on the oxygen reservoir. Humans and other animals don’t have the kind of impact on oxygen that we have on other things, especially carbon dioxide. Carbon dioxide is much rarer in the atmosphere than oxygen—only about one-twentieth of 1 percent, whereas oxygen is about 21 percent. But when the level of carbon dioxide goes up or down, even at that smaller scale, this can have dramatic effects. If we burned all those known fossil fuels right now, we would have a big impact, proportionally, on the carbon dioxide in the atmosphere, and that would make a difference to all sorts of things.

This idea that the forests and oceans are our lungs—the real-time source of the oxygen we need—is one that people are sometimes reluctant to correct, as it helps with our sense of the urgency and importance of environmental protection. In this book I want to cover every part of these problems as accurately as I can. The Earth has some features that life (the long-term and continually renovating tenant) affects over long spans, but human choices don’t, very much. Oxygen levels are an example. It also contains processes that human choices do affect—the production of carbon dioxide. And all this takes place against a background of processes on an even larger scale, with the sun getting brighter, the Earth slowing in its orbit, and the galaxies moving apart.

This part of the story of the Earth is often told in a way that positions oxygen as the crucial factor—oxygen making animal life possible. I don’t disagree, but just as fundamental is the way the invention of photosynthesis led to large amounts of the sun’s energy being captured and held in chemical form. The Earth became infused, crammed, with energy in a way that would not happen without life. Bonds were built between atoms in new kinds of molecules, bonds that stored the sun’s energy. That energy, with the aid of oxygen, could then be released, sending life swarming over the Earth.

A Circus of Forms

From single-celled life arose collections, colonies, and collaborations of many kinds. The multicellular organisms we are most familiar with are plants and animals, and also fungi (who are closer to animals than plants), but these are just some of the ways cells come to live together, forming blends and fusions, symbiotic and antagonistic.

Earlier I mentioned corals, animals with photosynthetic algae living inside their bodies. Those algae are of a kind called dinoflagellates. (This is the same group of algae that cause red tides.) The dinoflagellates themselves contain remnants of cyanobacteria, as you might expect, given that they engage in photosynthesis. But those are not the only layers in the system—it’s not just cyanobacteria, dinoflagellates, coral. To acquire their ability to photosynthesize, the dinoflagellates engulfed another kind of algae that had previously engulfed a cyanobacterium. Some corals have also been found with cyanobacteria living inside them “naked,” as if to remind the corals of the debt they owe to these tiny and more ancient organisms.

This willingness on the part of life to form collections and collaborations engenders a circus of forms, a great diversity of larger units, all making use of those basic ingredients—the living activity of cells, sensing and responding, taking material in and emitting waste, and changing over time. One lineage that pursued a multicellular experiment gave rise to animals. Cells came to live together, as they did often elsewhere, but in this case they did so in a way that invested in controlled motion, in action. To coordinate this, they evolved nervous systems and brains.

Animals arose initially in a low-oxygen environment, much lower than what we experience now. As oxygen levels increased, they were able to become more active. This is part of what happened around the “Cambrian explosion” about 540 million years ago. The explosion was a “coevolutionary” event, one in which evolution in one kind of animal provided an impetus to evolution in others, and vice versa, but all this may also have been set in motion, or at least facilitated, by an infusion of oxygen into marine environments. The amounts of oxygen were still well short of present-day levels, but enough to make a difference. And then, millions of years later, some animals moved onto land.

Much will follow from this—from the animal investment in action, the move onto land and the different lives possible there, and the environmental transformations that result. First, though, I want to return to a theme from the very start of the chapter.

I said back there that although the human species rushes on in the last small part of the total span of time, life is not like that, and we can think of ourselves as part of life as a whole. I want to say a bit more about the “part of” idea. Once cellular life exists (bacteria, animals, plants, and so on), cells come always from cells. It is cell giving rise to cell, over the millennia, through all the evolutionary changes and the new species that arise. And when a cell produces a daughter cell, there is a material continuity between them. Membranes, for example, come materially from other membranes; a new cell membrane has part of an old membrane within it. This process is mixed up and more complicated in some cases (as when sperm meets egg), but cells come from cells, with material continuation. It’s not that you will have inherited particular material parts from some ancient life-form, but that there is a chain of such relationships—the cells in you containing parts of earlier cells, which contained parts of earlier ones, and so on.

When we look back, then, it’s not just that we are living organisms, examples of life, and such things were around back then as well. And it’s not just that the link of reproduction has been present since then, with old organisms somehow making new ones. We are also a material continuation of what was here before. This relationship stretches back through our ancestors, through lizard-like beings, fish, worm-like animals, to single-celled life. Our ties to ancient life-forms are not just a matter of causal connection. A stronger bond connects old and new organisms, projecting life through time, and this bond has been in place for most of the history of the Earth and much of the history of the universe.

Gaia

The scene coming into view is one of a dynamic Earth. The Earth is not an inert stage, or something that only changes under its own steam; it changes as a result of the actions of life. Once we have come this far, a gestalt switch is possible, a way of taking things further. Back in the 1970s, James Lovelock and Lynn Margulis introduced the “Gaia hypothesis.” The Earth, they suggested, is itself an organism, or at least organism-like. It regulates itself, a huge system with a metabolism that spans the plant and animal world along with parts of the Earth that we usually think of as inorganic. Earlier in this chapter, we already found the need to talk about borderline and partial cases when thinking about the origins of life. Maybe there’s another phenomenon of this kind, on a much larger scale?

Lovelock, who died at 103 as I was finishing this book, was a chemist and inventor. He devised a detector that picks up tiny traces of pollutants in the air, and spent most of his career as an industry consultant rather than an academic. He was led to reflect on the ways that life, on any planet, will tend to modify its atmosphere. The atmosphere found on Earth, especially with all that oxygen, would be noticeable from far away in space as abnormal, as marked by life. We should be able to work out whether other planets contain life by looking for the same sort of signature.

From there, Lovelock introduced the idea that perhaps a planet like Earth can have the metabolism of a living organism—is something like a living organism. The novelist William Golding (Lord of the Flies) suggested the name Gaia for this idea, from the ancient Greek goddess of the Earth. The American cell biologist Lynn Margulis, like Lovelock a rebel in science, became an early supporter and co-developer of the theory. In this book, I’ve treated the idea that remnants of cyanobacteria found themselves inside plants just as something that happened, a historical fact, but it was Margulis who rescued this idea from near-oblivion in the 1960s. She made the same claim about mitochondria, the powerhouses found within the cells of animals, plants, and others—Margulis argued that they, too, are descended from free-living bacteria. Margulis didn’t invent these ideas about the origins of our cells, but she brought them back from the fringe. When I was a student in the 1980s, her revival of this view was just starting to triumph over skeptics.

The Gaia hypothesis, in its original form, held that Earth is a huge self-regulating system that acts to maintain life. This is, without question, a radical idea, but points that can be raised in support of it do look intriguing, to say the least. (Ford Doolittle, one of the early critics of Gaia, always acknowledged these; Gaia is not just gratuitous storytelling, or a poetic invocation of natural harmony.) One example is Earth’s temperature. Life requires liquid water—water on Earth can’t be all ice, or all gaseous vapor. And although we are a suitable distance from our sun for the temperature to be roughly in the right place, the sun’s own temperature is not a constant; it has become a good deal hotter over the time that life has existed on Earth. Somehow, the overall temperature on the planet has been kept in a fairly narrow and life-friendly range.

Another example is the saltiness of the sea. Salt is carried to the sea in runoff from rain and rivers. Although salt water is in many ways friendly to life, if the sea was much saltier than it is now, most life could not cope. When a history of liquid water was discovered on Mars, this seemed encouraging to the idea of Martian life. But it looks like that water might have been too salty. And while we can easily see how more salt will keep running into our oceans, it’s hard to see how much gets out. Might Earth build itself evaporation pans that trap salt in solid form and keep the ocean’s salinity at a reasonable level? In his original Gaia book, Lovelock wondered whether the Great Barrier Reef in Australia might be a “partly finished project for an evaporation lagoon.” That is a startling idea, but in this case and others, we do need some explanation for the ongoing life-friendliness of Earth.

Let’s look, then, at what it would be for Earth to be an organism. Not all versions of the Gaia idea are committed to this view, but let’s start here. We agree that Earth is a complex system, full of interactions. That is not enough for it to be an organism. A global war is an interconnected system, but not an organism. We need, at least, a high degree of cooperation. The parts of a system must work together to keep it going, to keep the arrangement in place, maintaining order in the face of tendencies to fall apart.

The biologists David Queller and Joan Strassmann take this idea further by classifying systems using two dimensions: cooperation and conflict. Systems are more organism-like—more “organismal”—when their parts show a high degree of cooperation and low conflict. Isn’t one of these the flip side of the other? No, they say; a system can contain a lot of cooperation and a lot of conflict at the same time. Human societies are like this.

The form of cooperation that is relevant here is also a special kind. It involves working together in a way that maintains the integrity of a system, maintains its organization in the face of forces of decay. A living animal, such as a giraffe, is a clear case, but if we take this approach, there will be a lot of partial or gray-area cases, rather than a sharp divide between organisms and non-organisms. We saw the same thing in the discussion earlier in this chapter of the origins of life. So, as a next move, the question we should be asking is not a simple “Is the Earth an organism?” Instead, we can ask whether the Earth has organism-like features, whether it organizes itself in something like that way.

A system with organism-like features can arise through a coming together of parts with diverse origins. Much research in recent years has been directed on the close relationships between human bodies and the beneficial bacteria living in our guts. Some people think the human organism itself is a combination of animal cells and bacterial cells. A better example might be a cow, which has a stronger dependence on internal bacteria to digest its food. A cow is a bit like the corals we encountered earlier with photosynthetic algae living inside them. An example with more “looseness” is a collaboration that has been found between ants and acacia trees. These acacias build living quarters and provide food for ant colonies that live inside them. The ants, in turn, protect the tree from other animals who would like to eat it. This is not as intimate a connection as the cow-bacteria case, but although the ant houses are clearly part of the tree, they are there for the ants to live in. If we ask whether the cow-plus-bacteria really is an organism in its own right, then whether the ant-acacia combination counts as well, or whether these are all just collaborations between separate organisms, this question looks for a definite dividing line where none exists. What we find is differences of degree, borderline cases as well as clear ones, and a great variety of ways that a system can be organism-ish, or organism-like.

When Lovelock and Margulis suggested that the Earth is an organism, they did not say much about how this situation might arise. That led to objections from evolutionary biologists. Organisms have to arise from Darwinian processes, unless deliberate design is going on. God is not supposed to be in the picture, so it would have to be evolution. Evolution by natural selection requires a population, one in which reproduction takes place. New variants arise by chance in the population, and the ones that are better at keeping themselves going and reproducing may proliferate and spread. This also requires that the quirks that give one type an advantage over others are inherited over generations. Those that do well can then become a platform on which further rounds of variation and selection take place. A Darwinian process is a kind of grand trial-and-error, spread over a population.

In the case of the Earth as a whole, the universe does have a collection of planets, but no reproduction, no inheritance, and no competition among them. So there’s no Darwinian process, biologists said, and hence no way for something like the Earth to become an organism.

This for a time seemed an important objection to Gaia, but I think now it is not so good. First, the way life initially arose on Earth, especially on a metabolism-first view, has to be somewhat different from the usual kind of Darwinian process, although the “many experiments, a few successes” side still has to be in place. As we saw a moment ago, organism-like things can also arise through the coming together of parts with different origins, and these will have their own distinct, but connected, evolutionary histories. The ant-acacia collaborations are an example. A version of the Gaia hypothesis might work along similar lines. It might say that the Earth as a whole includes organism-like cooperative setups; it is like an ant-acacia system on a huge scale.

Is this possible, at the scale of the entire Earth? It’s not impossible, but very unlikely. This is partly because there will be continual opportunities for breakout, for a loss of a cooperative balance. In cases like the ant and acacia, we have cooperation between initially surprising partners, but those cooperative relationships are forged in conflict with other organisms. The ant-acacia relationship makes sense because the plants would like a defense against other animals who would like to eat them. It is very hard for everyone to be—and especially for them to stay—part of the same cooperative project. The ants-and-acacias say: It’s us against the world! Against the herbivorous world, anyway. On close inspection, this is not much of a model for Gaia.

Living things do sometimes have mutually compatible needs, but often they have antagonistic ones. The Earth, and anything on a similar scale and inhabited by organisms that are products of Darwinian processes, will always be a different kind of system from an organism. A big, complex system in which life plays an important part does not itself have to be organism-like—another organism, with the same features seen again at the new scale. It can be a different kind of thing, one that organisms are a part of, along with other parts. This is an example of what I referred to as complementarity earlier in this chapter. In this view, we can still acknowledge that the Earth is a special kind of system, one where life matters and there’s much feedback between the living and the nonliving.

The Earth-as-organism idea might also be seen as a metaphor rather than a claim to be taken literally. Some people certainly find the image appealing—it takes us back toward a mother-like Earth. But then we have to ask whether the metaphor helps us more than it misleads. People often seem to think that the metaphor has a positive role because it gets us to recognize and care about the whole. I am not sure about that; it might be quite unhelpful. Talk of Gaia invites us to think the Earth will take care of itself, if given time to adjust. It invites us to think there’s something big and thoughtful in the neighborhood that can compensate for our errors. It will know what to do, and will find a solution. This is not a good way to be thinking, because there’s not something big and thoughtful around on this scale, and no reason to expect future changes to head in a helpful direction all by themselves. This sort of vague hope is encouraged by talk of Gaia.

In recent years, some defenders of Gaia have moved away from the Earth-as-organism picture. They think Lovelock and Margulis went too far in that respect. Sometimes people just want to use talk of Gaia to emphasize the connections between living and nonliving parts of the Earth system (a view sometimes called “weak Gaia”). Another way of seeing the Gaia hypothesis is not just an acknowledgment of connections, but a claim about a tendency for various processes on Earth to regulate conditions in a way that is helpful to life, without the system being organismlike. The British scientist Tim Lenton defends this view.

The life-friendly features of our planet do have to be acknowledged. In the case of temperature, one puzzle is that the sun gets warmer while the temperature on Earth does not change much. Earlier in this chapter, I said that carbon dioxide levels are affected both by a cycle that runs through living activity in plants and animals, and by a slower “geological” cycle—rain, weathering of rocks, seashells, more rocks, volcanoes. This cycle has “negative feedback” built into it. That means there’s a process in place where, as the level of some factor increases, it triggers events that push it down again, and vice versa (when its level drops, it triggers something that pushes it up). In this case, when conditions are warmer, this turns up the activity in the carbon-storing part of the cycle—more rain, more weathering, more carbon locked away. But the parts of the cycle returning carbon dioxide to the atmosphere are not much affected. Since carbon dioxide traps heat in the atmosphere, when its levels go down, the Earth’s surface is cooled. Given all this, hotter conditions tend to lock up more carbon, and that exerts (very slowly) a cooling effect. Cool conditions, on the other hand, reduce the weathering that locks carbon away, and this pushes temperatures up.

I also noted back then that plants and fungi on land increase the weathering of rocks. This is relevant to the stabilization of temperatures. Plants cool the Earth, both through this effect and through the laying down of coal. Our early atmosphere seems to have had a lot more carbon dioxide, leading to a strong “greenhouse effect” that warmed the Earth, preventing the seas from freezing when the sun was fainter. Since then, the sun has gotten hotter, but much of that carbon, thanks to life, has been taken out of the atmosphere. All this is a combination of negative feedback and an alignment between processes that are just going on their own path without life affecting them. The sun slowly gets warmer, no matter what we do.

How about the salt in the oceans? This case is interesting because scientists, at the moment, seem a bit uncertain about it. Sometimes people say the salt level is not held stable, but only changes very slowly. Either way, it has stayed in a life-friendly range for a long time. The main way that salt can be removed from the sea is through evaporation, in places where, for some reason, water does not get replenished and salt is left behind. If this happens on a large enough scale, it forms a “salt giant,” a deposit of solid salt whose depth is measured in hundreds or thousands of meters. Around 5.5 million years ago, much or all of the Mediterranean Sea dried out in this way, before water came rushing back in. Why should a combination of this sort of process, with the ferrying of salt to the sea in runoff, lead to a roughly stable salt level? Is there some potential for negative feedback here, or are the two processes just separate, and (from our point of view) helpfully aligned?

Some life-relevant features have not been stable at all. Oxygen levels in the atmosphere have gone up and down a lot, in part due to changes in those “cycles and burial” processes I looked at earlier. Sometimes more carbon is buried and oxygen levels increase; sometimes things tend the other way.

Our picture, as far as I can make out, is that the Earth has indeed tended to stay in a fairly life-friendly range for a long time. It won’t always do so—the sun, it seems, will eventually cook away the oceans completely. But for a fair while, conditions have been friendly. This is not because a vast organism is regulating things in a goal-directed manner. Instead, a collection of processes, quite disparate ones, operate in tandem and do keep things—or have kept things, at least—in a life-friendly state.

Why do they do this? Is it a matter of luck? This is not luck in the usual sense, not like a lottery or game of roulette. In each case, a huge mechanism lies behind things, one involving physical principles and the history of the Earth.

We might have a story for each factor (temperature, salt, etc.), but what about the fact that all of these helpful mechanisms are in place together—is that just luck? In a sense, yes; I can see what the word “luck” refers to there. It refers to the fact that there’s no overarching explanation of why all these processes (each big, and ancient) head in a direction that is suitable for life. Does this show a failure of understanding on our part? Must there be more to it?

Some of it does have to be luck, in this broad sense, including important parts. Tim Lenton, the Gaia-friendly scientist I mentioned earlier, notes that a planet with life has to have water, in good amounts, before life appears. I’m not talking here about whether the water is ice or liquid, but just its abundance. Earth had plenty of water. Why? Much of it was probably brought in on asteroids, Lenton says. If so, this has to be a matter of luck no matter what one thinks about Gaia. Even a living planet can’t call in asteroids bearing water. An articulate Gaia would have to say: “Yes, that thing with the water and asteroids is pretty good, but it’s not one of mine.”

A role here is also played by “observation selection effects,” as they are sometimes known. Suppose we do need water-bearing asteroids for the evolution of complex and intelligent life. Then intelligent beings will only be around to ask these questions on planets that have gotten lucky in this way. If there are many planets that can be seen as “experiments” of this kind, with different amounts of water coming in to each, and a small number of planets have a life-friendly quantity of water arriving, then any intelligent beings on those planets will say, “That’s remarkable; conditions here had to be just right.” But there were many such experiments. In such a scenario, someone will end up noting their good luck, and it happens to be us. This only helps remove a sense of mystery about the life-friendly conditions on Earth if there are a lot of experiments, and hence a reasonable chance of someone being alive somewhere and able to reflect on their good luck. Otherwise, we do have to accept that a low-probability event has occurred. (If a million people play a gambling game that has odds of success for each person of a million-to-one against, someone will probably end up happy. If only a hundred people play, then any win is surprising.)

Is it then likely, as in my scenario above, that someone, on some planet, will end up saying, surrounded by ample water and friendly temperatures, “Look how lucky we were!”? That is a good question. If an Earth scientist who knows more than I do about all this says no, it was not at all likely, then I must take that seriously. Then either there was, indeed, a stroke of something like luck, or perhaps we don’t know the whole story.

Where does this leave us? Some of our planet’s stabilizing, life-friendly effects are not just intriguing; they can look a little uncanny. The combination of factors that have kept our water from freezing or boiling away is like that—or seems so to me, anyway.

Lenton would say: To believe that these feedback connections are there, and helping life survive, is to accept the Gaia idea, in a more modern form. I would respond that the Gaia idea can certainly change—can evolve. An echo of pervasive cooperation does still come with the term, and that is misleading. So is the suggestion that there’s a single center of control at work, something like a single agent. Whatever language we decide to use, the right picture to have in mind is, I think, one that uses that idea of complementarity introduced earlier in this chapter. As you walk along, you are a living part of a larger system, one that has living and nonliving parts. That system is not much like an organism, but it is a system in which the nonliving is closely tied to the living. The Earth has been enlivened by its organisms, even though it is not itself alive.

Goals

One thing that can make the Gaia idea exciting is the suggestion that events in the atmosphere and broader environment happen for a reason, in a particular sense of that term. They don’t just have causes—things that make them happen—but purposes.

The idea of purpose in nature is occasionally criticized as unscientific, but some of what goes on is purpose-driven. Humans can act with a goal or purpose in mind. And, a bit more controversially, an event like a flow of adrenaline has a purpose within our bodies even when we don’t consciously decide to produce it. Adrenaline flows prepare the organism for fight or flight—that is what they are for.

If the whole Earth was an organism, this is part of what we’d get. All sorts of subtle activities might go on in order to keep conditions on our planet within a suitable range for life, just as all sorts of processes within us go on to keep our body temperature in the right range. I argued against this view of the Earth, but purposes and goals are important in their own right, and they are going to appear often in the chapters to come.

Goals, purposes, and functions (in one sense of that term) are often referred to as teleological concepts. In the ancient scientific/philosophical framework of Aristotle, influential for many centuries, just about all natural processes have something like a goal, a natural end. The model of a growing tree, going through a sequence of events that manifests its nature and purpose, was applied very broadly, even to inanimate objects. This way of thinking was readily taken over into a Christian framework, when it arose centuries later. Purpose was still everywhere, and now reflected God’s creation and will.

The scientific revolution in seventeenth-century Europe included attacks on the overuse of teleological ideas, even though the scientific revolutionaries were generally not atheists. This continued in the eighteenth-century “Enlightenment” period. A picture of the world as driven by physical mechanisms, impacts, and push-pull causes seemed to leave little or no place for purpose. Darwinism, when it appeared in the next century, was sometimes seen as reviving a role for purpose in nature, because “natural selection” is a bit like choice, and sometimes seen as continuing teleology’s banishment, because natural selection is a physical process with no conscious guidance. I think the right interpretation is that teleological ideas did get a low-key revival within Darwinism, and something similar happened within cybernetics, a field that arose a century or so later.

In outlining how all this works, I’ll make use of ideas from the American philosopher Larry Wright, who wrote about the scientific place of teleological ideas in a very insightful way. I augment his ideas with modifications of my own.

Some things, Wright said, happen because of their effects. That sounds odd, backwards, impossible—and in a pure form, it is. The actual effects something has can’t reach around through time and cause it to happen, or to come into existence. But there are a couple of approximations to that situation. One is where we think about an effect, something we want to happen, and act accordingly. We act in a way that we think will bring the effect about. I think that if I put this wedge of paper under the table leg, it will stop the table from rocking, so I put the paper there. Maybe it won’t work, but that was my goal, and that is why a bit of paper is now under the table leg.

Some non-conscious, less intentional processes can have a similar feature. Suppose you are looking at a biological organ like a heart, or an event like an adrenaline flow. What you are looking at is the latest member of a long line of similar objects or events. A long line of beating hearts was there in other animals before this one. A long line of adrenaline flows went before this one. In cases like this, earlier members of the line have done something, have had effects, that help bring newer members of the line into existence. Earlier hearts helped keep animals alive, and that has led to hearts being kept around, and also refined, in evolutionary processes. Hearts don’t directly give rise to more hearts, but they do play an indirect role in new hearts being produced. The same is true of adrenaline flows. The useful effects that earlier members of the line had can play a role in explaining why things are the way they are now—why hearts, and adrenaline flows, have continued on. Adrenaline flows, hearts, and mating displays exist, roughly speaking, because of their effects.

Hearts are there because they pump blood—that is their function within the body. The bit of paper is under my table’s leg because it will stop the table’s rocking. That is its function, and that was the goal of my action in putting it there. This is a low-key rehabilitation of concepts that used to be used in more adventurous, all-encompassing ways. This rehabilitation does not carry over the idea that for something to perform its function is good, in a moral sense. That is left behind.

So far I’ve talked about two kinds of processes that can give rise to functions, purposes, and so on. One is evolution by natural selection and the other is deliberate, conscious choice. There are a few others—processes that are similar to evolution, or similar to deliberate choice. One is learning by trial and error, doing something today because it worked yesterday. You might have done it entirely accidentally the first time (it was a “random mutation” in your behavior), but it worked, so you continue to do it. This case includes a role for choice, but it also has similarities to evolution, and the choice need not always be conscious.

Larry Wright suggested that many of our ways of talking about purposes and goals originate in what is sometimes called a “dead metaphor”—a metaphor that has lost its metaphoricalness and given rise to a new way of using language literally. He thought that the conscious cases, where we do something with a goal in mind, are the starting point, and the nonconscious processes of evolution by natural selection, and some others, are treated as analogous to the conscious case. When hearts have been kept around by evolution because of their useful effects, this is like a person choosing to install a fan because of its useful effects.

In none of these cases is it literally true that X happens because it leads to Y, or X comes to exist because it does Y. Something can’t exist because of what it will do, an effect it will have. That would be backward causation, or a kind of perfect teleology. Instead, the idea is that “X happens because it will lead to Y” gestures toward some phenomena that are real and that are close to the impossible case in different ways.

The history of our ways of talking about these things, according to Wright (and I think I agree), starts from the case of conscious planning. In nature, it goes the opposite way; it goes from unconscious evolutionary design, to learning, then to conscious planning and decision making. There’s been a transition in nature between different ways in which the world can be shaped by goals and purposes.

In this setting, once again, we can find borderline cases, faint glimmers of what is seen more clearly elsewhere. Think back to those negative feedback cycles, discussed earlier when we looked at the temperature on Earth. As the system moves away from some state it’s usually in, such as a temperature, it gets pulled back again. It gets pulled back because of an effect that the move away has had. The world gets warmer, that leads to more carbon being locked up in rock by the geological carbon cycle, and this (eventually) makes things cooler, heading back to where we started.

A heart, by pumping blood, does something that explains why it’s there. In the carbon cycle feedback case, an increase in temperature has effects that explain why, in the future, the higher temperature is not there. Some of the same shape is present—faintly and in a transformed version—in a different and larger system that involves the whole Earth. If we go back to Darwinian evolution, that process, too, is a kind of feedback, now using the term in a broader way. The useful effects of hearts (and adrenaline flows) help animals to stay alive, this leads to them having offspring who also have hearts, and so on. The Darwinian process is a kind of grand feedback system that depends on organisms and reproduction, and this is one of the great engines of creation on Earth.

Here is one more borderline or faint case. Some kinds of learning, I said, are similar to Darwinian evolution. Trial and error is like mutation and selection. Tim Lenton (the Gaiafriendly Earth scientist I mentioned earlier), drawing on conversations with the evolutionary theorist William Hamilton, noted that although the Earth does not exist within a population of the sort needed for evolution by natural selection, there might be a scenario in which life started up on Earth, then crashed, then started up again a bit differently, and so on, until it found a way of doing things that was stable. There could have been a series of trial-and-error episodes in the history of the Earth. This could happen, in principle, and then one might say, with poetic license, that the Earth learns, rather than evolves, to maintain life well.

Has there ever been anything like this in Earth’s actual history? Perhaps, very far back. There have been a couple of events in which the entire surface of the Earth, or much of it, was frozen over. A runaway global cooling led to a snowball, or ice slushy, on a planetary scale. Then Earth broke out of this and made its way back to a state friendlier to life.

Life wasn’t completely extinguished during these snowball misadventures. What crashed and recovered was some sort of overall setup involving living activity, the atmosphere, ocean chemistry, and other factors. One can imagine a situation where this happened several times—a series of failures and resets—before things got onto a more even keel.

I was surprised by this idea when I learned about it from Lenton. In principle it does make sense. This is a hint, a faint case, of a grand-scale selection process that the Earth could be part of. Whether the crash-and-rebuild sequence actually led to significant results is a further question. I wonder if the past events we have evidence for could have done much to shape the life-friendly processes we’ve been looking at in this chapter. The possible snowball Earth events were rare (two or three in total) and very far back. They were well before life on land, for example. But I don’t want to dismiss the idea.

When I think about that picture in which the Earth system makes its way to a healthy state in fits and starts—getting going, collapsing, and having to stumble back—I am reminded of David Hume, the great Scottish philosopher of the Enlightenment, whose Dialogues Concerning Natural Religion (posthumously and anonymously published) include a conversation in which one character concedes that perhaps some sort of deity made the Earth, but—he asks—what sort? Perhaps, he says, this world “was only the first rude essay of some infant deity, who afterwards abandoned it, ashamed of his lame performance.” Or, alternatively, our world “is the work only of some dependent, inferior deity; and is the object of derision to his superiors.” Or, lastly, it is perhaps the product “of old age and dotage in some superannuated deity.” That creator has now died, and since then his world has been left to go on its own way—“has run on at adventures.”

However this life-covered Earth came to be, and whether or not there were fits and false starts before our current track was finally established, “running on at adventures” is exactly what has happened since.