by Bruno Clarke
Darwin did not know, as we do now, that the air we breathe, the oceans, and the rocks are all either the direct products of living organisms or have been greatly modified by their presence. In no way do organisms just “adapt” to a dead world determined by physics and chemistry alone. They live with a world that is the breath and bones of their ancestors and that they are now sustaining. —James Lovelock, “Geophysiology,” Reviews of Geophysics 17 (1989)
Lovelock’s organic metaphors of “breath and bones” convey a composite image of Gaia’s living embodiment in the sum of life and its immediate environment at any given moment. The breath and bones currently at hand are the contemporary sky and earth—the air all around and the rocky parts of our own living world. The air that fills the sky that lies upon the Earth also fills us, whirling through every living thing in some fashion. The bones beneath our feet weather down and recycle their elements in the continuous regeneration of bodily forms.
There was a time when the Earth and its atmosphere were strictly physico-chemical entities, abiotic residues of the cosmic formation of our planet over 4.5 billion years ago. We call this primal time the Hadean eon. But ever since the Archaean eon, when microbial life took over the planet, its air has gradually become increasingly biogenic, produced by life: Gaia’s breath.
The key thing to realize about the air we now breathe is that, as Lovelock affirms, it has been pumped up and held in place by “our ancestors”—all past life from the moment life on Earth began some four billion years ago, the myriad metabolisms of innumerable living organisms over geological time. And the greater proportion of organic emissions has always been microbial.
All life forms produce gaseous by-products of some sort. In the earliest times, the predominant biogenic contribution to the atmosphere was methane. But since the beginning with the Proterozoic eon, the exhalations of photosynthetic organisms have raised the level of oxygen far beyond that of methane or carbon dioxide. In Phanerozoic times, the evolution and spread of forests pushed atmospheric oxygen up to its current Gaian state, a stable proportion of 21%.
The concept of Gaia developed by James Lovelock and Lynn Margulis is grounded in her deep knowledge of the microcosm, the leading role of the microbes in the scheme of life, including the unfolding of their evolutionary histories, their places in the phylogenetic network. In their earliest formations, Gaia’s breath and bones coalesced as the Archaean eon of prokaryotic archaea and bacteria gave way to the Proterozoic eon of eukaryotic cells.
Credited in its modern form to Lynn Margulis’s work on symbiosis as a source of evolutionary innovation, this diagram tells the story of the serial endosymbiosis of the eukaryotic or nucleated cell, the cell-type at the basis of the subsequent four biological kingdoms—Protists or Protoctists, like amoebae or algae, Fungi, Plants, and Animals.
The earliest living cells underwent a primordial divergence into two domains, the archaea and the bacteria. Members of the archaean domain were typically anaerobic fermenters like the methanogens. Within the bacterial domain, however, two exquisite innovations arose to be gifted to later life-forms: the development of oxygen-respiration in the family of alphaproteobacteria, coded purple, and the development of photosynthesis in the cyanobacteria, coded green.
We can pick up the story of endosymbiosis told here with entry of the purple bacteria into a supportive archaeal host. These microbial actors may have been uninvited guests to begin with, but they eventually settled into a sheltered life inside their host cell. This endosymbiotic merger was an evolutionary success, and in time, as this consortium evolved into the eukaryotic cell, the enclosed purple bacteria evolved into the mitochondria, the eukaryotic organelles that handle the work of oxygen-respiration in all aerobic organisms.
Meanwhile, long before the eukaryotic consummation of these evolutionary events around the middle of the Proterozoic eon, in the middle of the prior, Archaean eon, some three billion years ago, cyanobacteria, the bacterial organisms previously mislabeled “blue-green algae,” evolved the faculty of photosynthesis. A second moment of endosymbiotic merger then set the stage for the robust radiation of subsequent photosynthetic organisms.
Photosynthesis grabs solar photons to split hydrogen from its oxygen partner in water molecules and bind it to ambient carbon atoms, thus producing the carbohydrates—sugars and starches—on which the photosynthesizers feed themselves, and which composes the primary substance of plant bodies.
Additionally, the cellular organelles responsible for photosynthesis in all plants and other green organisms, the chloroplasts, evolved from the incorporation of cyanobacteria into the early extant eukaryotic cell types. The evolution of cyanobacteria in the Archean eon bequeathed to almost all subsequent photosynthetic life forms an exquisite metabolic process—autotrophy—by which they supply their nutritional needs on nothing more than water, air, and sunlight.
Gaia’s bones are the rocky forms deposited by the skeletons of expired organisms. The eukaryotic form we will focus on for its role in constructing many stony parts of the biosphere is the coccolithophore, a modern form of shell-producing protist. Its distant ancestors began laying down chalky sediments soon after their appearance sometime in the long Proterozoic eon, a service the coccolithophore carries on to this day.
Coccolithophores are protists, eukaryotic microbes on the way to plants properly speaking. This form of phytoplankton grows a distinctive coating of overlapping calcium carbonate plates called coccoliths. Coccoliths and other carbonate extrusions of living beings represent a major sink for carbon dioxide. Sometimes they dissolve, releasing their carbon back into their ambient seas, sometimes they fall intact to the ocean floor in sedimentary layers hundreds of meters thick, eventually to be uplifted, as famously revealed in Dover’s white cliffs or in the elongated limestone shelves of Salento.
Moreover, coccolithophores are photosynthetic protists. They are the main photosynthetic forms of the open ocean and make a significant contribution to the oxygen production to the biosphere. That is, coccolithophores make generous donations to Gaia’s breath on the way to building up Gaia’s bones, in the form of limestone.
Limestone and Karst
Geologists estimate that the equivalent of 23,000 contemporary biospheres are locked up in the rocks of Earth's crust and mantle. For instance, almost all limestone is the post-organic product of the activity of living things. The growth of coral reefs creates biogenic limestones, while sediments of calcium carbonate organic debris such as coccoliths produce bioclastic limestone, like the chalks formed from accumulations of carbonate shells in shallow seas.
Most sedimentary rock—which form makes up a quarter of all rocks, sedimentary, igneous, or metamorphic—is limestone. What we see all around us in Salento is a massive uplifted platform of sedimentary limestone long exposed to surface weathering. Because rainwater dissolves limestone, it slowly erodes to create karst landscapes full of sinkholes, springs, buried streams, caverns, and coastal caves, such as the Grotta Gaia in Porto Selvaggio, just to the north of Santa Caterina di Nardo.
Moreover, it turns out, limestone is the key ingredient of cement, a primary substance with which civilization has constructed its built environment:
The most common type of cement in the world . . . is made from limestone. . . . Baked . . . at . . . around 2,700 degrees Fahrenheit — limestone transforms into carbon dioxide and calcium oxide. That calcium oxide, or lime, is a key ingredient in the cement that holds our bridges, apartment buildings and roads together. (Washington Post 27.6.23)[i]
All of these stones—even the artificial and utilitarian forms of molded cement manufactured to human specifications—are ultimately based on the recycling of bygone life. As we revere the sacred stones of Salento laid down and carved out by natural processes over geological time, let us also express our gratitude for the humble microbes whose incessant lust for life has given us the larger part of these worldly gifts.
Nevertheless, it is good to recall that not all evolutionary developments are happy ones for all concerned. For instance, the arrival of photosynthesis set the stage for a major extinction episode variously called the Great Oxidation Event (GOE), the Great Oxygenation Event, the Oxygen Catastrophe, the Oxygen Revolution, the Oxygen Crisis, or the Oxygen Holocaust.
The GOE is credited with igniting the subsequent development of more complex life-forms. The increasing accumulation of biogenic oxygen in Earth's atmosphere massively boosted the energy budgets of the biosphere. However, these innovations came at a considerable ecological price for the life extant at that moment: the microbes at home in the environment prior to the evolution of photosynthesis were mostly, like cyanobacteria itself, anaerobic. For them, free oxygen was not a feedstock but a lethal poison. Many early organisms likely went extinct in this conjectured Oxygen Holocaust. Yet this outcome also favored the evolutionary success and longevity of the aerobic organisms that could use the new stores of oxygen to their advantage.
However, those anaerobic organisms that survived often did so, and their descendants are still thriving today, because they found or formed oxygen-free niches, workable Gaian environmental solutions, such as the mud of alluvial soils or marine sediments, the hindguts of insects, or the digestive tracts of mammals.
From this perspective, the arrival of the Anthropocene—the name given to the observation that human activities, especially those related to agriculture and resource extraction and supercharged by colonialism and fossil-fuel use, have altered the breath and the bones of the entire planet, refashioning Gaia’s composition in a matter of mere centuries, mere millennia at most—should not seem all that improbable.
We modern humans often like to think that we’re above or exempt from all this nonhuman business. But that notion is on its deathbed. Instead, it is increasingly clear that we are inextricably inside and a part of the Gaian system. And in that case, our currently lopsided and noxious anthropogenic contributions to the Anthropocenic biosphere are just some of the latest tricks up Gaia’s many sleeves. However, here is the inhuman fact we must face: Gaia does not care if humans live or die.
If Gaia may be said to have an aim—one that rises above sheer thermodynamic tendencies toward the dissipation of energy throughout the cosmos—then that aim runs counter to the drift of entropy and seeks out the persistence of life, period. When our human activities threaten the habitability of this planet and the persistence of life at large, those actions threaten their own viability, for without effective and viable alliance with the biosphere, modern humans are becoming Gaian liabilities.
No human technology or quick fix can hope to enfold and overcome such fundamental contingencies. Instead, it’s time for us to seek out species-level Gaian alliances and put them into practice. Human beings must become Gaian beings, conspiring with life on life’s behalf, breathing more easily when breathing together. Artists in particular are well positioned to take the lead in this existential campaign. Let’s get started.
Poly's Sacred Stones
Seas of time
stones of lime
If you go
you will know
how it flows
down the wells
of the sea
Turn to stone
that turns to bone
that holds the breath
that kills the death
Of life on Earth
and what it's worth
when seas of time
make stones of lime
We're not on the outside looking in
We're on the inside looking out
—for Polyxene Kasda
Presented on June 30, 2023, at the Baroque Blue artistic residency curated by Dores Sacquegna, "The Sacred Stones of Salento," Santa Caterina di Nardo, Italy