A Planetary Perspective

This is the second article in a series about the Earth-system – how our planet has shaped us as human beings, and how we, in turn, have shaped it. Read the series: Part 1 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10 | Part 11 | Part 12

We live on a madly unstable planet. For billions of years it has never stopped wobbling and wandering, freezing and thawing, cracking and flowing, blooming, dying, regenerating. Small changes in its chemical composition, geological cycling, orbit, or orientation in space cascade into extreme changes felt on its surface. Despite this—maybe because of it—our planet has also given birth to a complex, interwoven, and forever changing family of living beings, a rarity in the cosmos as far as we know. Understanding this bigger picture of the relationship between the planet itself and human life can help us gain some perspective on our modern predicament. So let’s begin at the beginning.

Illustration: Pariplab Chakraborty

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Earth was born about four and a half billion years ago, alongside several sibling planets, all of them formed from fragments of material left over after the birth of the Sun. It began as a glob of fiery, fluid rock spinning amidst other bits of ice and stone. But over the course of Earth’s first half billion years, its molten surface cooled and solidified; its dry, sulfurous skies became unending showers of acid rain; its empty basins filled with water. In these new oceans simmered a rich elemental brew of carbon, hydrogen, nitrogen, phosphorous, iron—to mention but a few star ingredients—absorbing heat-energy from deep-sea hydrothermal vents and joining together into chains of chemicals. These complex molecules then clustered together, generating higher order structures, eventually forming cells that ate and breathed and reproduced—living avatars of dust, born of Earth’s own dynamically interconnected biological, geological, and chemical processes, or biogeochemistry.

While these earliest lifeforms were coming into being, Earth’s surface was still being relentlessly pummelled by meteors, some of them the size of mountains. Great impacts injected new material into our planet, cracking and reshaping its biogeochemistry. In this environment, about 3.5 billion years ago, some of those early cells evolved a new ability to convert light-energy from the sun into chemical fuel through a process, called photosynthesis, that takes carbon-dioxide from the air and emits oxygen as metabolic waste. By two billion years ago, the buildup of oxygen was turning Earth’s skies blue above its oceans of rust-red. Many forms of life in that nascent biosphere were poisoned by this atmospheric oxygen and went extinct.

But life itself was not defeated. The cells that did survive continued to diversify into myriad new forms that could withstand and even make use of this corrosive new gas. Some of these found ways to work together, joining into multicellular lifeforms, mixing genes through sexual reproduction, eventually filling the oceans with early forms of fungi, plants, and animals by around one billion years ago. These creatures grew and died, burrowed and crawled, built residual structures, stirred sediments, altered rocks and seabeds in ways that further changed Earth’s biogeochemistry. This changing geochemistry also affected the structure and motion of Earth’s crust, now sliding and tearing, bumping and churning up the landforms into barren mountains and valleys and volcanic alleys, exposing new, unweathered rock—which further changed biogeochemical systems.

At some point in the midst of this lively flourishing, the changes led to the entire planet freezing over. By around 720 million years ago, floating icebergs and islands of slush were drifting over the tropical seas of a ‘snowball Earth,’ as the planet lay cocooned within vast sheets of ice. In the stagnant, anoxic waters beneath, life barely carried on—but it did carry on. And after the ice truly loosened its grip, about 630 million years ago, the quiescent biosphere once again surged. As the seas turned bluegreen, saturated again with oxygen, undersea life burst forth in a stunningly rapid increase in biodiversity.

By 450 million years ago, some of these new creatures had crept out onto the land—the first multicellular beings ever to colonize the barren surfaces of rock pocked by meteors, scarred by rain, wind, glaciation, volcanic and tectonic churn. Over the next tens of millions of years, an ever-evolving profusion of life would spread throughout the shape-shifting continents, the rising and tumbling mountains, the expanding and shrinking islands, even as the planet rebounded again and again between different climatic states. Global average temperatures sometimes rose or fell by several degrees—magnitudes that dwarf the climate change already threatening modern human civilization. But such shifts took thousands of years, far slower than the changes we’re observing today. Sometimes extreme changes in climate and biogeochemistry triggered global mass extinction events, when more than half of all known extant lifeforms were wiped out. Still, whenever the oceans were warm—and most of the past half a billion years have been very warm—algae and phytoplankton proliferated in tremendous blooms, billions of photosynthesizing cells, reproducing and dying with vigorous rampancy; copious quantities of their remains piled up on the bottom of the ocean.

By four-hundred million years ago, forested swamplands had grown across large parts of Earth’s landmasses. Great, primeval trees sucked the abundant carbon-dioxide from the sky and used the carbon to build their trunks—another novel biochemistry: wood. When the trees died, they didn’t quite rot away, for creatures fully able to digest wood had not yet evolved. Instead, the woody plants fell into heaps that sunk slowly into the mud. Folded deep into the ground, the detritus of those first trees and all those algae and plankton were pressed over many millions of years into beds of new, carbon-dense mineral formations: the ancient wood became coal; the residues of algae and plankton turned into petroleum and natural gas. These minerals held hundreds of millions of years’ worth of captured sunlight-energy stored within the chemical bonds of carbon-based compounds.

Though we humans remain yet a long way off in this chronicle, we might note this as the genesis of the fossil fuels we would eventually take as the lifeblood of our modern global civilisation, whose happenstance distribution around the world would become a primary driver of modern geopolitics—even impacting responses to the present war in Gaza. We might also observe that the Earth-system continuously reinvents itself, never returning to an identical previous state. Nor is there any final or fixed steady state, no destination or goal. There have been many different versions of this one Earth, and many more will come. There’s no reason to expect that the way we’ve known it since the time of written histories describes its normal or neutral state, the way it’s meant to be, or the way we might be able to maintain it in support of our present way of life. Earth has known nothing but continuous change. And we humans are but one aspect of these ongoing changes: we too are recently grown forms of Earth’s living crust, who someday will be gone or changed unrecognisably by the forces of evolution. That is a law of life. It ebbs and flows. It transmutes itself. It is never static.

The first mammals—small, shrew-like creatures—appeared around 225 million years ago. At the time, nearly all the world’s land was connected together into a single, giant landmass, a supercontinent we call Pangaea. Around 180 million years ago, Pangea began to break up, drifting into continents somewhat more familiar to us today. As the climate kept on fluctuating, new organisms continuously arose, some of whom were much better at digesting the woody trunks of trees, rotting them away and thus curtailing newer formations of coal.

Meanwhile, reptiles had grown gigantic. Dinosaurs, in their various forms, great and small, remained the dominant beasts on land for 250 million years. Their heyday only ended, one fine day about 66 million years ago, when the giant Chicxulub meteor crashed into the sea. The heat of the collision instantly incinerated large swathes of the planet’s surface. Fire and rocky debris rained down upon the land. The impact unleashed a mega-tsunami with waves hundreds of meters high. It forced a seismic response that caused swarms of volcanoes around the globe to belch and spurt continuously for tens of thousands of years, burying millions of kilometres of land under burning lava—and shaping the geography of many lands, including the Deccan Plateau. This magnitude of devastation once again utterly revised life on Earth, in what’s referred to as the Fifth Great Mass Extinction Event—the fifth time that most species then existing on Earth rapidly died out.

But as before, life eventually bounced back with even greater diversity. New mixes of forests grew. Birds now filled the skies. By forty million years ago, mammals were emerging as the most dominant class of creature on land, spreading themselves under steamy jungle canopies around the planet, some even sliding back into the oceans, evolving into whales and dolphins. This exuberant flowering of life was not quelled even as the planet embarked upon yet another cooling trend. Snow began to fall again after many millions of years, first in the mountains of Antarctica. Over the following millions of years, the world’s highest peaks, north and south, slowly donned mantles of ice. Earth had been encrusted in ice before, but not when it was also adorned by forests and grasslands swarming with legions of warm-blooded creatures, birds and mammals, amid the richest, densest, most diverse biota it had ever created.

In this cooling, drying climate, some forests in Africa were burning away into grasslands. It was then that our earliest hominid ancestors—two-legged apes of short stature—climbed down from their shrinking tree cover and strode onto the African savannas, some six million years ago. Like other primates, they probably used some tools, such as digging sticks. But over the next few million years, as their cognitive capacities were reshaped by their changing environment, they began noticing the properties of different types of stone, how best to break them into shapes useful for chopping things. They also came to understand some things about fire.

By two million years ago, when massive fields of ice had well taken hold of Antarctica and the far northern latitudes, our early hominin ancestor, Homo erectus, had become recognisably human, though not quite identical to us (we, Homo sapiens sapiens, appeared a mere three hundred thousand years ago). With their well-developed stone tools and mastery of fire, H. erectus ventured beyond Africa, eventually colonizing much of southern Eurasia from what today is Spain to India to China and the farthest reaches of peninsular southeast Asia (which would later become the islands of Indonesia). None of what followed would have been possible without both fire and a propensity to build thick social worlds, two aspects of being human that appear tightly linked and are the subject of the next essay.

Usha Alexander trained in science and anthropology. After working for years in Silicon Valley, she now lives in Gurugram. She’s written two novels: The Legend of Virinara and Only the Eyes Are Mine.