This is the second article in a 12-part series about the Earth-system, how our planet has shaped us as human beings, and how we, in turn, have shaped it The article appeared here first, in Unraveling Climate Change, a series for The Wire.
Ancient lava flows, part of the Deccan Traps, visible today in Panna National Park, MP. Photo credit: shunya.net.
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.
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 pummeled 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.
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