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A Song of Fire and Ice
Once upon a time, a giant star was dying. It had been burning for millions of years; now the fusion furnace at its core had no more fuel to burn. The star created the energy it needed to shine by fusing hydrogen atoms to make helium. The energy produced by the fusion did more than make the star shine. It was vital to counteract the inward pull of the star’s own gravity. When the supply of available hydrogen began to run low, the star began to fuse helium into atoms of heavier elements such as carbon and oxygen. By then, though, the star was running out of things to burn.
The day came when the fuel ran out completely. Gravity won the battle: the star imploded. After the millions of years of burning, the collapse took a split second. It prompted a rebound so explosive that it lit up the universe—a supernova. Any life that might have existed in the star’s own planetary system would have been obliterated. But in the cataclysm of its death were born the seeds of something new. Even heavier chemical elements, forged in the final moments of the star’s life—silicon, nickel, sulfur, and iron—were spread far and wide by the explosion.
Millions of years later, the gravitational shock wave of the supernova explosion passed through a cloud of gas, dust, and ice. The stretch and squeeze of the gravitational wave made the cloud fall in on itself. As it contracted, it started to rotate. The pull of gravity squeezed the gas at the cloud’s center so much that atoms began to fuse together. Hydrogen atoms were pressed together, forming helium, creating light and heat. The circle of stellar life was complete. From the death of an ancient star emerged another, fresh and new—our Sun.
* * *
The cloud of gas, dust, and ice was enriched with the elements created in the supernova. Swirling around the new Sun, it also coagulated into a system of planets. One of them was our Earth. The infant Earth was very different from the one we know today. The atmosphere would have been to us an unbreathable fog of methane, carbon dioxide, water vapor, and hydrogen. The surface was an ocean of molten lava, perpetually stirred up by the impacts of asteroids, comets, and even other planets. One of these was Theia, a planet about the same size as today’s Mars.1 Theia struck the Earth a glancing blow and disintegrated. The collision blasted much of the Earth’s surface into space. For a few million years, our planet had rings, like Saturn. Eventually the rings coalesced to create another new world—the Moon.2 All this happened approximately 4,600,000,000 (4.6 billion) years ago.
Millions more years passed. The day came when the Earth had cooled enough for the water vapor in the atmosphere to condense and fall as rain. It rained for millions of years, long enough to create the first oceans. And oceans were all there were; there was no land. The Earth, once a ball of fire, had become a world of water. Not that things were any calmer. In those days the Earth spun faster on its axis than it does today. The new Moon loomed close above the black horizon. Each incoming tide was a tsunami.
* * *
A planet is more than a jumble of rocks. Any planet more than a few hundred kilometers in diameter settles out into layers over time. Less dense materials such as aluminum, silicon, and oxygen combine into a light froth of rocks near the surface. Denser materials such as nickel and iron sink to the core. Today, the Earth’s core is a rotating ball of liquid metal. The core is kept hot by gravity and the decay of heavy radioactive elements such as uranium, forged in the final moments of the ancient supernova. Because the Earth spins, a magnetic field is generated in the core. The tendrils of this magnetic field reach right through the Earth and stretch far out into space. The magnetic field shields the Earth from the solar wind, a constant storm of energetic particles streaming from the Sun. These particles are electrically charged and, repelled by the Earth’s magnetic field, bounce off or flow around the Earth and into space.
The Earth’s heat, radiating outward from the molten core, keeps the planet forever on the boil, just like a pan of water simmering on a stove. Heat rising to the surface softens the overlying layers, breaking up the less dense but more solid crust into pieces and, forcing them apart, creates new oceans between. These pieces, the tectonic plates, are forever in motion. They bump against, slide past, or burrow beneath one another. This movement carves deep trenches in the ocean floor and raises mountains high above it. It causes earthquakes and volcanic eruptions. It builds new land.
As the bare mountains were thrust skyward, vast quantities of the crust were sucked back into the depths of the Earth in deep ocean trenches at the edges of the tectonic plates. Laden with sediment and water, this crust was drawn deep into the Earth’s interior—only to return to the surface, changed into new forms. The ocean-floor sludge at the fringes of vanished continents might, after hundreds of millions of years, reemerge in volcanic eruptions3 or be transformed into diamonds.
* * *
Amid all this tumult and disaster, life began. It was the tumult and disaster that fed it, nurtured it, made it develop and grow. Life evolved in the deepest depths of the ocean, where the edges of tectonic plates plunged into the crust and where boiling hot jets of water, rich in minerals and under extreme pressure, gushed out from cracks in the ocean floor.
The earliest living things were no more than scummy membranes across microscopic gaps in rocks. They formed when the rising currents became turbulent and diverted into eddies and, losing energy, dumped their cargo of mineral-rich debris4 into gaps and pores in the rock. These membranes were imperfect, sievelike, and, like sieves, allowed some substances to cross but not others. Even though they were porous, the environment inside the membranes became different from the raging maelstrom beyond: calmer, more ordered. A log cabin with a roof and walls is still a haven from the arctic blast outside, even if its door bangs and its windows rattle. The membranes made a virtue of their leakiness, using holes as gateways for energy and nutrients and as exit points for wastes.5
Protected from the chemical clamor of the outside world, these tiny pools were havens of order. Slowly, they refined the generation of energy, using it to bud off small bubbles, each encased in its own portion of the parent membrane. This was haphazard at first but gradually became more predictable as a result of the development of an internal chemical template that could be copied and passed down to new generations of membrane-bound bubbles. This ensured that new generations of bubbles were, more or less, faithful copies of their parents. The more efficient bubbles began to thrive at the expense of those less well-ordered.
These simple bubbles found themselves at the very gates of life, in that they found a way to halt—if temporarily and with great effort—the otherwise inexorable increase in entropy, the net amount of disorder in the universe. Such is an essential property of life. These foamy lathers of soap-bubble cells stood as tiny, clenched fists, defiant against the lifeless world.6
* * *
Perhaps the most amazing thing about life—apart from its very existence—is how quickly it began. It stirred itself into existence a mere 100 million years after the planet itself formed, in volcanic depths when the young Earth was still being bombarded from space by bodies large enough to create the major impact craters on the Moon.7 By 3.7 billion years ago, life had spread from the permanent dark of the ocean depths to the sunlit surface waters.8 By 3.4 billion years ago, living things had started to throng together in their trillions to create reefs—structures visible from space.9 Life on Earth had fully arrived.
These reefs were not composed of corals, however. They still lay almost 3 billion years into the Earth’s future. They consisted of greenish, hair-thin threads and scuts of slime made from microscopic organisms called cyanobacteria—the same creatures that form the bluish-green scum on ponds today. They spread in sheets over rocks and lawns on the seabed, only to be buried by sand in the next storm: but conquering again and being buried once again, building cushion-like mounds of layered slime and sediment. These mound-shaped masses, known as stromatolites, were to become the most successful and enduring form of life ever to have existed on this planet, the undisputed rulers of the world for 3 billion years.10
* * *
Life began in a world that was warm11 but soundless apart from the wind and the sea. The wind stirred an air almost entirely free from oxygen. With no protective ozone layer in the upper atmosphere, the Sun’s ultraviolet rays sterilized everything above the surface of the sea or anything less than a few centimeters beneath the surface. As a means of defense, the cyanobacterial colonies evolved pigments to absorb these harmful rays. Once their energy had been absorbed, it could be put to work. The cyanobacteria used it to drive chemical reactions. Some of these fused carbon, hydrogen, and oxygen atoms together to create sugars and starch. This is the process we call photosynthesis. Harm had become harvest.
In plants today, the energy-harvesting pigment is called chlorophyll. Solar energy is used to split water into its constituent hydrogen and oxygen, releasing more energy to drive further chemical reactions. In the earliest days of the Earth, however, the raw materials were just as likely to have been minerals containing iron or sulfur. The best, however, was and remains the most abundant—water. But there was a catch. The photosynthesis of water produces as a waste product a colorless, odorless gas that burns anything it touches. This gas is one of the deadliest substances in the universe. Its name? Free oxygen, or O2.
To the earliest life, which had evolved in an ocean and beneath an atmosphere essentially without free oxygen, it spelled environmental catastrophe. To set the matter into perspective, however, when cyanobacteria were making their first essays into oxygenic photosynthesis—3 billion years ago or more—there was rarely enough free oxygen at any time to count as more than a minor trace pollutant. But oxygen is so potent a force that even a trace spelled disaster to life that had evolved in its absence. These whiffs of oxygen caused the first of many mass extinctions in the Earth’s history, as generation upon generation of living things were burned alive.
* * *
Free oxygen became more abundant during the Great Oxidation Event, a turbulent period between about 2.4 and 2.1 billion years ago, when, for reasons still unclear, the concentration of oxygen in the atmosphere at first rose sharply—to greater than today’s value of 21 percent—before settling down to a little below 2 percent. Although still unbreathably tiny by modern standards, this had an immense effect on the ecosystem.12
An upsurge in tectonic activity buried vast quantities of carbon-rich organic detritus—the corpses of generation on generation of living things—beneath the ocean floor. This kept it away from oxygen’s reach. The result was a surplus of free oxygen that could react with anything it touched. Oxygen etched the very rocks, turning iron to rust, and carbon to limestone.
At the same time, gases such as methane and carbon dioxide were scrubbed from the air, absorbed by the abundance of newly formed rock. Methane and carbon dioxide are two of the gases in the downy filling of the insulating blanket that keeps the Earth warm. They promote what we call the greenhouse effect. Without them, the Earth plunged into the first and greatest of its many ice ages. Glaciers spread from pole to pole, covering the entire planet in ice for 300 million years. And yet the Great Oxidation Event and subsequent so-called Snowball Earth episode were the kinds of apocalyptic disasters in which life on Earth has always thrived. Many living things died, but life was spurred on to undergo its next revolution.
* * *
For the first 2 billion years in the Earth’s story, the most sophisticated form of life was built on the bacterial cell. Bacterial cells are very simple, whether single or glued together in sheets across the ocean floor or in the long, angel-hair filaments of cyanobacteria. Each one, on its own, is tiny. As many bacteria could fit on the head of a pin as there were revelers who went to Woodstock—and with room to spare.13
Under a microscope, bacterial cells appear simple and featureless. This simplicity is deceptive. In terms of their habits and habitats, bacteria are highly adaptable. They can live almost anywhere. The number of bacterial cells in (and on) a human body is very much greater than the number of human cells in that same body. Despite the fact that some bacteria cause serious disease, we could not survive without the help of the bacteria that live in our guts and enable us to digest our food.
And the human interior, despite its wide variation in acidity and temperature, is, in bacterial terms, a gentle place. There are bacteria for which the temperature of a boiling kettle is as a balmy spring day. There are bacteria that thrive on crude oil, on solvents that cause cancer in humans, or even in nuclear waste. There are bacteria that can survive the vacuum of space, violent extremes of temperature or pressure, and entombment inside grains of salt—and do so for millions of years.14
Bacterial cells may be small, but they are famously gregarious. Different species of bacteria swarm together to trade chemicals. The waste products of one species might make a meal for another. Stromatolites—as we have seen, the first visible signs of life on Earth—were colonies of different kinds of bacteria. Bacteria can even swap portions of their own genes with one another. It is this easy trade that means, today, that bacteria can evolve resistance to antibiotics. If a bacterium doesn’t have a resistance gene for a particular antibiotic, it can pick it up from the genetic free-for-all of other species with which it shares its environment.
It was the tendency of bacteria to form communities of different species that led to the next great evolutionary innovation. Bacteria took group living to the next level—the nucleated cell.
* * *
At some point before 2 billion years ago, small colonies of bacteria began to adopt the habit of living inside a common membrane.15 It began when a small bacterial cell, called an archaeon,16 found itself dependent on some of the cells around it for vital nutrients. This tiny cell extended tendrils toward its neighbors so they could swap genes and materials more easily. The participants in what had been a freewheeling commune of cells became more and more interdependent.
Each member concentrated only on one particular aspect of life.
Cyanobacteria specialized in harvesting sunlight and became chloroplasts—the bright green specks now found in plant cells. Other kinds of bacteria devoted themselves to releasing energy from food and became the tiny pink power packs called mitochondria that are found in almost all cells that have nuclei, whether plant or animal.17 Whatever their specialism, they all pooled their genetic resources in the central archaeon. This became the nucleus of the cell—the cell’s library, repository of genetic information, its memory, and its heritage.18
This division of labor made life for the colony much more efficient and streamlined. What was once a loose colony became an integrated entity, a new order of life—the nucleated, or “eukaryotic,” cell. Organisms made of eukaryotic cells—whether singly (unicellular) or lots together (multicellular)—are called eukaryotes.19
* * *
The evolution of the nucleus allowed for a more organized system of reproduction. Bacterial cells generally reproduce by dividing in half to create two identical copies of the parent cell. Variation from the addition of extra genetic material is piecemeal and haphazard.
In eukaryotes, by contrast, each parent produces specialized reproductive cells as vehicles for a highly choreographed exchange of genetic material. Genes from both parents are mixed together to create the blueprint for a new and distinct individual, different from either parent. We call this elegant exchange of genetic material sex.20 The increase in genetic variation as a consequence of sex drove an uptick in diversity. The result was the evolution of a wealth of different kinds of eukaryotes and, over time, the emergence of gatherings of eukaryote cells to make multicellular organisms.21
Eukaryotes emerged, quietly and modestly, between around 1,850 and 850 million years ago.22 They started to diversify around 1,200 million years ago into forms recognizable as early single-celled relatives of algae and fungi and into unicellular protists, or what we used to call protozoa.23 For the first time, they ventured away from the sea and colonized freshwater ponds and streams inland.24 Crusts of algae, fungi, and lichens25 began to adorn seashores once bare of life.
Some even experimented with multicellular life, such as the 1,200-million-year-old seaweed Bangiomorpha26 and the approximately 900-million-year-old fungus Ourasphaira.27 But there were stranger things. The earliest known signs of multicellular life are 2,100 million years old. Some of these creatures are as large as twelve centimeters across, so hardly microscopic, but they are so strange in form to our modern eyes that their relationship with algae, fungi, or other organisms is obscure.28 They could have been some form of colonial bacteria, but we cannot discount the possibility that there once lived entire categories of living organisms—bacterial, eukaryote, or something entirely other—that died out without leaving any descendants and that we should therefore find hard to comprehend.
* * *
The first rumbles of an oncoming storm came from the rifting and breakup of a supercontinent, Rodinia. This included every significant landmass at the time.29 One consequence of the breakup was a series of ice ages that covered the entire globe, the like of which had not been seen since the Great Oxidation Event. But life responded once again by rising to the challenge.
Life entered the lists as a range of peaceable seaweeds, algae, fungi, and lichens.
It emerged tough, mobile, and looking for trouble.
For if life on Earth was forged in fire, it was hardened in ice.
Copyright © 2021 by Henry Gee.
