How Did Life Recover From The First Mass Extinction?

Fires rage for thousands of kilometres. They spread across the landscape, consuming grasslands and fern forests alike. Ahead of these relentless blazes, life, large and small, flees in thundering herds or scattered and dropping from the air, desperately screeching alarm. An eerie, permanent twilight claims the sky, thick with dust. Without the heat from the sun, temperatures plummet and a global winter descends, freezing solid the lush forests that escape the burn, and depriving the remaining food chains of the vegetation they need to survive. This is the KT extinction event, aftermath of a catastrophic asteroid impact that left a crater 150 kilometres wide and deep enough to contain Mount Everest twice over. From this millennia-long night, there would be no dawn for the dinosaurs. Life hangs by a thread. When the dust finally clears and the sun shines once again, it does so on a largely empty world. With the big dinosaurs gone, only the hardy scavengers and survivalists are left. Some dinosaurs did make it through, those for example that could eat tough, long-lasting nuts and seeds, or which had a soft, fluffy down to protect them from the cold. Scavenging mammals, too, clung on, who were used to scraping through on short measures. It's a scorched, battered, frozen, post-apocalyptic landscape. But for them, it's a world of opportunity. Suddenly, in little more than the blink of an eye, the predators are gone. The competition is wiped out and the resources of the entire globe are theirs for the taking. With the deadly hunting pterosaurs now gone, the skies no longer threaten sudden, swooping death. So the fluffy nut-eaters can roam and climb in safety. They can even learn to fly themselves, without danger from above. Eventually, their fluff evolves to feathers and their beaks are specialised to their food source of choice. They become the wildly successful avies. Birds, of which there are tens of thousands of species, flying, running and even swimming all over the globe. Meanwhile, the mammals, once very much the underdog in a world ruled by terrible lizards, now have the run of an empty home. As the plants of the plains and forest grow back, without competition, they have the pick of the crop. They can eat more, grow larger, change their lifestyles to nurture their young, rather than endlessly looking for food. There is now more than enough energy available for the mammals to support carnivores and even apex predators. They expand into habitats all over the globe, wherever food is to be found, even into the oceans. In the few million years after the K-T extinction, mammals diversified more than in the preceding 160 million years. But of course, this was not the first time a catastrophic event had revolutionised life on the planet. Some two billion years earlier, the Great Oxidation Event, the largest extinction of all time, brought life to the brink. And when it bounced back, it too was fundamentally transformed. Indeed, without the first extinction, life may never have made it beyond simple bacteria, at all. Before the K-T extinction that wiped out the dinosaurs, before even the end Permian extinction, known as the Great Dying, there was another mass extinction that almost wiped out all life on Earth. Around two and a half billion years ago, at the fringes of thriving microbial communities, some groups had evolved to use photosynthesis instead of chemistry to acquire their energy. These new photosynthesizers could colonise everything the light touched, but the gift of photosynthesis came with an unwanted by-product. Oxygen, which in the Archean Ocean was as lethal as mustard gas. Deadly oxygen had filled the oceans and then the air. The reactive element poisoned everything in its path, and then transformed the atmosphere, stripping it of its insulating blanket of methane and plunging the planet into an ice age for three hundred million years. Thick ice extended from the pole to the equator, and along the shallow shorelines, photosynthesizers died in their droves. The architects of their own destruction. But we know that this wasn't the end of life on Earth. Our very existence on the surface of the planet today is testament to life's tenacity through the most catastrophic of events. Indeed, the vast majority of our ecosystems today rely on the very photosynthesis that brought about such destruction. Life clung on by the thinnest of threads. Just one percent of all life in Archean Oceans persisted into the next age. The Proterozoic. And it's from this one percent that the living world was rebuilt. After the dinosaur decimating KT extinction, the dust cleared on an empty world. And so too, after the great oxygen extinction. The one percent of life that survived did so in isolated refugia, perhaps where volcanoes punched through the ice like in Iceland today. The warmth of magma so near the surface maintained shallow water oases in the midst of a vast ice desert. Small communities of photosynthesizing bacteria could cling on here in their own lonely worlds. Some more ancient bacteria survived too. The anaerobes, still feeding off hydrothermal minerals in deep dark ocean crevices, is yet untouched by the toxic oxygenated waters, unaffected by the persistent darkness. Perhaps for them, the great oxygen extinction and ensuing ice age had passed by unnoticed. As long as tectonic forces maintained their supply of volcanic minerals and the poison gas didn't reach them, it was business as usual for these ancient chemotrophic bacteria. These isolated communities persisted where they could, and the volcanic activity that helped keep them alive also helped bring the ice age to an end. Untroubled by the rise and fall of bacterial civilizations at their feet, volcanoes continued to erupt, explode, and effuse carbon dioxide into the atmosphere. Gradually, an insulating layer of this greenhouse gas built up and the atmosphere could once again trap heat and warm the surface. Eventually, after a seemingly unshakable winter, the ice receded. Patches of blue ocean emerged and grew, and the warmth they absorbed helped to chase the ice back to the poles. The surviving lifeforms witnessed as the planet uncleared itself, casting off the chains that had gripped it for so long. They emerged from their tiny warm oases into oceans of untapped riches. But these oceans were empty, just as the post-dinosaur diorama would be. Only the lucky few remained to enjoy the spoils of this post-apocalyptic world. For those that had already survived the worst, this empty world was a gift of opportunity. It took little time for the shallows to become colonized once more. As the ice melted, sea levels rose, drowning the shorelines of every tiny island and making brand new habitats for opportunistic sun-loving microbes. Photosynthesis was still an ideal source of energy, since it required only the inexhaustible glow of the sun and ubiquitous carbon dioxide. And, as is the natural way of things, microbial predators evolved too, to take advantage of the energy captured by the producing photosynthesizers. Instead of capturing the sun's energy themselves, they consumed the green producers and extracted the energy contained within their organic structures. But, with such riches in store and an entire planet as their playground, the survivors had the chance to innovate further. Just as the fluffy, feathered dinosaurs learned to fly in the safety of a rich, empty world, so too could the microbial survivors figuratively spread their wings. In the late 19th century, Imperial Germany is a hotbed of science and innovation. Young, inquiring minds are drawn to the University of Bonn in the west of the empire. Palatial buildings scattered along the banks of the Rhine, set in expansive grounds, each home to a dedicated institute, and hundreds of focused scholars. In a small, overfilled office in a corner of one such institute, a finely dressed young man adjusts the gas lamp on his desk, pulling it closer to the microscope before him. The man's name is Andreas Schimper, and he is a phytogeographer, tasked with understanding the distribution of plants all over the globe. Although, on this evening in 1883, his mind is troubled by revelations more microscopic in nature. Schimper is an adventuring naturalist, like so many of the men in his family. At the tender age of 27, he has already taken steamships across the Atlantic to Venezuela and the West Indies. Traveling to the corners of the globe in the late 19th century was a thrilling and dangerous affair, and Andreas Schimper has a taste for botanical adventure. His exploration and research will give rise to the term tropical rainforest, and many species of plant will come to be named in his honor. But Andreas believes that to truly understand the nature of an exotic plant, one must study it at the smallest possible scale. And so, he spends hour upon hour poring over paper-thin slices of his specimens, staring at their box-like cells and the green speckled chloroplasts inside them. He sees this same fundamental organization in every green leaf he places beneath the objective. Whether lovingly returned from the Venezuelan jungle or plucked from the lawns of his own institute, chloroplasts and their surrounding cells, each distinct, but their fates inextricably intertwined. How did this come to be? Schimper has a theory, but it's almost too outrageous to consider. He pauses with his pen hovering above the paper, and slowly begins to write. Back in the protoerozoic ocean playground, life is rosy for the photosynthesizing bacteria. They have resources in abundance, and all the resistances they need to survive in this brave new oxygenated world. But other survivors of the great oxygen extinction are still struggling. Anaerobic bacteria still shrink from the damaging reactive oxygen, and are still dependent on the chemical energy sources that have supported them for the last billion years. Perhaps gene transfer could have been their savior once, the same gene transfer that allowed the knowledge of photosynthetic pathways to be shared among microbial communities at the beginning of the great oxygenation. But these anaerobes have been separated from their oxygen-loving sisters for hundreds of millions of years, and the rift is too wide to be crossed by genetic trading alone. But that's not to say that they can't benefit from each other in a different way. In the bountiful world of the protoerozoic, these two disparate groups do something extraordinary. They cooperate. Cooperation in the living world is commonplace today, if you know where to look. On the African plains, small dowdy birds spend their days perching around, and often on, the large game that roam the savannah. They are oxpeckers, and the clue to their lifestyle is in their name. They live among rhinoceroses, zebra, and other large mammals, and feast almost exclusively off the insects they find on the mammals' backs. In this way, the birds get an easy meal, and perform near-constant pest control in return. This kind of symbiosis, where both parties benefit in some way, occurs time and time again in the animal kingdom. Coteries of cleaner fish hover around the bellies and open mouths of deadly sharks, eating dead skin, parasites, and cast-off food. The service they provide renders them immune to the shark's attention, and they are protected in turn by their ferocious patrons. Mutualism even extends between animals and plants, as flowers adapt to attract insect pollinators, producing sweet nectar to draw them in, and using them to spread their sex cells to every other flower the insect may visit. The symbiosis between bacteria at the dawn of the Proterozoic was a springboard for survival in the brave new oxygenated world, but its origins are somewhat sinister. In the basic food chains of the microbial past, some anaerobes acquire their energy from the organic molecules of their oxygen-loving cousins. They engulf them whole, swallowing them as a snake might consume an entire deer, to digest it slowly over many days. But in a one-in-a-billion chance, occasionally, the prey isn't digested, but continues to live on inside the predator's cells. The predator doesn't get the nutrition from the meal it swallowed, but it gets something even better, the fruits of its prey's ongoing labor. In a sense, the consumer has enslaved its prey inside itself, keeping it captive and taking some of the energy it releases for its own. But the relationship isn't so one-sided. Although it may seem like a diminished existence, the prey microbe actually has been transported into a life of luxury. Protected from environmental stress and the threat of further predation, the bacteria that lives inside another is infinitely richer than one struggling along in the outside world. This is the most intimate of symbiotic relationships, and eventually they even learn to grow and reproduce together. Soon, this so-called endosymbiotic relationship becomes a permanent one, with the two irreversibly reliant on one another. Indeed, this relationship persists to this day in practically every cell of our own bodies. The engulfed bacteria have become our mitochondria, the powerhouses of our cells, which take the sugars we eat and break them down to release energy for our metabolism. Our mitochondria even still retain some of their original DNA, distinct from the DNA of the cells themselves. But endosymbiosis was such a successful trick that protoerozoic cells found use for it a second time. The newly paired endosymbiotes could now break down organic matter in an oxygen-rich environment, allowing them to quickly rise to dominance throughout the oceans. A second pairing between these new dominant chimeras and successful photosynthesizing cyanobacteria changed the playing field once again. Through the same process of engulfing, preserving and protecting, the endosymbiotes acquired the cyanobacteria's ability to capture the sun's energy to make their own organic matter. With this new relationship, the trinity transcended. The cell could survive anywhere, on any food source or on the sun's light alone. The photosynthesizing endosymbiotes become the chloroplasts, the photosynthesizing organs in the green cells of algae and plants today. This was the revelation that the phytogeographer Andreas Schimper had in Bonn in 1883. He had little evidence beyond the ubiquity and likeness of chloroplasts in plants from around the world but their resemblance to tiny, free-living cyanobacteria was too great to go unnoticed. Yet this was outrageously new thinking. Indeed, Charles Darwin's concepts of evolution were still controversial and hotly debated amongst natural scientists of the time. He opted for a tentative footnote. Writing of the chloroplasts he saw, their relationship with the organism containing them would somewhat suggest a symbiosis. His ideas were pursued by scientists throughout the 20th century and endosymbiosis is now widely accepted as the origin of an entire domain of life. The eukaryotes. All plants, animals and fungi are eukaryotes and are made from an entirely different kind of cell to the bacteria that ruled the Archean oceans. They contain the mitochondria and chloroplasts of the original symbiosis, but also a self-contained nucleus and other cellular machinery to help streamline an increasingly complex existence. It's likely that only an event as catastrophic as the Great Oxygen Extinction could have created the unique conditions to allow this remarkable transformation to happen and create complex cells capable of supporting the next stage of evolution on Earth. Some 100 years before Andreas Schimper sat musing over the chloroplasts in exotic plants, another naturalist was putting pen to paper to express his own novel thoughts on botanical matters. His name was Erasmus Darwin and he was grandfathered to Charles Darwin. A talented medical man from a wealthy English family, Erasmus spent much of his life working as a physician in Lichfield, England. But he was a natural philosopher and a poet at heart, choosing to express his ideas through elegant verse and florid prose. So it was that in 1789, Erasmus Darwin penned a poem called The Loves of the Plants, which personified the parts of a flower specialized for sexual reproduction as an amorous bride and groom. But Erasmus also uses the poem to put voice to his own views on the purposes of sex in the plant kingdom. He asserts that the romance between flower bride and groom, the joining of pollen and egg after successful pollination, is the very thing that creates new types of living thing and is the source of evolution and of progress. Erasmus Darwin was among the first to consider sex and evolution together and unwittingly hinted at a process whose genesis lay two billion years deeper in time. The empty world following the great oxygen extinction laid the foundation for the emergence of eukaryotes. But the newly oxygenated proterozoic earth was no paradise. Although the archaic bacteria had largely evolved a resistance to reactive oxygen, neither bacteria or novel eukaryotes were entirely immune. In fact, two billion years later, even we are not immune to its slow chemical attack. Oxygen inside the cell gradually attacks chemical machinery, breaking down large complex molecules like DNA. This so-called oxygen toxicity is one of the reasons that our bodies begin to break down as we age. Our cells are gradually damaged by oxygen over our lives to the point where they can no longer effectively do their jobs. This, of course, had never been a problem for anaerobic archaean bacteria. But in an increasingly oxygenic world, the risk of toxic oxygen became one that could no longer be ignored. Bacteria reproduce in a simple way. An individual clones its DNA and splits the cell in two, producing a sister-daughter clone, identical in every way to its parent. But if oxygen damages the DNA of the parent, then the daughter inherits that damage unchanged. Over many generations, the oxygen damage acquired over individual lifetimes accumulates and grows, until even a simple cell can no longer function. The lineage of clones is doomed. This was the fate that befell probably the most famous clone of all, Dolly the Sheep. Dolly was cloned from a six-year-old sheep in 1996. Genetically identical to her sister mother, Dolly died at just six and a half, despite her breed having a usual lifespan of 12 years. She had developed diseases associated with old age, having inherited the oxygenically aged DNA of her mother. In a sense, she was already six years old when she was born. And the first eukaryotes put themselves at even greater risk, through the house guests living within them. Mitochondrial symbiotes need oxygen to release energy from food, and chloroplast symbiotes actually pump oxygen into the cell as they perform photosynthesis. So a eukaryote not only had to survive in oxygenated waters, they had to survive the toxic oxygen within themselves. In the name of adaptation, they had invited the killers in through the front door. Those that had survived the great oxygen extinction now faced a slower, but no less terminal fate, thanks to the ongoing attack of oxygen. So another extraordinary, evolutionary innovation was needed to ensure the survival of the survivors. That innovation was sex. In modern animals and plants, sexual reproduction is a complex affair, with specialized anatomy and instinctive behavior to ensure the continuation of the species. Take, for example, the male black widow spider, who must walk into the jaws of death just to approach his mate. To avoid being mistaken for prey, he performs a curious dance, vibrating his abdomen at just the right frequency. The female knows not to eat him straight away, allowing him to approach and complete the act. Before eating him after all. But sex on the cellular level is little more than a modification to the dance already being done by bacteria worldwide. Instead of copying the entire genome and cleanly splitting a cell in two, the eukaryotes copy just half their DNA and create sex cells much smaller than themselves. Thus depleted, these sex cells cannot survive alone, but must join with another, from another individual, to complete the sequence and make themselves whole once again. Fully restored, the daughter is therefore a blend of two parents, rather than a clone of just one. By complicating reproduction in this way, requiring two parents to make new life rather than just one, the early eukaryotes won themselves vital protection against the toxic oxygen in their cells. Chance determines which section of DNA a daughter will inherit from her two parents, and there's a good chance that those sections will be as yet undamaged by oxygen. For her, the new generation, the oxygen clock is reset. The risks posed by oxygen over time are so great, and the protection offered by sexual reproduction so complete, that scientists believe eukaryotes could simply not have survived without the invention of sex. Today, 99.99% of all eukaryotes, from single cells to the largest of multicellular animals, reproduce sexually. To be a eukaryote is to have sex, and that link was likely forged right at the beginning of eukaryotic history, in the brave new world revealed after the oxygen extinction. And so Erasmus Darwin, in his modest English townhouse in the 18th century, was unknowingly peering through two billion years of history as he considered the courting dance of plants. Sexual reproduction is the source of variation in all living things, and right at the beginning, that variation was critical for the survival of an entire domain. When Erasmus looked around him, he saw a world dominated by multicellular eukaryotes. All eukaryotes are dependent on the oxygen machinery inside their cells, and the sexual reproduction that injects variation into every generation. All this was made possible by an apocalyptic event more than two billion years previous. The Great Oxygen Extinction. Just as we can thank the KT extinction for our mammal line's dominance over the dinosaurs, so we can thank the oxygen extinction for the diversity and dominance of eukaryotes across the globe. It took extraordinary strife to produce the extraordinary diversity we enjoy in the world today. Even the darkest nights end, letting the sun rise on a world of new opportunity. You've been watching the entire history of the Earth. Don't forget to like and subscribe, and leave a comment to tell us what you think. Thanks for watching, and we'll see you next time.