Origin Story: A Big History of Everything

by David Christian

Oxygen-producing photosynthesis, the sort of photosynthesis used by all modern cyanobacteria, may have evolved as early as three billion years ago. This is suggested by evidence for brief “whiffs” of increased oxygen levels even before the end of the Archean eon, two and a half billion years ago. But at first, any oxygen they released would have been quickly absorbed by iron or hydrogen sulfide or free hydrogen atoms, because oxygen is an electron thief and will combine eagerly with any element that has spare electrons. That is why atoms that have had their electrons stolen are said

there was a limit to how much oxygen these mechanisms could absorb, and starting about 2.4 billion years ago, levels of atmospheric oxygen began to rise fast, from less than 0.001 percent of today’s levels to perhaps 1 percent or more.

The appearance of an oxygen-rich atmosphere beginning about two and a half billion years ago (the “great oxygenation event”) transformed the biosphere. Rising oxygen levels altered the chemistry of the biosphere and even of the upper levels

The exceptional chemical energy of free oxygen powered new chemical reactions that created many of the minerals on today’s Earth.15 High up in the atmosphere, oxygen atoms com...

oxygen buildup was a profound shock to living organisms because, for most of them, oxygen was poisonous. So rising oxygen levels caused what the biologist Lynn Margulis called an “oxygen holocaust.” Many prokaryotic organisms perished, and those that didn’t die retreated to protected environments in the deeper, oxygen-poor levels of the oceans or even into rocks. Rising oxygen levels messed up Earth’s thermostats

Free oxygen broke down atmospheric methane, one of the most powerful of greenhouse gases, while photosynthesizing cyanobacteria consumed huge amounts of the other crucial greenhouse gas, carbon dioxide. As oxygen levels rose and levels of greenhouse gases fell, early in the Proterozoic eon, Earth froze in the first of several snowball-Earth episodes. Glaciers spread from the poles to the equator, turning the Earth white, and a white Earth reflected more sunlight, cooling it even further in a terrifying positive-feedback loop. Eventually, most of Earth’s oceans and continents were covered by ice. The Makganyene glaciation lasted for one hundred million years, from around 2.35 to 2.22 billion years ago. This was a close shave. Organisms

Life hung by a thread, as most life-forms retreated beneath the ice and huddled around the warm fires of suboceanic volcanoes. But Earth did not go the way of Mars and get too cold for life. This was thanks to the geological thermostat driven by plate tectonics, now renovated and supplemented by new biological techniques that depended on the activity of photosynthesizing organisms. Glaciers blocked photosynthesis, which slashed oxygen

Meanwhile, under the glaciers, oceanic volcanoes kept pumping carbon dioxide and other greenhouse gases back into the oceans. Greenhouse gases began to accumulate beneath the ice until, eventually, they broke through the glaciers, and Earth’s surface warmed again. Oxygen levels plummeted to about 1 or 2

The appearance of a third domain of life-forms, Eukarya, matters a lot to us because all large organisms, including ourselves, are built from eukaryotic cells. These were the first cells that could use oxygen systematically, exploiting its fierce chemical energy in a process known as respiration, which is what we do when we breathe. Respiration is the reverse of photosynthesis and is really a way of releasing solar energy that has been captured and stored within cells through photosynthesis. While photosynthesis uses energy from sunlight to turn carbon dioxide and water into energy-storing carbohydrates, leaving oxygen as a waste product,

uses the chemical energy of oxygen to pilfer the energy warehoused in carbohydrates, leaving carbon dioxide and water as waste products. The general formula for respiration is CH2O (carbohydrates) + O2 → CO2 + H2O + energy.

Respiration gives you the energy of fire without its destructiveness. By using oxygen cleverly, respiration can extract at least ten times as much energy from organic molecules as earlier non-oxygenic ways of breaking down food molecules.16 With more energy to power their metabolism, rates of primary production—the production of living organisms—may have increased by anything from ten to a thousand times.17

common for different species to collaborate through what is known as symbiosis. Today, humans have vital symbiotic relationships with wheat, rice, cattle, sheep, and many other species. But Margulis was talking about a much more radical type of symbiosis, one in which once independent bacteria, including the ancestors of modern mitochondria, ended up living inside a cell from the Archaea. Margulis called the mechanism endosymbiosis. At first, her idea seemed crazy, because it ran counter to some of the most fundamental concepts about evolution by natural selection. But most biologists now accept her arguments. The most

Margulis realized that organelles such as the mitochondria that manage energy in animals and the chloroplasts that manage photosynthesis in eukaryotic plants look as if they were once independent prokaryotic cells. Exactly how they ended up inside other cells remains unclear, and some have argued that such mergers must be extremely rare. If so, that probably means that even if bacterialike organisms are common in the universe, large organisms like us may be extremely rare, because, on our planet at least, only eukaryotes can build large organisms. Margulis’s discovery of endosymbiosis tells us something more about the history of life. Evolution is not just a matter of competition.

eukaryotes, the genetic material is locked inside the protected vault of the nucleus. That material is released only under the most stringent conditions, using rules less promiscuous and more orderly than those of prokaryotes, and these rules affect how eukaryotic cells evolve. When eukaryotes produce germ cells—eggs and sperm, the cells from which their offspring will be formed—they don’t just copy their DNA. They stir it around first. They swap some of their genetic material with another individual of their species so that the offspring of the two parents gets a random selection of genes, one half from one parent and the other half from the other parent. Both the genetic and the physical mechanisms involved in this elaborate

genetic variations were guaranteed every generation, because even if most of the genes were the same (after all, both parents are from the same species), a tiny number were always slightly different. With more variation to select from,

Life-friendly conditions are not enough. You also need those conditions to persist for a long time so that life can keep evolving and experimenting. A stable sun helps here, and our sun fit the bill nicely. By stellar standards, it’s a solid citizen, unlikely to do anything too unpredictable. Erratic orbits mean wild climatic gyrations, so stable planetary orbits help. Our Earth ticks this box, too. Our unusually large moon helped stabilize Earth’s orbit and tilt. And, as we have seen, plate tectonics, erosion, and then life itself provided thermostats that stopped temperatures from wobbling too much at the Earth’s surface. So much could have gone wrong. A supernova in a neighboring star system could have blown up. Or we could have collided fatally with another planet. Somehow or other, our Earth avoided these dangers and remained life-friendly for more than three billion years. That was enough time for big life to evolve. And big life really is big. We are to bacteria what Dubai’s 830-meter-tall Burj Khalifa is to an ant crawling past the doorman’s shoes. Once it appeared, big life would transform the biosphere as much as little life did,

Ecologists talk of a food chain, a sort of queue of energy consumers with plants at the front, followed by herbivores (or creatures that consume plants), then by carnivores, which can consume herbivores, then by fungi, which bring up the rear by feasting on the dead. The whole process delights entropy, which exacts a garbage tax at every step. Approximately 90 percent of the energy captured by photosynthesis is lost at each trophic level, so much less energy is available for the later links on the food chain. That’s why you find fewer animals than plants on Earth, and fewer carnivores than herbivores. But the fungi do well either

impact of the first forests was particularly significant because as yet, there were no organisms that could break down the lignin in wood. That’s why forests from the Carboniferous period (from 360 to 300 million years ago) were mostly buried beneath the soil, along with the carbon they had drawn down from the atmosphere. Over time, they fossilized to form the coal seams that later powered the industrial revolution. About 90 percent of today’s coal deposits were buried during the period of high oxygen levels, from around 330 to 260 million years ago. With plenty of oxygen, forest fires were easily ignited by lightning strikes. So the Carboniferous and early

Plants tweaked Earth’s geological thermostat, because they sped up the weathering of rocks by grinding and dissolving them into soils that could carry buried carbon more easily into the oceans; from there, some carbon was subducted deep into the mantle. Buried carbon could no longer react with oxygen to form carbon dioxide, so oxygen levels rose. This is why the amount of free oxygen depends roughly on the amount

carbon subducted into the mantle, so levels of atmospheric oxygen and carbon dioxide tend to move in opposite directions. Rising oxygen levels also allowed new chemical reactions in the crust, creating many of the four thousand different types of minerals found on Earth today.14 Between

Natural selection equipped large organisms with a desire for more information, because good information was vital to their success. That’s why, when a human solves a puzzle, the brain gets the same buzz it gets from food and sex.16 Natural selection also gave large organisms more sensors and more types of sensors: for sound, pressure, acidity, light.

Memory can store the results of decisions made consciously and use them for fast, automated responses. Once you’ve learned how to drive a car, you don’t need to think through a long to-do list when you see a red light. Your body just gets on with it. You won’t even notice your foot pressing on the brake. These elaborate decision-making and modeling systems

Mammals diversified in a new evolutionary radiation, as small businesses would today if every large corporation declared bankruptcy overnight. Many mammal species went big. Within half a million years, there were cow-size herbivorous mammals and equally large mammalian carnivores. There were also primates, members of the order of tree-dwelling, fruit-eating mammals from which we are descended. Though the first primates already existed in the world of dinosaurs, they flourished only after the dinosaurs had left the scene. There

PETM is of interest today because it is the most recent period of rapid greenhouse warming in Earth’s history, so it may help us understand climate change today. The parallels are eerie. The amounts of carbon dioxide released into the atmosphere during the PETM were similar to those being released today by the burning of fossil fuels, and fifty-six million years ago, the result was an increase of between five and nine degrees Celsius in average global temperatures.24 What drove this sudden warming? Volcanic activity was unusually intense between fifty-eight and fifty-six million years ago, and carbon dioxide from volcanoes would have increased levels of atmospheric carbon dioxide. But then something happened fast, over a period of perhaps just ten thousand years, about the time that has passed in human history since the appearance of agriculture.

The best bet at present is that polar oceans warmed to the point where methane clathrates (frozen balls of methane, which look like ice but ignite if you put a match to them) suddenly melted, releasing large amounts of methane, a greenhouse gas even more powerful than carbon dioxide. That would have heated things up very fast. If this story is correct, we need to keep a very wary eye on methane clathrates in today’s polar oceans. After a climatic spike lasting perhaps two hundred thousand

Whatever the precise causes, the cooling trend that began about fifty million years ago has continued to the present day. About 2.6 million years ago, at the beginning of the Pleistocene epoch, the world entered the current phase of regular ice ages. The world had not been this cold for 250 million years, since Pangaea itself had split apart at the end of the Permian period. Fifty million years ago, in this post-dinosaur, post-PETM world

of chilly and erratic climate changes, our primate ancestors evolved.

Genetically, though, we are more homogenous than our closest living relatives, the chimps, gorillas, and orangutans. We just haven’t been around long enough to diversify much. Besides, we are extraordinarily sociable, and we love to travel, so human genes have moved pretty freely from group to group. We belong to the mammalian order Primates, which includes lemurs, monkeys, and great apes. And we share a lot with our primate relatives. The earliest primates almost certainly lived in trees, and young humans (I include my young self here) love climbing trees and are good at it. To climb trees, you need hands and fingers

you’re going to leap from branch to branch, it’s a good idea to have stereoscopic vision so you can judge distances. That means having two eyes at the front of your face, with overlapping lines of sight. (Don’t try jumping from branch to branch with one eye closed.) So all primates have hands and feet that can grip and flattish faces with eyes at the front.

Their brains are unusually large relative to their bodies, and the top front layer of the brain, the neocortex, is gigantic. In most mammal species, the cortex accounts for between 10 percent and 40 percent of brain size. In primates, it accounts for more than 50 percent, and in humans for as much as 80 percent.2 Humans are exceptional for the sheer number of their cortical neurons....

Aren’t brains obviously a good thing? Not necessarily, because they guzzle energy. They need up to twenty times as much energy as the equivalent amount of muscle tissue. In human bodies, the brain uses 16 percent of available energy, though it accounts for just 2 percent of the body’s mass. That’s why, given the choice between brawn and brain, evolution has generally gone for more brawn and less brain. And that’s why there are so few very brainy species. Some

bipedal species, the big toe is no longer used for gripping, so it aligns more closely with the other toes, while the spine enters the skull from below, not from the back (get down on all fours and you’ll understand why). Walking on two legs required rearrangements of the back,

There were many indirect effects of bipedalism, but we’re not yet sure exactly why hominins became bipedal. Perhaps bipedalism let our ancestors walk or run farther in the grassy savanna lands that had spread around a cooling world in the past thirty million years. It also freed human hands to specialize in manipulative tasks including, eventually, the making of tools. There are no signs

This would have increased the range of foods they could eat, because many foods are indigestible or poisonous until cooked. Cooking would also have reduced the time they spent chewing and digesting their food. Use of fire may have had other important consequences. For example, cooking reduced the digestive work required of the gut, so the gut shrank (and, yes, there is fossil evidence for this), releasing some of the metabolic energy needed to run larger brains. As yet, this interesting hypothesis remains unproven, because good evidence for systematic control of fire appears only from about eight hundred thousand years ago and becomes quite common only after about four hundred thousand years ago.12 We also know that the stone technologies of H. erectus changed little over a million

the past million years, hominin evolution accelerated. About six hundred thousand years ago, new species appear in the fossil record, with brains and bodies more and more like modern humans’. Not surprisingly, they apparently lived in larger groups, too, groups that linked as many as 150 individuals, which seems to have been the upper limit among our hominin ancestors.13 There are

Although the size and structure of the human brain have not changed since Homo sapiens first appeared in East Africa… the learning capability of individual human beings and their historical memory have grown over the centuries through shared learning—that is, through the transmission of culture. Cultural evolution, a nonbiological mode of adaptation, acts in parallel with biological evolution as the means of transmitting knowledge of the past and adaptive behaviour across generations. All human accomplishments, from antiquity to modern times, are products of a shared memory accumulated over centuries.

The great world historian W. H. McNeill constructed his classic world history The Rise of the West around the same idea: “The principal factor promoting historically significant social change is contact with strangers possessing new and unfamiliar skills.”21 Living in

Whole groups could die out suddenly, along with the technologies, stories, and traditions they had built up over many centuries. The largest catastrophe of this kind occurred about seventy thousand years ago. Genetic evidence shows that the number of humans suddenly fell to just a few tens of thousands, only enough to fill a moderate-size sports stadium. Our species came close to extinction. The catastrophe may have been triggered by a massive volcanic eruption on Mount Toba in Indonesia that pumped clouds of soot into the atmosphere, blocking photosynthesis for months or years and endangering many species of large

During the Pleistocene epoch, which encompasses the two million years since the evolution of Homo erectus, there were many ice ages. They normally lasted for one hundred thousand years or more, with briefer warm periods, or interglacials, between them. The period we live in now is a warm interglacial that began ten thousand years ago, at the start of the Holocene epoch. The previous interglacial occurred about a hundred thousand years ago and may have lasted for twenty thousand years or more. After it ended, global climates got steadily colder and drier, though with many temporary reversals and local variations. The coldest period of the last ice age was from about twenty-two thousand to eighteen thousand years ago. As climates cooled,

One hundred thousand years ago, during the last interglacial, almost all humans lived in Africa, though a tiny number had left for the Middle East. At sites such as the caves of Skhul and Qafzeh in modern Israel, they may have encountered and occasionally interbred with Neanderthals. (We know this because today, most humans who live outside Africa have

Then, as climates cooled, our ancestors seem to have left the Middle East to the Neanderthals, whose bodies were better adapted to colder climates. They didn’t return until about sixty thousand years ago. However, some humans may have traveled east into Central Asia and South Asia. One reason for thinking this is that humans reached Sahul (the ice-age continent that included Australia, Papua New Guinea, and Tasmania) between fifty thousand and sixty thousand years ago. Migrants leaving Africa sixty thousand

The earliest migrations into Siberia and northern Europe were probably short-lived exploratory probes during brief warm periods. But sites such as Mezhirich show that by twenty thousand years ago, our ancestors could cope with extremely cold environments. Some may have settled permanently in Siberia as early as forty thousand years ago. Twenty thousand years later, at the coldest phase of the last ice age, some Siberians trekked east across the land bridge of Beringia, which was crossable because so much water was locked up in polar glaciers that ocean levels were lower than today. From Beringia, humans spread into the Americas,

From there, some migrated into South America, probably reaching as far south as Tierra del Fuego within two or three thousand years. At present, the earliest firm evidence for the presence of humans in North America dates to about fifteen thousand years ago.

The English anthropologist Robin Dunbar has argued that 150 people represents the largest group size that human brains can normally cope with, so it may be that communities naturally split if they got any larger. Dunbar has argued that even today, most humans are embedded in intimate networks that are no larger than 150, even if they have more fleeting relationships with many other people. Modern communities are huge, but only because of the creation of special new social structures to hold them together.

Australia, Siberia, and North America, the megafauna vanished not long after the arrival of humans. Perhaps, like

Mauritius, the megafauna didn’t fear our ancestors enough, unlike African megafauna, which had coevolved with humans and knew how dangerous we could be. In any case, megafauna, like all large animals (including the dinosaurs), are particularly vulnerable to sudden changes. There are many modern examples of megafaunal extinctions, such as the disappearance of the large New Zealand birds known as moas within a few centuries

ice age, just over twenty thousand years ago, was followed by several thousand years of erratic warming until, starting about twelve thousand years ago, global temperatures settled into the warmer and more stable regime that dominated human history during the Holocene epoch. By the end of the last ice age, our alien scientists would already have been very interested in the strange events

Farming was a mega-innovation, a bit like photosynthesis or multicellularity. It set human history off on new and more dynamic pathways by helping our ancestors tap into larger flows of resources and energy that allowed them to do more

people and more links between communities generated positive feedback cycles that accelerated change. For all these reasons, farming counts as our seventh threshold of increasing complexity. The potential for transformative innovations had existed since collective learning first took off, but now that potential began to be realized as a result of three main Goldilocks conditions: new technologies (and increasing understanding of environments generated through collective learning), increasing population pressure, and the warmer climates of the Holocene epoch.

The basic principle of farming is simplicity itself. Farmers use their environmental knowledge to increase the production of those plants and animals they find most useful and to reduce the production of those they can’t use. Farmers weeded and watered the land to help grow the plants they wanted, such as wheat and rice, and fenced in animals they valued, such as sheep and goats, but they removed weeds and shooed away or killed animals they didn’t like, such as snakes and rats. These activities changed entire landscapes, and plants and animals responded to these new environments, as they respond to all environmental changes, by adapting genetically, by evolving. That is why