David Christian is one of five expert speakers on Scientific American’s 175th Anniversary Cruise to the Americas in March 2020. For more information on our 175th Anniversary Cruise, including a detailed itinerary, seminar descriptions and speaker bios, click here.
During my lifetime—since the mid-20th century—we humans have suddenly become so powerful that what we do in the next 50 years will shape the future of all the species with which we share this planet. Whether we realize it or not, we have become the custodians of planet Earth.
Those who are young today will have to learn how to grow into the new role of planetary custodians. But how do you do that? No species has ever played this role in the four billion years since life first appeared on our Earth (though groups of species, such as the first oxygen-producing organisms have transformed the biosphere). And we have no idea if any other species are trying to look after entire planets elsewhere in our galaxy or in other galaxies.
One thing we can be sure of is that in order to become skillful planetary custodians, future generations will need new types of education so they can think in new ways. Above all, they will need a vision wide enough to support them in this huge, multidisciplinary challenge. The siloized educational system that I grew up in—a system that let you focus on Latin literature or economics or biochemistry, but rarely helped you see the links between disciplines—will no longer work. We will need all the intellectual wealth generated within different academic disciplines in recent centuries. But we will also need educational structures that help us see what holds that knowledge together, so we can see the very large patterns. In short, we will need a modern, science-based origin story.
Such courses are emerging today, and being taught to increasing numbers of students in universities, high schools and even a few primary schools. One variant, which I have taught for 30 years, is known as “big history.” Big history courses use the best knowledge available within many different disciplines to tell a rigorous, science-based story about the whole of the past. They begin with the origins of our universe, 13.8 billion years ago, in what cosmologists call the “big bang.” And they tell how, within a very simple early universe, new structures and entities appeared, creating forms of increasing complexity until, eventually, they generated the fantastically elaborate and interconnected global community we live in today on planet Earth.
The next few paragraphs will offer a synopsis of this modern origin story, and show how it can bring into focus the strangeness of the moment we live in right now.
Big history courses tell how, from a simple mist of hydrogen and helium atoms, gravity sculpted the first stars and galaxies, a few hundred million years after the big bang. Then they tell how new chemical elements were forged within dying stars, and scattered through space. The new elements, from oxygen and carbon to gold and uranium, started combining chemically to form the astonishing variety of different materials and structures in today’s universe. Those structures included the first planets; these were much more complex than stars because they included all the elements of the periodic table, along with complex molecular substances such as ice and dust and even tiny blobs of organic matter.
Some planets, including our own home planet, offered exceptionally rich “Goldilocks conditions” for chemical experimentation. Gassy planets such as Jupiter or Saturn were still dominated, like stars, by the lightest elements, hydrogen and helium. But closer to the sun, these lighter elements had largely been swept away, leaving environments populated by a wider variety of the periodic table’s elements. This is where the rocky inner planets formed. Venus, Earth and Mars were all chemically rich. Unlike Mercury, they were also far enough from the sun that oceans may have formed in their early histories. And liquid water is the perfect medium for complex chemical experimentation, and eventually for life.
Sadly, Venus and Mars both lost any early oceans they may have had. On Venus, a runaway greenhouse effect cooked the planet and boiled away any early seas, while Mars’ weaker gravitational field allowed greenhouse gases to leak away, leaving a planet so cold that water froze. Only on Earth have seas and oceans survived for billions of years, creating the perfect conditions for the most complex chemical experiment of all: the evolution of life. Greenhouse gases play a crucial role in the story of why and how our planet became and remained life-friendly.
This is the point in the big history story when future planet managers really need to take notice! Because we now know that life could easily have perished on Earth, too. So, what kept our planet life-friendly for four billion years? There’s a puzzle here (Carl Sagan called it the “faint young sun paradox”), because our sun was much cooler when it was young, and has gotten warmer and warmer over four billion years. That means that the early Earth should have been too cold for life, but later on it should have gotten so hot that its seas and oceans would have boiled away, along with their precious cargo of living organisms. What stopped Earth becoming as hostile to life as the rest of the universe?
We still don’t fully understand all the mechanisms involved. But in recent decades geologists and climate scientists and biologists have pieced together a remarkable story about the emergence of natural thermostats whose effect was to regulate levels of greenhouse gases in the atmosphere so that oceans and life could survive.
The young Earth was wrapped in a warm blanket of greenhouse gases such as carbon dioxide and methane, that were burped up from deep under the crust. A greenhouse atmosphere kept its oceans liquid even though solar emissions were much weaker than today. Surface temperatures were regulated by geological feedback mechanisms that tweaked the levels of greenhouse gases. When temperatures rose, more water evaporated from the oceans and there was more rainfall. More rainfall washed large amounts of carbon dioxide from the atmosphere. Dissolved carbon dioxide formed carbonic acid, which eventually buried lots of carbon in carbonate rocks in the oceans. Some got buried even deeper, beneath the crust.
These processes reduced levels of atmospheric carbon dioxide, and that lowered surface temperatures. In contrast, when surface temperatures fell, there was less rainfall, so less carbon dioxide was removed from the atmosphere, which kept surface temperatures warmer. Meanwhile, volcanoes provided a steady supply of new greenhouse gases by pumping them into the atmosphere from under the crust. These were crude feedback mechanisms, but their long-term effect was to keep Earth’s surface within the narrow Goldilocks range for water and life, between zero degrees Celsius and 100 degrees C.
Eventually, as solar emissions increased, the Earth’s blanket of greenhouse gases began to thin, and new biological thermostats emerged to buffer the crude thermostats of geology. When plantlike organisms began to breathe in large amounts of carbon dioxide and exhale large amounts of oxygen over two billion years ago, the Earth’s atmosphere was transformed. Oxygen built up, and the level of greenhouse gases began to fall. For a time, the fall was dangerously fast. For millions of years the Earth was covered with ice during several so-called “snowball Earth” episodes. But volcanoes came to the rescue each time, by pumping up new supplies of greenhouse gases beneath ice sheets that covered the planet. And eventually, new oxygen-breathing organisms evolved and proliferated, creating a new atmospheric balance. Taken together, these mechanisms, and perhaps others that we do not yet understand, kept the Earth’s surface temperatures within the Goldilocks range for life, even as heat from the sun kept increasing.
Within the protected temperature-controlled cocoon of the young Earth, life flourished and evolved, creating a huge variety of homes for living organisms in the planet’s oceans and seas. As oxygen levels increased with the spread of photosynthesizing plants, oxygen’s fierce chemical energy drove more extravagant and exotic forms of evolution. Multicellular life forms really began to flourish only in the last 600 million years, and only in the last 400 million years did large organisms begin to colonize the Earth’s continents, eventually greening large parts of their once rust-coloured landscapes, and filling the new jungles and savannas with the roars and screams of vast new creatures such as the dinosaurs.
We humans are very much part of this story. Like all our mammalian relatives today, we flourished because of a cosmic accident that struck down most of the large reptiles that dominated the continents 65 million years ago. An asteroid the size of a modern city crash-landed somewhere near Mexico’s Yucatán peninsula. (We can see its traces today). Its violent arrival punctured the Earth’s crust and generated dust clouds that shut down photosynthesis for a year or two. Large, dinosaurlike organisms that needed lots of food and reproduced slowly were most at risk.
But the small, scurrying, rodentlike organisms that were our mammalian ancestors generally did better. In a post-asteroid world, mammals flourished, diversified and became the dominant animal species on land. Our primate ancestors were among the survivors. Mammals were warm-blooded, so they needed lots of energy, so they needed plenty of food, so they needed more smarts, which is one reason why they were generally brainier than dinosaurs. Primates were particularly brainy.
Humans evolved as part of this trend to larger brains. Relative to our body size, our brains are spectacularly big. But what really makes us humans different is some piece of rewiring in our brains that we don’t fully understand, which opened up entirely new forms of communication. We are different because we can tell each other stories, pass on new information, share jokes and even explain what we are feeling and imagining.
Or rather, we can do these things with a virtuosity that no other species can approach. And that makes all the difference. We share so much information with such precision that information can accumulate, community by community, across many generations. We are the first species in four billion years that can accumulate information over many generations, through what we can call “collective learning.”
And information, of course, is power. More information means a greater ability to manage our environments, to hunt or gather food, and to manipulate our surroundings. Over some 200,000 years, at a rate that was barely perceptible at first but has gradually accelerated, we learned more and more powerful ways of managing, exploiting and eventually transforming the landscapes, plants and animals around us. Today, this increasing ecological power explains why we are so populous, why we dominate all terrestrial environments and why, since discovering how to tap the colossal energy locked inside fossil fuels, we have begun to transform the Earth’s surface, its oceans, its atmosphere and the other species that share the planet with us.
This is the trajectory that has led us to a turning point in planetary history. Suddenly, for the first time in four billion years, a single species has enough power to transform the biosphere, for better or worse. The scale of the change became apparent only in my lifetime. Nuclear weapons were first created over 70 years ago. They give us the power, if we are foolish enough to use it, to degrade the biosphere in a catastrophe as swift and destructive as the asteroid landing that wiped out the dinosaurs.
Even if we avoid nuclear war, the global bonfire of fossil fuels is beginning to mess with the ancient thermostats that kept planet Earth life-friendly for four billion years. Since 1750, humans have pumped more carbon dioxide into the atmosphere than was released by the Chicxulub asteroid, whose impact about 65 million years ago is widely believed to have ended the dinosaurs' reign. Indeed, we are now pumping carbon dioxide into the atmosphere more than 80 times faster than the rates at which the geological and biological thermostats can produce or absorb the gas. These are calculations that should make all future planet-managers sit up and blink!
I hope this short summary makes it clear why the big history story is very much about today’s world; and how, by linking insights from many different disciplines, it can help us understand the extraordinary importance of this moment in the planet’s history. The wide lens of big history should also help us figure out what we need to do to maintain a life-friendly Earth for future generations.
Big history courses have been taught, under a variety of names, for several decades. But their importance is growing, as we realize the need to balance knowledge in depth with knowledge in width, in order to fully understand the challenges we face today. Big history courses offer a crucial resource for a younger generation that is taking on the challenge of managing our planet for the sake of future generations of humans and millions of other species