Origin Story, page 15
Add memory to these decision-making systems, and we have the foundations for complex learning, the ability to record the results of earlier decisions and use those records to make better decisions in the future. A species of fish known as cleaner wrasse, for example, clean the teeth of fish that could easily eat them. But they have to learn which clients will not eat them and may provide a free feed from between their teeth. 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 evolved throughout the Phanerozoic eon. They evolved most spectacularly in animals, because animals have to make many more decisions than plants do. In most invertebrates, neuronal networks remained distributed throughout the body, though they were often concentrated in particular nodes or ganglia. Some invertebrates, such as the octopi, have built powerful information-processing systems from such networks; most of an octopus’s neurons are in its arms. In the vertebrate line, too, many neurons reach deep into the body, where they keep in touch with sensor cells and the motor cells that carry out decisions. But as sensors multiplied and processing became more critical, increasing numbers of neurons gathered together in brains, where they became specialist information processors. Information processing was particularly important in the complex, energy-guzzling lineages of birds and mammals, though these very different types of organisms evolved different subsystems to handle big data.20
In mammals, the increasing importance of information processing helps explain the evolution and growth of the cortex, the gray, outer layers of the brain. The cortex provides lots of space for calculations and a lot more calculating ability, so it allowed better problem-solving in unfamiliar situations or when other decision-making systems were deadlocked. Eventually, the brainiest mammals would evolve general information-processing and problem-solving systems that were to those of the bacterial world what the Internet is to an abacus. The evolution of enhanced problem-solving and information-processing systems would eventually lead to the information explosion unleashed by our own remarkable species.
An Asteroid Lands—A Lucky Break for Mammals
For a long time, dinosaur brawn seemed to trump mammalian brains. Then, sixty-five million years ago, everything changed in a flash.
The world of the dinosaurs vanished in just a few hours when a ten-to fifteen-kilometer-wide asteroid crashed into Earth.21 The crash caused a major extinction event, during which about half of all genera disappeared. Geologists refer to this as the K/T event because it occurred at the border between the Cretaceous period (often abbreviated K, from the German word for “chalk,” Kreide) and the Tertiary period, an older name for the Cenozoic era, which began sixty-five million years ago.
When the asteroid hit, it was moving at thirty kilometers a second (about one hundred thousand kilometers an hour), having taken just seconds to fly through Earth’s atmosphere. We know exactly where it fell: in the Chicxulub (pronounced “Chikshulub”) crater in the Yucatán Peninsula of modern Mexico. The asteroid evaporated as it punched through the crust, leaving a crater almost two hundred kilometers across. Molten rocks were hurled into the air, where they formed dust clouds that blocked sunlight for many months. Limestone evaporated, spraying carbon dioxide into the atmosphere. An area hundreds of kilometers around the impact point was stripped of life. Hundreds of kilometers beyond that zone, forests lit up in massive firestorms. At sea, a tsunami formed a wall of water that crashed down on the shores of the Gulf of Mexico and killed fish and dinosaurs hundreds of kilometers away. In the Hell Creek Formation, in Montana and Wyoming, you can find fossils of fish whose gills are full of glass from the asteroid impact.22
Farther away, the immediate impacts were less extreme. But within weeks, the whole biosphere had changed. Soot blocked sunlight, creating what we might describe today as a nuclear winter. Nitric acid rained from the sky, killing most of the organisms it touched. The surface of Earth would have been in total darkness for a year or two, shutting down photosynthesis, life’s lifeline to the sun. When the dust thinned, and light began to return through the haze, Earth warmed fast, because the atmosphere now contained a lot more carbon dioxide and methane. A few years after the impact, the wretched survivors could start photosynthesizing and breathing again, but they did so in a hot greenhouse world.
It must have taken thousands of years for the biosphere to return to something like normalcy. Meanwhile, perhaps half of all previously existing genera of plants and animals had vanished. As is typical in such crises, large species were particularly hard hit, because they need more energy, are less numerous, and reproduce more slowly than smaller creatures. This is why the large dinosaurs perished. But modern birds are descendants of smaller dinosaurs, some of which just made it through. Smaller organisms, such as the rodentlike mammals, did slightly better, and some of these would become our ancestors.
The first evidence of the asteroid impact was picked up in rocks in Italy by geologist Walter Alvarez and his team. Geologists already knew that there were striking differences between the rocks before and after the dividing line at the end of the Cretaceous period. Fossils of plankton known as foraminifera are common in the older strata just prior to that date, but they vanish after it. What was not clear was whether the change had taken tens of thousands of years or just a year or two. In 1977, at a site near Gubbio, Italy, Alvarez’s team found very high levels of the rare element iridium, dating from the very end of the Cretaceous. That was odd, because iridium is rare on Earth, although it is common in asteroids. Alvarez and his colleagues found equally high levels of iridium at many other sites in Italy, and we now know of at least a hundred similar sites around the world. It began to look as if the iridium must have been brought in by an asteroid. That suggested a catastrophic event.
At the time, most geologists were committed to the idea that all geological change was gradual, so few bought the idea. They wanted direct proof, a geological smoking gun. That turned up in 1990 when it was shown that the Chicxulub crater was of just the right size and had been created at just the right date. Since then, most geologists have accepted not only that an asteroid impact wiped out the dinosaurs, but that such catastrophic events may have occurred many times in the history of Earth. True, there is also evidence of massive volcanic eruptions around the K/T boundary, and these may have undermined the health of the biosphere, but there can be little doubt now that the fatal blow was delivered by an asteroid.
The post-Chicxulub world was the world in which our mammalian ancestors would evolve. This is the world of the Cenozoic era, the past sixty-five million years of Earth’s history.
After the Asteroid: A Mammalian Adaptive Radiation
As mammals, we human beings share 90 percent of our genes, or about three billion base pairs on our DNA, with other mammals, from rats to raccoons. Somewhere among the other 10 percent of our DNA lie the genes that make us different.
Like all mammals, we are warm-blooded, which means we need more energy than most reptiles to keep our body temperature up and our brains humming. Our brains need to be powerful, because they have to generate a lot of ecological tricks to maintain these large flows of food and energy. Though the earliest mammal-like creatures were no larger than mice, they probably already nursed their young, like today’s mammals, and had unusually large brains in comparison to their body size. The basic division between marsupials (mammals whose young need special protection and nourishment, often in pouches) and placentals (mammals whose young are fed within the womb through a placenta) goes back at least 170 million years.
Through the long 150 million or so years of the Jurassic and Cretaceous periods, most mammal species remained small, scuttling through the moonlit undergrowth.23 They came in many different forms. Some were doglike, such as repenomamus, a creature large enough to eat small dinosaurs and their babies. Some swam, returning to the oceans. Some were batlike, some ate insects, some climbed trees. About 150 million years ago, the world of mammals was changed by the evolution of new types of plants to rival the conifers and ferns that had dominated the plant world so far. These were the angiosperms, plants with fruits and flowers, the types of plants that dominate the forests and woodlands, the parks and backyards of today. Flowering plants provided a food bonanza for those mammals with teeth designed to munch on fruit and seeds or on the many insects that also munched on flowering plants or helped pollinate them.
The asteroid impact that brought down the dinosaurs may also have killed off three-quarters of all existing mammal species. But most mammals were still small, so some sneaked through the evolutionary crisis. After the planet returned to something like normalcy, survivors of the Chicxulub asteroid found themselves in a strange new world. With the dinosaurs gone, there were new opportunities. 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 was one more crisis to be survived before mammals could take over the Earth. That was the Paleocene-Eocene thermal maximum (PETM, for lovers of acronyms), a short, sharp shock of greenhouse warming at the border between the Paleocene and Eocene epochs, about fifty-six million years ago. It was damaging enough to drive many species to extinction. The 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. By the end of that period, many species of plants, animals, and sea-dwellers had vanished. 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 years, global temperatures began a long, slow descent toward colder temperatures, with a few brief reversals. Carbon dioxide levels began to fall once more, while oxygen levels rose. Differences in temperature between the equator and the polar regions increased, and ice spread across the Arctic and Antarctic, locking up water in glaciers, so ocean levels fell.
The cooling was caused in part by changes in the orbital cycles and tilt of Earth itself. These changes are known as Milankovitch cycles, after the scientist who first described them. As Earth’s orbit and tilt altered, the amount of energy reaching Earth from the sun shifted in subtle ways. Tectonic processes may also have been at work, as the Atlantic Ocean widened, and the large southern continent of Gondwanaland cracked into separate modern continents. Antarctica settled over the South Pole, providing a platform for the buildup of huge ice sheets, while the northern continents circled the polar ocean, insulating the northern polar region from warm equatorial currents. Meanwhile, the collision of the Indian plate with Asia pushed up the Himalayas, which accelerated weathering, increasing the rate at which carbon was moved from the air to the sea and into the crust.
Living organisms may also have helped chill the biosphere. In the past thirty million years, as carbon dioxide levels fell, new types of plants evolved, including the grasses that cover modern savannas and suburban lawns. They used a new form of photosynthesis—C4 photosynthesis—that was more efficient than the C3 photosynthesis used by trees and shrubs. Because it was more efficient, it sucked more carbon out of the atmosphere.25
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.
PART III
Us
CHAPTER 7
Humans: Threshold 6
A common language connects the members of a community into an information-sharing network with formidable collective powers.
—STEVEN PINKER, THE LANGUAGE INSTINCT
Humanity entire possesses a commonality which historians may hope to understand just as firmly as they can comprehend what unites any lesser group.
—WILLIAM H. MCNEILL, “MYTHISTORY”
The appearance of humans in our origin story is a big deal. We arrived just a few hundred thousand years ago, but today we are beginning to transform the biosphere. In the past, whole groups of organisms, such as the cyanobacteria, have changed the biosphere, but never before has a single species wielded such power. And we’re doing something else that’s utterly new. Because we humans can share individual maps of our surroundings, we have built up a rich collective understanding of space and time that lies behind all our origin stories. This achievement, apparently unique to our species, means that today, one tiny part of the universe is beginning to understand itself.
Our account of human history will barely touch on the things historians usually discuss: the wars and leaders, the states and empires, or the evolution of different artistic, religious, and philosophical traditions. Instead, we will stay with the main themes of our modern origin story. We will watch the appearance of new forms of complexity, created, this time, by a new species that used information in new ways to tap into larger and larger flows of energy. We will see how humans, linked first in local communities but eventually across the world, began to transform the biosphere, slowly at first, then more rapidly, until today we have become a planet-changing species. How we humans will use our power remains unclear. But we already know that humans, and indeed the entire biosphere, stand at a moment of profound and perhaps turbulent change.1
How did we get here? Our modern origin story can help us get our bearings by placing human history within the much larger story of planet Earth and the universe as a whole. The view from the mountaintop can help us see what makes us different.
Primate Evolution in a Cooling World
Culturally, we humans are astonishingly diverse, and that is part of our power. 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 or feet and toes that can grip. If 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.
Primates are exceptionally brainy. 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. They have about fifteen billion, or more than twice as many as chimpanzees (with about six billion).3 Whales and elephants, the next in line after humans on the most-cortical-neurons list, have about ten billion cortical neurons, but they have smaller brains than chimps relative to body size. Large brains mean that primates are wizards at acquiring, storing, and using information about their surroundings.
Why are primate brains so big? This may seem (pardon the pun) a no-brainer. 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 species are so disdainful of brains that they treat them as an expendable luxury. There are species of sea slugs that have mini-brains when they are young. They use them as they voyage through the seas looking for a perch from which they can sieve food. But once they’ve found their perch, they no longer need such an expensive piece of equipment so… they eat their brains. (Some have joked, cruelly, that this is a bit like tenured academics.4
