As we lumbered along at a ground speed of thirty miles per hour, I felt as if our plane would drop from the sky. With this strong headwind, the five-hundred-mile trip from Iceland’s capital, Reykjavík, to our remote landing strip on east Greenland could take nearly half the day. The craft we were flying, a DeHavilland Twin Otter, is the workhorse of the Arctic. With a stall speed of fifty-five miles per hour and fitted with huge balloon tires or skis, it can land on tiny patches of rocky tundra or ice nestled in remote Arctic valleys. Hunched in a compartment big enough for only four crew, pilots, and gear, I could only imagine how early Arctic explorers—those who set out in the nineteenth century with wool coats, leather shoes, and salt pork dinners—felt on their first sight of the North. With the slow ride and a window seat, I was able to linger on the view.
During the trip north, the vista transforms as plants recede and the extent of ice expands. The sea ice appears in patches at first, then forms a solid sheet over the ocean. Seen from ten thousand feet, the ice grades from a clean, almost pure white to shades of blue, green, and teal. With shapes like no other place on Earth, it fractures in cubes in some spots and in long sticks and crystalline diamonds in others.
The slow, low-altitude approach to Greenland is defined by an ominous wall of fog that lingers in the windshield for hours. As one gets closer, the fog is revealed to be a massive sheet of ice that extends as far as the eye can see. The center of the island is filled by one of the largest glaciers on the planet. The sheet extends six thousand feet high and six miles deep, over an area the size of Texas. Exposures of bedrock of the island are restricted to cliffs that line the coast; the rest of Greenland’s rock lies buried deep under ice. The ice cap is a lifeless glacial desert touched by humans only rarely.
In one bout of activity, this desert of ice sprang to life in the 1950s, when Greenland took strategic importance during the Cold War. On the northwest corner of the ice cap a secret project run by the U.S. Army was launched with a name right out of Dr. Strangelove: Project Iceworm.
The plan, concocted somewhere in the seventeen miles of Pentagon hallways, was to carve silos for six hundred nuclear warheads in the ice in northern Greenland. Connecting these silos were to be tunnels that contained an entire underground city given the futuristic name Camp Century.
The project was started secretly in 1959, when twenty-one tunnels were dug with heavy equipment flown from bases far to the south. In its heyday, this city under the ice housed over two hundred people and contained a shop, hospital, theater, even a church. Power was supplied by the world’s first portable nuclear reactor, the plucky Alco PM-2A. Heat from the reactor melted ice to provide the subglacial city with water. Self-sufficient and mostly belowground, the whole operation was something like a human ant farm.
Being close to perceived threats in the U.S.S.R., Camp Century had the makings of a perfect military base. The plan worked well, except for one problem: ice moves. By 1966, it had become clear that the ice was shifting so extensively that warping tunnels would destroy expensive equipment. Pictures of Camp Century taken today reveal twisted machinery and abandoned huts, all artifacts of schemes and fears inside an ancient block of ice.
Work at Camp Century did have value, though not of the type Pentagon planners could have ever foreseen.
Louis Agassiz was born in 1807 with charm, intelligence, and an unrelenting passion to study nature. Even as a child, he fed his insatiable curiosity by making his own collections of animals and plants, often drawing each of their organs in exquisite detail. He believed in learning by seeing, a dictum that was to become his catchphrase throughout his career. Sensing his proclivities at an early age, his parents set him up to apprentice with an uncle who had established a successful business. They wanted Louis to develop into a successful “man of affairs,” not a collector of bugs and rocks. But they underestimated the influence of his charm. Young Agassiz would have none of his parents’ designs: he enlisted one of his teachers to lobby his parents for him to stay in school and become, as he said later, “a man of letters.”
While Louis was in his late teens, he and his brother were studying in Zurich and found themselves without a ride home, a distance of over a hundred miles. They started walking until a stranger, a well-to-do Swiss, offered them a lift. So impressed was he with Agassiz’s acumen that this gentleman later wrote to his parents offering to pay for his full education. Thus began a steep career trajectory that ultimately propelled him to the United States, where he took part in the founding of two major scientific centers: the Museum of Comparative Zoology at Harvard and the National Academy of Sciences.
As a young family man in 1837, Agassiz took his brood on a summer vacation to the picturesque town of Bex. Lying along the Rhône River, Bex is bordered on the east and west by the Alps. Today it is home to the only working salt mine in all of Switzerland. A narrow-gauge train takes visitors hundreds of feet beneath the earth. This vast hole was originally dug in the 1820s to quarry salt that in those days was literally worth its weight in gold. At the time of Agassiz’s visit the mine was new, and its director took great pleasure in showing summer visitors the local geology, which in this part of the Alps is hard to miss and very easy to appreciate.
Some time before Agassiz’s visit, the director and a friend had discovered a number of puzzles in the local rocks. With the arrival of Agassiz, the two were excited to quiz a visiting luminary on the meaning of these geological oddities.
Giant boulders dotted the landscape, some the size of a caravan. That is not unusual; large boulders can be a common occurrence. But these were completely out of place, because the rock that composed the boulders was different from the local bedrock. In fact, the closest match to the boulders was in cliffs hundreds of miles away. Something had transported them, but what?
Closer inspection of the boulders revealed other clues. Scrape marks, almost as if made by a pickax, etched their surfaces. And the marks didn’t run willy-nilly; they extended in parallel lines.
More mysteries came from a bird’s-eye view of the valleys from the scenic overlooks that lie along the roadsides of this part of the Alps. Each of the mountainous valleys was bordered by ridges of gravel that looked scrunched, almost as if they had been moved by a plow or a steam engine. Since the ridges were perched on hillsides in rural valleys, these causes were obviously ruled out.
Boulders and gravel mounds told the same story: something was moving rocks around. But what?
Flowing water could be ruled out. Floods large enough to move the giant boulders would have left very obvious markings across the landscape. Of course, human activity could be ruled out as well. That left the one obvious cause—ice.
At the time of Agassiz’s visit, the ice was nestled in glaciers high up on the mountains. But what if that was only its most recent position? What if at some point in the past the ice covered the valleys below? If the levels of ice waxed and waned in and out of the valleys, then the boulders would move, and the rubble would be plowed about to make mounds and carve scrapes.
After this grand show-and-tell, Agassiz’s friends tried the ice idea out on him. To Agassiz—whose life’s modus operandi was to learn by observing—the visit sparked an epiphany. It was a set of observations that changed his world. At every scale, Switzerland’s rocks made sense when considered in the light of moving ice: scrapes on rocks told the same story as the mounds of gravel and the shapes of the valleys themselves. Agassiz’s heart raced at the thought of something even more general. His travels revealed these features weren’t limited to the Alps; they were common all over Europe, even south to the Mediterranean. Moving ice wasn’t confined to picturesque Swiss cantons; it must have covered virtually all of Europe.
Unbeknownst to his friends in Bex, Agassiz set off to test his grand idea. In 1840, he published a book, dedicated to his friends from that fateful summer vacation, called Studies on Glaciers. In it he proposed the radical notion that ice at one point in time extended from the North Pole all the way to the Mediterranean and then retreated, only to extend again. A friend came up with a catchy name for these cold intervals: “ice ages.”
Agassiz, with his personal charm, set off to convince the great eminences of the time of his notion. He took visitors out in the field as his friends from the summer vacation had done for him, encouraging them to see a past rich in ice. It took many trips, and even more arguments, but Agassiz succeeded. The ice age theory became widely accepted.
The beauty of this theory was that, like most great scientific ideas, it made specific predictions. Agassiz’s notions could be tested simply by looking at the rocks in the world. Exotic boulders, mounds, and linear gashes on rocks should be widespread. If it is one thing to find a widespread pattern, then it is the clincher to find the cause.
But a problem for enthusiasts was that Agassiz’s ice ages lacked any plausible mechanism. In fact, the idea even flew in the face of existing dogma that Earth has been cooling over time. If Earth was cooling, then glaciers should not have retreated to where they are today; they should have expanded. Moreover, Agassiz’s layers of gravel and boulders were showing not a single shift but a rise and fall of Earth’s temperatures over time. What caused the waxing and waning of the ice?
Born and raised on a farm in Scotland, James Croll (1821–1890) lacked any formal education. Like Agassiz, he lived for the life of the mind: great ideas, puzzles, and intellectual problems. To support himself, he tried selling insurance, but with a natural aversion to people he couldn’t stomach the job. Leaving that, he set up a tea shop. While he still couldn’t manage to avoid people altogether, the shop did offer one salient advantage over the other gig: it left him plenty of time to study. And studying was the one thing he absolutely loved to do.
Croll’s physiognomy, revealed by the best-known picture of him, shows the thousand-mile stare of one whose mind is transported to a faraway place or working on a deep mathematical problem. His mouth, set firm with a Scottish obduracy, also reveals a decided lack of humor; one can’t imagine many jokes emerged from those lips. By all accounts, Croll had an exceptional focus that, coupled with a passion for learning, would allow him to spend an entire year reading a single book, often lingering on one page for a day or more to digest each idea. His driving passion was to get to the bottom of intellectual problems. Not satisfied with seeing only patterns, he wanted to figure out how the world actually worked.
Agassiz’s ice ages provided a puzzle ripe for the solving. Croll’s approach was decidedly different from that of Agassiz before him. Thinking of fundamentals, Croll asked, “What was the cause?” He set off with a pad and pen to solve the problem. His search for a cause demanded thinking about the factors that changed the amount of heat on Earth. The source for much of that heat is the sun. Is there some regular variation in heat from the sun that could trigger ice ages?
Soon after launching into this research, Croll read a paper by a brilliant French scientist that set his mind in motion. The idea was that regular variation in Earth’s orbit could change the amount of heat that hits Earth’s surface. Earth spins around the sun, and its tilt brings the seasons. The orbit depends on the proximity of other big celestial bodies nearby: Mars, Jupiter, Venus, and Saturn are all rotating in space as well. As they approach Earth on regular cycles, their large masses warp the orbit and tilt of our planet. In times on the order of thousands of years, Earth’s orbit will wobble and change, thereby influencing the amount of sunlight that warms the planet. Croll reasoned that ice ages happen during regular intervals when the orbit causes the planet to receive less heat from the sun.
Here was a cause that made a specific prediction: the ice ages should happen at regular intervals defined by the orbit of the planet. Unfortunately for Croll, his theory became just a passing fad. Because he lacked any firm way of matching the timing of the ice ages to orbits, Croll’s theory remained just a good idea.
A few decades after Croll’s death, a young Serbian concrete engineer got the notion that he could use the mathematical talents that were so helpful in designing buildings to uncover how the universe worked. His thinking was revealed in a toast he gave a poet friend after the two shared a bottle of wine in a Belgrade café. The poet had hoisted his glass to proclaim, “I want to describe our society, our country, and our soul.” The concrete engineer countered with the salute, “I want to do more than you. I want to grasp the entire universe and spread light into its farthest corners.”
Soon after the boast, the engineer, Milutin Milankovitch, switched jobs. Leaving his building firm, he took a professorship at the University of Belgrade. Not easily intimidated, he proceeded to announce that he was out to solve the problems of the planet by pure mathematics. Global climates were his first problem. But not just Earth’s. He wanted to devise a mathematical theory for climate all over the face of Earth and for every other planet in the solar system as well.
This ambition puzzled a few of his colleagues. Why would you need to calculate global temperatures if we can simply set up weather stations to measure them? Milankovitch’s answer revealed his thinking. If, armed with only pencil and paper, he could predict temperatures mathematically, then we would truly understand their causes. Off he went, looking at the planetary rhythms that so captivated Croll.
Croll’s ideas were a natural starting point, but Milankovitch brought a huge new twist to the problem. Using orbital calculations similar to Croll’s, Milankovitch explored how sunlight could change the heat of the planet. To elucidate this relationship, he modeled the different ways that heat gained by the ocean can be transferred to the atmosphere and back. A brilliant mathematician, he was able to calculate the magnitude of temperature changes during the seasons, resulting in a remarkably specific set of predictions.
Earth’s orbit changes in three major ways. Over 100,000 years Earth’s orbit goes from the shape of an oval to a more circular pattern. During 41,000 years Earth rocks back and forth about 2 degrees. And in the course of 19,000 years Earth’s tilt wobbles like a top.
Milankovitch realized that these are not huge changes, and in fact they would not alter the amount of heat received by Earth much. What they could do, as his equations showed beautifully, is change the duration and intensity of the seasons. And the reason is straightforward: if the seasons depended on the degree Earth is tilted and the manner in which the planet rotates around the sun, then changes to the shape and orientation of the planet and its orbit will affect the heat of summer, the cold of winter, and everything in between.
The rocks reveal the occurrence of ice ages. Mathematical calculations show that Earth’s climate can change in a cyclic manner, matching the orbital changes of our planet. But do the cycles of ice ages and those of Earth’s orbits march together? Answers would have to wait for new scientific quests—namely, the effort to make the atom bomb.
The Manhattan Project was a short-term war effort that pulled together a unique cadre of scientists to focus on a single goal. With the war’s end the U.S. government found itself with a problem, but one of those problems that is good to have. It had teams of scientific geniuses housed in different places, from New Mexico to New York, with no long-term infrastructure to continue their work. To make matters more challenging, no longer was there a single goal to their work, like developing a bomb; there were now many. Not wanting to lose the talent, or the momentum generated from fundamental breakthroughs in physics, the government supported a number of labs around the country, including one at the University of Chicago. Chicago was home to the group, led by Enrico Fermi, that launched the first controlled nuclear reaction (today the spot is marked by a Henry Moore sculpture across the street from the gym). After the war, the government helped the university establish a number of institutes exploring the big questions of physics and chemistry. One of those big problems was the history of our planet.
Two people who benefited from this transition from war to peace science at Chicago, Willard Libby and Harold Urey, shared a passion and a belief. The passion was for expanding knowledge. The belief was that trapped in the dynamics of single atoms—in their electrons, protons, and neutrons—were clues to the origin and history of the planet and perhaps even the entire solar system.
Driving this exploration of the atom was the development of new devices that could measure particles in parts per billion. With this resolution, new kinds of answers to old questions were now possible.
Libby set up two junior scientists in his lab with five thousand dollars to carry out a research program on carbon. Like most atoms, carbon exists in a number of different forms in the natural world. All carbon atoms have the same number of protons inside their nuclei; the different versions are distinguished from each other by the number of neutrons inside. Libby’s insight was that all living things will have the same amount of carbon 14 in their bodies as the atmosphere in which they live. Living creatures breathe, eat, and drink carbon atoms in their daily lives and thus share the same balance of carbon with the atmosphere. Once organisms die, this balance with the atmosphere is disrupted, and no new carbon enters as food or nutrients. Whatever carbon atoms remain in the body begin to decay into other forms. As we’ve seen with other atoms, this reaction happens at a constant rate set by the laws of physics and chemistry. Knowing this, Libby ventured that if you can measure the amount of carbon 14 in a sample of old bones, you can, with some assumptions, calculate how long ago the animal died. This was a huge advance: it was like finding clocks inside ancient bones, teeth, shells, and wood.
To Harold Urey, who worked in a lab just steps away, atoms were imagined to be clues to the history of the planet, solar system, and universe. One of his main objects of fascination was an atom familiar to us all—oxygen. An abundant player in our air, water, and skeletons, oxygen has some distinctive properties that make this infinitesimal atom a window into our past and a much larger world.
Urey knew that oxygen, like carbon, exists as heavy atoms with extra neutrons and light atoms with fewer. On purely theoretical grounds, he guessed that the balance of these forms in any substance depends on temperature. The timing of his guess could not have been better, because accurate machinery could test his ideas.
And it worked: the ratio of heavy and light oxygen atoms in a material was dependent on temperature. To Urey and his team, this success meant that if you could measure the infinitesimal amounts of the different forms of oxygen in any substance—water or bone, for example—you might be able to guess the temperature of the environment in which it formed. The trick was to find the right kind of record that could reveal the details of Earth’s climate with precision. Only then could the tool kit derived from the work of Libby, Urey, and their colleagues pull together cause and effect.
Seashells are durable and hard because they contain a crystal, calcium carbonate. This molecule, so vital to their hardness, also fortunately contains oxygen. Urey and others saw that as seashells develop during the life of the animal, the molecules that make the shell are ultimately derived from the water in which they lived. The relative amounts of the different forms of oxygen in the shell could, then, reflect the temperature of the waters that the creatures grew in. And since shells preserve well, they could contain an excellent record of ancient events.
With oxygen atoms as the thermometer, carbon atoms as the timekeeper, and the regularity of the layers as a guide, the teams set off to see how climate changed over the ice ages. One group looked at the most continuous record of seashells they could find, to map the temperature changes over time. The bottom of the sea is ideal: it contains layer after layer of sediment that drifts down the water column. By looking at the oxygen composition of the seashells inside these layers, the researchers could get an approximation of how climate changed over time. The team found that the planet’s temperatures waxed and waned with peaks of high temperature and valleys of low temperature. What’s more, the temperature seemed not to change randomly over time: if you squinted really hard at the graphs they made, you could see that the peaks and valleys seemed to rise and fall every 100,000 years. This was not some random number but one of those proposed by Milutin Milankovitch years before. One-hundred-thousand-year pulses started cropping up in other people’s data as well. Maybe astronomical events were influencing things after all?
The problem was that the data were messy; the plots of temperature versus time have lots of wiggles, not just the 100,000-year one. Then three scientists, one British and two American, took a new look and applied a method developed by one of Napoléon’s regional governors after his conquest of Egypt. The bureaucrat, bored on the job, set off to understand heat and its transfer among different materials. It wasn’t heat that was to help geologists over a century later; it was a new mathematical approach he devised. If you have a graph with lots of different wiggles in it, perhaps that mess is made by several different rhythms superimposed on one another. The mathematical technique, known as Fourier transform analysis, is a way of revealing how a complex pattern can be made by a number of regular and more simple ones.
With that simple analytic tool, the data revealed not chaos but a deeply buried signal. The pattern emerges from a number of rhythms superimposed on one another: 100,000-year cycles onto cycles of 40,000 and 19,000 years. Milankovitch and Croll were right: ice ages are correlated in a broad way to the changing orbit, tilt, and gyration of Earth.
Graphs of climate, with peaks and valleys reflecting the rise and fall of temperature over the millions of years of geological time, look something like an EKG of a human heart. The heartbeat of our planet has drummed on for countless eons, beating to rhythms in Earth’s orbit and the workings of air and water. Before the global cooling 45 million years ago that so fascinated scientists such as Maureen Raymo, these orbital changes did not often lead to ice ages. With a newly cool Earth, orbital wiggles became written in the waxing and waning of sheets of polar ice. And it is the ice itself that reveals the biggest surprises.
In 1964, during the heyday of Camp Century, a Danish geologist, Willi Dansgaard, visited the major air base in the region, Thule Air Base—the supply station for the camp—to look at local snow. Dansgaard spent some time in Chicago, even working in Urey’s lab. Students then remember his fondness for the cold, leaving windows open during the long Chicago winters.
While on base, he heard buzz of the military project going on a hundred miles to the east. Asking permission to visit Camp Century, he was rejected on the grounds that it was a top secret operation. With some luck, in the form of a visionary senior administrator in the U.S. Army’s Cold Regions Research and Engineering Laboratory, he was given access to the pristine cores of ice that the air force dug up to make the city under the glacier. Perhaps within these chunks of ice were keys to understanding the planet’s climate?
Dansgaard had yearned to see a huge uninterrupted column of ice for much of his professional life, and now the most complete ice cores yet known were within his grasp. Two features of ice cores are immediately apparent. They are colorful, varying from iridescent green to blue. And they are layered, with thick layers, thin ones, and everything in between. Almost anything in the atmosphere or in the water can get caught in ice. Debris of all sizes and kinds can get trapped: not only seeds, plants, and ash, but vintage World War II planes. Air from the atmosphere can get caught as bubbles. The layers of ice themselves can reveal the extent of the seasons. Arctic winters are dark and cold, whereas the summers are bright and less cold. With the sun come melt, flowing water, and the detritus water brings. Summer bands in the layers are darker and messier than the ones made in winter. Dust blown by the winds can make some layers darker than others. With so much trapped in the ice, it becomes a very precise and informative record of ancient climates.
Dansgaard’s breakthrough came from applying the tools developed by Harold Urey to the Greenland ice core. Since his focus wasn’t shells but ice, the work required a few modifications, but he nevertheless was able to see a climate record. He measured oxygen along an ice core over half a mile deep, representing more than 100,000 years. Dansgaard saw the remarkable chilling taking place 17,000 years ago, during the ice ages first seen by Agassiz. He also encountered a warming period 500 years ago, corresponding to when humans first settled Greenland. And he found a cooling period extending from 1700 to 1850, when much of Europe was cold and Hans Brinker was ice-skating in the canals of Amsterdam.
Dansgaard’s was a rough first effort because his core, having been dug for missiles and churches, didn’t allow for great scientific resolution. A scientifically useful core is drilled, sectioned, and kept in conditions that allow long stretches of unbroken ice to be analyzed. Needed were new, more precise cores. And if these data were to have meaning, he’d need to see ice from different places on the planet: from both poles and from mountaintops of different continents.
Drilling scientifically accurate cores requires collaboration among engineers, scientists, and governments working on the planet’s largest ice sheets. This is expensive science: rigs need to be set up, and teams housed, in some of the most remote places on Earth. Since the 1970s a number of cores have been drilled, and to date the most complete of these are several drilled into the Greenland ice, the glaciers in Antarctica, and several mountain glaciers from around the world.
The fine-grained view of climate and ice reveals surprises. Earth’s climate during the past 100,000 years has swung wildly on occasion. The ice ages weren’t just long invariant cold periods: glacial periods have witnessed warm intervals, and warm intervals have seen glacial conditions. The emerging picture is that Earth’s climate depends on the heat balance of the planet—the amount of heat coming in from the sun minus the heat that escapes into space—and the ways that this heat is transferred among the oceans, land, air, and ice. Music is an analogy for what drives climate: a composition can be heard as one entity but be decomposed into rhythms, backbeats, and harmonies of different instruments acting on their own cycles. Orbital motions of the kind revealed by Milankovitch define the main cadence. The movement of heat through ocean currents, winds, and ice floes form other beats. The result of the interacting effects of these components is a system that has a long-term rhythm and short-term riffs.
Climate at the end of the last glacial period, about 12,500 years ago, exemplifies one of the riffs. At this time, when by all accounts things should have continued to warm, there was a dramatic shift to a sharp cold spell that happened in the blink of an eye in geological terms—over decades. The record from pollen, oxygen atoms, and other markers implies a climate that converted from warm to cold on a dime. Global mean temperatures changed 15 degrees in as little as a decade. If wiggles of the climate curves are like an EKG, fluctuations like this are the equivalent of planetary heart attacks. When you think of the extent to which coastlines, arable land, and deserts can be transformed by changes in global temperature of just 2 or 3 degrees, the prospect of a 15-degree shift is staggering. Yet that is the kind of change that has taken place during the history of our species.
Orbits, climates, and ice define the way living things spread across the globe and through time. Changes in global climate fragment some populations into isolated groups separated by ice. Others are offered new migration routes, enabling them to reach portions of the globe inaccessible under previous climatic conditions. DNA of Native Americans reveals that they are derived from a single male who likely crossed the Bering Strait when an ice bridge formed during the last ice age. European populations, too, carry the signal of ice in their family trees. The DNA of many Europeans derives from populations that formerly lived in Ukraine and spread out during the last recession of ice. Ice is carried deep inside our human family tree, in the DNA we share with our diverse human cousins.
Some populations do not change; they die. The end of the last ice age in North America was a double whammy for the mammals that lived there. First, they had to deal with changing climatic conditions. On top of that, they had a new competitor and predator to deal with: people. The change in climate and the arrival of humans from Asia spelled the end for North America’s saber-toothed tigers, mammoths, and ground sloths.
Still other populations change their way of life altogether.
Dorothy Garrod was known to her colleagues at Cambridge as being “cripplingly shy” and “difficult to know.” Yet she was anything but shy. “My dear Jean,” wrote Garrod to her cousin in 1921, “The last week in France was great fun. It was really almost too moving to be true. You crawl on your stomach for hours … climbing up yawning abysses (lighted only by an acetylene lamp…) and get knocked on the head by stalactites and on the legs by stalagmites, and in the end arrive at all sorts of wonders.” Here was a woman who explored ancient worlds, experienced raw adventures, and had a lot of fun doing it. Discoverer of Neanderthal bones in caves and new archaeological sites around the globe, this “shy” woman became the first female occupant of a chaired professorship at both Oxford and Cambridge.
Digging in Shukba Cave and the surrounding fields near Jerusalem, Garrod discovered odd stone tools shaped like crescents. Nothing like them had been seen before. Then she unearthed a series of mortars, grinding stones, and figurines. The people who lived there had ground wheat and practiced religion.
More digging yielded more discoveries: carefully buried dog skeletons, shelters, bodies in graves with intricate decorations, even elaborate stone sculptures. These people, whom Garrod called Natufians, had the first domesticated dogs, the first sculptures of people having sex, and elaborate burial rituals. The Natufians had settlements with hundreds of people interacting in complex societies that changed over time. Previously, human populations were nomadic: populations adapted to changing climates and food supplies by moving. Natufians exemplify novel strategies: the development of a largely sedentary culture that ranged from mobile camps to semipermanent settlements over several thousand years—ranging from fifteen thousand to eleven thousand years ago.
No population is insulated from changes to the planet, particularly the kinds of decadal climate shifts recorded in the polar ice. The Natufians lived during a period of rapid climate change about thirteen thousand years ago: a cold plunge brought glaciers to high latitudes and cold, dry, weather to lower ones. This cold snap meant that traditional grains likely became more scarce. The Natufians and their contemporaties were almost certainly stressed by this shock to the global climate system, let alone to their food supply and way of life. How did they and the cultures that followed manage?
Plump seeds, typical of domesticated plants, have been found in the remains of Natufian settlements from about eleven thousand years ago. Beginning as rare components in Natufian sites, kernels and grains become common in later human settlements. The seeds are evidence of agriculture; the mortars and pestles are signals of a society using their crops for food. With these inventions, humans no longer needed to rely on the vagaries of migrating animals for subsistence. With the development of agriculture, and more permanent settlements seen in places such as those with Natufian culture, humans could now establish institutions and cultural practices associated with stable societies.
Just as Dorothy Garrod dug in the earth to discover Natufian culture, Jonathan Pritchard, my colleague at Chicago, peers within DNA to see patterns in its structure and sequence. By comparing the DNA sequences of living humans, he can tell if our differences are due to the vagaries of chance or have been sculpted by the action of natural selection. If a particular gene offered an advantage in survival or reproduction to the people who possessed it, it should leave a signal in DNA—one that he could see using statistical techniques he developed for just this purpose. All else being equal, if selection has operated on a gene, it should be more common and less varied in a population than it would be by chance alone.
Jonathan has found stretches of human DNA that carry the signature of natural selection; these are genes that in some way affected the survival or reproduction of our ancestors. This is a kind of holy grail for biologists, because they can tell what biological traits were important. And what do these genes do? Some relate to color pigment. If the spread of human populations across the globe brought them to areas with different light levels, the genes affecting pigmentation would change, with lighter pigmentations found in populations more distant from the equator.
Other genes reflect changes to the diet. Genes that became common in some human populations relate to digesting milk, carbohydrates, and alcohol. The ability to process these products involves special enzymes that break down the characteristic sugars inside. The genes involved with these functions gained a new importance in the past ten thousand years. The ability to digest milk is evidence of the domestication of cows; processing alcohol relates to fermentation. Both are traits of agricultural and, to some degree, sedentary human communities.
The effects of rotating planets and past chills are everywhere—from the sand on the beach to exotic boulders in the landscape, even to parts of our own DNA that persist, like the tunnels of Camp Century, as artifacts of changing climates and cultures.
