by Neil Shubin
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.
The 100,000-year cycle relates to changes in the shape of Earth’s orbit: ice ages tend to occur more in eccentric periods.
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.
SEEDS OF CHANGE
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.
Dorothy Garrod (right) in the field. (Illustration Credit 9.5)
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.
CHAPTER TEN
MOTHERS OF INVENTION
By 8 million years ago the shapes of the continents, oceans, and seas would be recognizable to an elementary school class today. The planet looked decidedly modern, except for one impo
rtant omission: it lacked a big-brained species walking on two legs.
Hints to the revolution afoot are first seen inside rocks about 7 million years old from what are today Chad and Kenya. A French team, working at the margin of a lake bed, unearthed a chunk of a skull that has a remarkable mix of traits. With large brow ridges above the eyes and a small cranium, it looks something like a chimpanzee. But the snout and face are far too small for any chimp: these traits are decidedly humanlike. More clues come from slightly younger rocks in Kenya. The portions of femurs and other leg bones that have been found are straight, much like those of a creature that spends time moving about on two legs. Something was happening, as new kinds of apes lived, and perhaps even walked, on the planet.
These creatures certainly didn’t know it, but the ground under their feet was changing. The continent of Africa was beginning to fracture. Upheavals deep within Earth caused the crust to tear, opening up a rift that began to unzip the continent from north to south. The rip started small and widened to extend about two thousand miles, from Egypt south to Mozambique. As the process continued, these rifts, like those we chased in our own hunt in Greenland’s 200-million-year-old rocks, caused bulges and depressions in the surface of Earth that formed a series of valleys with mountains.
