by Neil Shubin
This chain of events removes carbon from the atmosphere, taking it from the air and moving it to the hot internal crust of Earth. Alone, these steps would pull all the carbon out of the air, leaving Earth freezing with no atmospheric insulation. The good news is that there is a recycling mechanism for carbon. Carbon in the interior of Earth gets injected back into the atmosphere by volcanoes that eject gases. That is the long-term source of much of the carbon we breathe: while acid rain and the weathering of rocks remove carbon from the air, volcanoes emitting gases return it. Volcanoes typically release huge amounts of water vapor, carbon dioxide, and other gases; by some estimates they send over 120 million tons of carbon dioxide into the air each year.
The cycle of carbon. Over long time periods—millions of years—carbon enters the atmosphere as it ejects from volcanoes, only to return to rock at the bottom of the ocean so that the cycle can begin anew. Acid rain is the link: it takes carbon from the air, allows it to drain into the ocean, and get incorporated into rock.
Like a sequence of events in which each step makes sense but the end points are counterintuitive, the conclusion to draw from carbon’s movement is that rock erosion and weathering is linked to climate. Rock erosion by acid rain is like a giant sponge that pulls carbon dioxide from the atmosphere. Lowering the amount of carbon dioxide in the air will drop the planet’s temperature. On the other hand, planetary events that increase the amount of carbon in the air—enhancing volcanic activity or slowing removal of carbon from the air—will, of course, serve to raise temperatures. All else being equal, increasing erosion of rocks leads to lower temperatures, decreasing erosion to higher ones.
The movement of carbon links rocks to climate and ultimately answers Sagan’s paradox about the sun. The planet’s temperatures are kept within a narrow range by the movement of carbon molecules through air, rain, rock, and volcano. Hot weather leads to more rock erosion, which leads to more carbon being pulled out of the air and thus colder weather. Then, just as things get colder, the cycle moves the planet’s temperatures in the opposite direction: colder weather leads to less erosion, increasing amounts of carbon in the air, and hotter temperatures. Liquid water is possible on our planet only because of this balance; neither we nor the landscapes we depend on could exist without it. But liquid water is like the miner’s canary. Too much of it, or too little, reveals a long-term shift in workings of the planet, changes that amount to planetary fevers and chills.
What happened when the poles started to freeze about 40 million years ago? The shift from hot to cold occurred at the same time that the levels of carbon in the atmosphere dropped precipitously. But this begs the question: What changed the levels of carbon in the air?
Maureen “Mo” Raymo went to school to study climate and the kinds of geological changes that could have an impact on it. And, like Arrhenius, she produced a thesis that elicited memorable comments from advisers. One went so far as to comment that her Ph.D. dissertation was “a total crock.”
Her path to that fine moment began like any other graduate student’s: she took a string of classes representing the core knowledge of her field. In geology seminars of the 1980s, much of the buzz was about global carbon and Earth’s thermostat. A classic paper, read by every student at the time, written by Robert Berner, Antonio Lasaga, and Robert Garrels, described this link in chemical detail. The paper became affectionately known as BLaG after the initials in the last names of each author. Everybody read BLaG and everybody was tested on BLaG, even though virtually everybody, including the BLaG authors themselves, realized key details of their brilliant model had yet to be filled out.
Raymo took the standard class for graduate students where the details of BLaG were presented. She also took classes on modern rivers, mountain formation, and tectonics. But unlike the rest of us who sat through this kind of curriculum, she began to connect the dots.
Everybody knew that the climate cooled drastically starting 40 million years ago, but there was no known geological mechanism that could possibly have done this. What could drop the temperatures? Only a major planetary change could possibly have removed enough carbon to allow such cooling.
Then Raymo looked at a globe and remembered her plate tectonics. The period of drastic cooling commenced at a pivotal time in the history of the planet. This was when the continental plate of India, which had been traveling north for hundreds of millions of years, began to slam into Asia. The result of this collision is like sliding two stacks of paper along a tabletop until they scrunch together: they crinkle and rise. A similar kind of mashup of the continents led to the rise of the Tibetan Plateau and the Himalaya mountains.
Raymo’s lead adviser (not the one who called her thesis a crock) was thinking about how a new mountain range would affect global wind currents or serve to make a shadow that could foster storms. Raymo’s insight came from thinking about how a massive mountain range and plateau could affect Earth’s thermostat.
The Tibetan Plateau is a vast barren face of virtually naked rock. It contains over 82 percent of the rock surface area of the planet and reaches over twelve thousand feet high. With the rise of such a plateau came ever growing amounts of rock erosion on its surface. When we look at the Himalayas, most of us see a dramatic series of mountains, but Raymo saw a giant vacuum that removes carbon dioxide from the atmosphere—and rivers that flush the carbon into the sea. With decreasing carbon in the atmosphere came a cooler planet. The rise of the Tibetan Plateau led to the shift from a warm Earth to a cold one; it did so by pulling carbon from the air via erosion of rock.
Raymo’s theory makes sense of an enormous amount of data, but gaining support for an idea like this is more like winning a criminal case on circumstantial evidence than it is a mathematical proof; only the agreement of a heap of independent lines of evidence can nail the case. Raymo made a very specific prediction: the test lies in using tools that can correlate measurements of the rates of uplift of the plateau—and the levels of weathering of rock—with the amount of carbon in the air. There are altimeters in ancient rocks—altitude-sensitive plants. Carbon in the air dropped at the time of uplift, but we still do not have the precision to tie the different variables together in the fine detail needed for a test. Whether erosion of the plateau alone is sufficient for the climate change we see or if this acted in concert with other mechanisms remains to be seen.
By 40 million years ago, the map of the world was on the move, and with it the environment that supports life. India slamming into Asia may have heralded an era of dropping levels of carbon dioxide and global cooling, but details of the timing of the freezing of Antarctica suggest other contributing factors. Forty million years ago, it went from being a rain forest to having a climate much like southern Patagonia today. Then, by about 30 million years ago, the fauna and flora started to diminish, until 20 million years ago, when the first permanent sea ice appeared. The vegetation was largely that of stunted tundra by this time. Ten million years ago desolation was complete.
Look at a map, and you’ll notice that the Northern Hemisphere is mostly brown land and the Southern Hemisphere is mostly blue ocean: the northern half of Earth is composed of large and mostly connected continents, and the southern one, vast oceans. Inside this simple observation lie clues to the freezing of the planet, the vanquishing of life on Antarctica, and the environmental changes behind much of human history.
By the early 1970s, as the reality of plate tectonics was being widely acknowledged, one huge patch of ocean floor remained virtually unexplored: the waters of the southern oceans made famous by the great explorers of Antarctica such as Robert Falcon Scott, Ernest Shackleton, and Roald Amundsen. Rough seas separate icebergs and barren rocky islands, so much so that the latitudes from 40 degrees to 70 degrees in which this southern ocean sits are given nicknames: the “roaring 40s, furious 50s, and shrieking 60s.” This was among the last regions of the ocean floor to be sampled for a good reason. The ocean currents and winds make for a forbidding sea.
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bsp; Coming off their successes mapping the floors of the Atlantic and the Pacific, the ocean-mapping teams that supported the work of people like Heezen and Tharp turned to this southern patch. From 1972 to 1976, expeditions were sent to core about twenty-six sites to look at the sediments of the ocean bottom. At each stop in the ocean, at a site determined by studying maps at home, a winch returned a plug of seafloor captured from a core at the bottom. Each rock was studied chemically to reveal its age and origins, and the structure of the ocean bottom was mapped just as Marie Tharp and Bruce Heezen had done a decade before in the Atlantic.
The cores from the sea bottom changed the way we look at the southern world. Surrounding Antarctica like a ring is a huge rift valley with a molten core. This, like the rift at the bottom of the Atlantic, is a place where new seafloor is being made, where the plate is actually spreading. The jigsaw puzzle of the South, contained in the shape of the continents and Colbert’s Lystrosaurus discovery, became clear. At one time in the past, the entire southern part of the globe was indeed one giant super landmass composed of what are today all the southern continents: Antarctica, Australia, South America, and Africa. The distinctive blue oceans that define our South today weren’t there.
Then, with the birth of this volcanic ring surrounding Antarctica, the continents separated and moved away from their southern neighbor. Three things happened at once: Africa, Australia, and South America moved north, Antarctica became isolated at the South Pole, and vast seas opened up separating all the southern continents. None of these changes boded well for life near the South Pole.
Just as isolation is bad for people, so too is it for polar continents. The ocean current that has so vexed mariners for years runs as a ring around Antarctica from east to west. It was born as space was created for it by continents cleaving from Antarctica. Oceans are wonderful ways to transport heat. As an example, Britain lies at the same latitude as northern Labrador. One place is relatively mild, the other quite fiercely cold. The reason? Warm currents coming up from the equator keep Britain’s climate mild, whereas the western Atlantic has no such current. Before Antarctica became isolated, ocean currents running from the equator brought heat to the continent. When Antarctica separated from the other southern continents, this conveyor of heat stopped, only to be replaced by the ring current. This change spelled cold for Antarctica: whatever heat existed at the South Pole just escaped into the air, never to be replenished by warm waters. Life on Antarctica literally froze to death or skedaddled to greener pastures elsewhere.
The emerging map of the world changed climate and life. Moving continents and expanding seafloor brought new patterns of ocean circulation, erosion, and levels of carbon dioxide in the atmosphere, thereby dooming an entire continent. The consequences extend as far as the eye can see.
SEEING IT ALL
Humans are visual animals built to detect patterns in a messy world. Bush pilots’ eyes, like those of Paul Tudge, are trained to spot objects on a flight. Children can find hidden objects in puzzles or pictures, fly fishermen learn the water by seeing shadows below the ripples in streams, and radiologists save lives by deciphering shadows on images: our species has survived by finding patterns hidden in the apparent chaos around us. This ability lies in the interplay between our eyes and our brains: together they help us learn to see, survive, and thrive.
We live in a world so awash in vivid hues that it is easy to forget we perceive only a tiny fraction of the colors in front of our eyes. Light arrives to us in a wide spectrum of wavelengths, from ultraviolet to infrared. Gadgets such as night-vision goggles provide only a glimmer of these hidden frequencies. Other creatures can see a broader range of colors naturally. Birds perceive many more shades of blue, as do some species of fish. Each species—whether eagle, trout, or human—is tuned to experience and perceive its world in a particular way. And our perception has its roots in the forces that froze the poles of Earth.
Human eyes, like those of other mammals, have a postage-stamp-sized retina in the back that receives light from the lens. Plastered on the retina are about 5 million specialized cells that are like little receivers to detect red, yellow, and blue—the three primary colors of light. This ability is conferred on each cell by a specialized protein inside that undergoes a distinctive change in shape when the right color hits it. The cells in the retina can discriminate about a hundred different shades of light. When these signals hit the brain, they are combined, allowing us to perceive a palette of about 2.3 million different colors.
Our closest primate cousins of the Old World—monkeys, gorillas, chimps, and orangutans—can see the same palette of colors as we do. We share a very similar makeup, which extends to the proteins inside the retina we use to perceive color. More distantly related primates, such as those that live in South America, do not see in color exclusively: in some species the males are color-blind. Ever since the nineteenth century, primatologists have known about a big split in our primate family tree: all Old World monkeys have full color vision, whereas this trait is lacking in their New World cousins. Is there also a difference in lifestyle that explains the ability to see in vivid color?
The first hint came from a surprising finding. Howler monkeys, as the name implies, have a distinctive cry. They were described by the great explorer Alexander von Humboldt in the nineteenth century as having “eyes, voice, and gait indicative of melancholy.” Scientists studying their behaviors and visual structures in the 1990s discovered that unlike South American monkeys, all howlers are able to see in the same spectrum of color as we do. There is a huge difference in the diets of howlers and their South American cousins. All other monkeys eat mostly fruit, whereas howlers exist on leaves.
This observation motivated a young graduate student, Nathaniel Dominy, a former football player at the Johns Hopkins University, to think in a new way about how color vision arose. Perhaps the lessons of the howlers is general, he thought, and there is a major difference in diet that explains why our branch of the primate family tree sees in color.
Kibale National Park in western Uganda sits among a rich forest landscape of evergreen and mixed deciduous trees. Leopards, hornbills, and distinctive forest elephants—unusually hairy and small—dwell there. So too does an astounding di-versity of primates. A whopping thirteen species—including chimpanzees—make the park their home.
Kibale is also home to a fourteenth species of primate—humans—many of whom live in the Makerere University Biological Field Station to study their primate cousins. In 1999, Dominy traveled there with the simple goal of watching the monkeys eat.
Dominy and his research adviser, Peter Lucas, had a plan: they were going to look at each type of primate in the reserve and quantify exactly what it ate and when. If there was a pattern to the diets, they were going to find it. The crew wasn’t just armed with notepads; they carried a backpack laboratory that was described in a later scientific paper with a title that says it all: “Field Kit to Characterize Physical, Chemical, and Spatial Aspects of Potential Primate Foods.” Inside the backpack was a materials testing device designed to measure toughness of foods; a spectrometer to quantify color and basic nutritional properties of foods; and a number of other gizmos to record the shape and weight of whatever the monkeys gobbled up.
Dominy, Lucas, and their team spent ten months watching primates. When they weren’t interrupted by the threat of bandits or terrorists (at one point they were forced to retreat to the American embassy in Uganda), they worked around the clock, eventually logging 1,170 hours of observations. They would watch the animal as it consumed its meal, then hit the leftovers with their backpack lab. In the end, they found that the monkeys consumed about 118 different kinds of plants.
Back home, as they crunched the data, a pattern emerged. Species with color vision preferentially selected leaves that varied on the red-green color scale. That is, they were differentiating foods that animals lacking color vision could never even perceive. And what of the foods that they selected using their
color vision? These morsels uniformly had the highest amount of protein for the least amount of toughness. The primates’ mothers must have been pleased: they ate things that were both good for them and easy to digest. And the biggest cue, red color, was something that only species with full color vision could detect.
To Dominy and his colleagues, a hypothesis emerged: color vision enabled creatures to discriminate among different kinds of leaves and locate the most nutritious ones. This advantage gained new prominence when climates changed and plants responded.
More clues to color vision are nestled inside DNA. Mammals that lack color vision have only two proteins to perceive color; we and the Old World apes that perceive colors have three. In 1999, as DNA technology became cheaper and more powerful, the actual composition of these proteins could be compared, giving a detailed look at their chemical structure. Hidden inside the sequences was a major clue to the origin of color vision. The three proteins that allow us to see colors are duplicates of the two seen in other mammals. By comparing the sequences in the new copies with the old ones, we can get an estimate of when the duplication happened. All creatures with the three genes trace their lineage back to about 40 to 30 million years ago, the likely time when color vision arose in our closest ape ancestors.
What happened to the planet at the time of this genetic change? Earth got cooler. The forests of Antarctica and the North retreated, to be replaced by ice. Grasses spread to new places around the world. The fruit-bearing palm and fig trees, so common in Wyoming and throughout the warm world, declined, yielding forests mostly of leaves—some tough, some soft, some nutritious, some inedible. The skills that are now so useful to the primates with color vision in Kibale, Uganda, were a key to their success during the period of global cooling. The cold brought a new flora, one that put a new kind of color vision at a premium.