by Neil Shubin
ALL TORN UP
If Wegener’s continental drift evokes gradual movement and Hess’s geopoetry a sublime relationship between parts of Earth, then the word that describes the merger of the two, “tectonics,” conjures an idea that rattles our world to its foundations. The 1960s had revolution in the air in music and politics, but arguably the most lasting change was the emergence of a new way of seeing the planet. Patterns of rocks and fossils that were once bizarre started to make total sense. Scientists were gleefully revising centuries of scientific dogma, and foremost among these revolutionaries was John Tuzo Wilson, a Canadian geologist. Wilson, a physicist by training, had a personal quality that was to serve him well during this time. Late in his career he summed it up: “I enjoy, and have always enjoyed, disturbing scientists.”
John Tuzo Wilson, when he wasn’t disturbing scientists. (Illustration Credit 6.6)
In the late nineteenth century, paleontologists recognized that North America and Europe both have distinctive mountain ranges that run north to south. The Appalachian Mountains extend from Maine to North Carolina in the United States, and the Caledonians course from Morocco to Scotland in Africa and Europe. One place in northern Scotland has Appalachian fossils inside its rocks. And in a few places Appalachian rocks have European fossils inside. How did the creatures get there? One could invoke swimming, except that most of these shelly animals live fixed to the seabed and do not travel far.
To Wilson, armed with the new theory, the answer to this puzzle was akin to what happens when a child plays with a peanut butter and jelly sandwich. What happens when the child takes the two separate halves of the sandwich, puts them together, and then opens them up again? The mushed jelly and peanut butter reflect what happened when the sandwich closed up. The bits of peanut butter left on the jelly side and jelly on the peanut butter side reflect the opening.
To Wilson, Europe, North America, and the Atlantic Ocean behaved the same way as that sandwich. He proposed that there was an ocean that separated the continents hundreds of millions of years ago. This ocean closed, and as the continents from either side rammed into each other, a chain of mountains formed. When the continent reopened, the chain broke into two pieces, leaving what are today the Appalachians and the Caledonians. And those odd American fossils in Europe? They were just patches of the old continent that got left behind when the Atlantic Ocean opened.
Wilson, and the many scientists who followed, found the globe that we learn in school is only a snapshot in time: there have been innumerable globes in our past, and there will be many more in the future. Earth’s crust is composed of a number of plates, each containing ocean, continent, or both. These plates move relative to one another as the convection under the crust causes the seafloor to spread at Tharp’s ridges and to ultimately be destroyed at the deep ocean trenches.
In 1984, over half a century after Wegener’s death, NASA released the first direct measurements of continental drift. About twenty stations around the world were established, each capable of bouncing lasers off satellites equipped with reflectors. A telescope next to the laser on the ground picked up the reflection by the satellite. By measuring the time that laser light took for the round-trip to each station, NASA calculated the distance to the satellite. If the plates move, then the distance to the satellite should change over time. Using this technique, NASA showed that North America and Europe are getting farther apart by 1.5 centimeters per year. Australia is heading for Hawaii at about 7 centimeters per year. The plates on our planet move about as fast as hair grows on our scalps.
Connecting the dots between the rocks and rifts in North America and Africa. The shaded areas hold matching rocks.
The motion of the plates is slow over the pace of our lifetimes, but over geological time it can be cataclysmic. This dance of the plates takes several major steps. Plates can move against one another. As they rub, the plates can experience earthquakes, like those at the famous San Andreas Fault in California. Some plates smash into each other. When the plates are continents, this collision results in new mountain ranges. The Tibetan Plateau came about when India started colliding with Asia over 40 million years ago. Plates can also move apart. If, for some reason, upwelling of the convection current under the earth happens in the ocean, we see Heezen and Tharp’s ridges and rifts. When this upwelling happens under a continent, this single patch of land can rift into several.
Plate tectonics reveals connections everywhere. Farish and the team were led to Greenland, as we saw in the first chapter, by similarities to the rocks we knew from our work in Connecticut and Canada. In each location ancient faults, lake sediments, and sandstones point to one event in eastern North America 200 million years ago: an opening rift in the earth. Those similarities extend all the way across the Atlantic Ocean to the rocks in Morocco and Europe. The pattern of the rocks and fossils allows us to connect the dots: at one time eastern North America, Greenland, and Morocco were the same continent, which then broke apart during the formation of the Atlantic Ocean 200 million years ago.
Maps and rifts, however, link more than the features of the globe.
THE MAP WITHIN
In 1967, the Levingston Shipbuilding Company of Texas laid the keel for the Glomar Challenger, a ship that looked like any other save a giant drilling tower that rose nearly fifty feet from its center. For the next fifteen years the Glomar Challenger traveled the seas drilling cores in the seafloor. Making more than six hundred stops, it was able to dredge up cores of rock from almost two thousand feet beneath the bottom of the ocean. Each long core would come to the surface as fifty or more thirty-foot-long strips of rock and sediment looking something like gray-brown flagpoles. These cores, almost twenty thousand of them in all, became a bonanza for science: they told us the age of the seafloor, its composition, and its history. Today, they lie in repositories around the world, where they are still being studied long after the Glomar Challenger was scrapped.
Locked inside the cores are layers of minerals with combinations of atoms that allow us to reconstruct the atmosphere, temperature, and workings of the planet for the past 200 million years. Looming large among these are atoms that reflect the levels of oxygen in the atmosphere. Parts of the analysis might seem counterintuitive, but the central idea is that oxygen concentrations in the atmosphere can be approximated by measuring carbon in its different forms. Carbon and oxygen exist in a balance on the planet: carbon ejected from volcanoes interacts with and influences the levels of oxygen in the air and water. By measuring the different atoms of carbon in any given layer of sediment in a core, we can approximate the levels of oxygen in the air.
Two worlds lie inside the cores drilled by the Glomar Challenger: one from before the Atlantic Ocean started opening 200 million years ago and one that emerged after. The oxygen in the atmosphere increased dramatically after the Atlantic formed. By about 40 million years ago, the atmosphere went from one in which we would pant merely to sit still to the one we run around in today.
The rift that began to open over 200 million years ago and split the supercontinent into multiple bits created enormous amounts of new coastline. Each coast is an area where land meets the sea. As every coastal homeowner knows, these areas are subject to erosion. A dramatic increase in erosion can set off a chain reaction. Imagine entirely new coastlines dumping sediment into the sea. With this sediment comes the burial of very special mud that covers the bottom of the ocean shelf. This muck is extremely important, because every day trillions of single-celled creatures die and sink to the bottom; as they decay, they consume oxygen. Left alone, this mass of waste eats enormous amounts of oxygen from the water—and ultimately from the atmosphere—as it rots. But when these layers get buried, oxygen is no longer consumed as quickly, allowing it to build up in the water and the air. This is what the rifts and new coastlines have wrought: increasing levels of oxygen in the air brought about by the burial of oxygen-consuming muds.
The opening of rifts and the chain reaction that enhanced o
xygen in the atmosphere provided opportunities for our ancestors.
A new world began with the rift, one with ever-rising levels of oxygen. And, as we’ve seen, with oxygen come opportunities.
Mammals like us are committed to a very high-energy lifestyle. We manufacture our own heat. The action of our muscles, coupled with the insulation provided by our hair, fat, and, in the case of humans, clothes, keeps our body temperature stable relative to the outside world. Cold-blooded creatures, like lizards, also can keep their body temperatures relatively stable, but they use mostly behavioral mechanisms to do this: sunning on rocks or hiding in shade. In the cold, a lizard cannot be active. I don’t need to worry about snakes on my expeditions north; polar bears are our major concern. Mammals can remain active in climates that would kill cold-blooded animals. Our warm blood disconnects us from the vagaries of the temperature of the outside world. Fueling these fires requires oxygen.
Not only are we insulated from the outside world as adults; we begin our lives inside a womb surrounded by membranes that protect the embryo and provide it with connections to the mother’s blood supply. Since the fetus receives all of its oxygen from the mother, there needs to be a way that oxygen can be transferred from the mother’s blood. The transfer is facilitated by a steep gradient between the concentration of oxygen in the maternal blood and that of the fetus: under these conditions, oxygen will travel into the fetus. Importantly, the oxygen content of the mother’s blood has to be sufficiently high to enable this transfer in the first place. This constraint means that mammals with a placenta do not easily develop above fifteen thousand feet altitude. Tellingly, the oxygen at these altitudes is equivalent to that in the atmosphere at sea level 200 million years ago, before the Atlantic Ocean formed.
Insulation of bodies from changes to the outside world comes at a cost: a big warm-blooded mammal needs fuel to maintain a constant body temperature, develop in the womb, and thrive outside it. Oxygen is the key link: animals like these could never have emerged in the low-oxygen world that existed before the continents split apart. Marie Tharp’s rift didn’t only open up an ocean; it opened up a whole new world of possibilities for our ancestors.
CHAPTER SEVEN
KINGS OF THE HILL
Just one more step,” Paul Olsen kept repeating like a mantra. He was urging me on, but I was frozen like a cat in a tree.
We were on the shores of Nova Scotia taking a break from fossils to collect geological samples. The coastline is made of spectacular red, orange, and brown sandstones that are reminiscent of the Hopi and Navajo reservations of the deserts of the American Southwest. The beauty of this place is magnified by water: rocky bluffs erode into a natural sculpture garden of caves, arches, and pillars. Paul, a geologist at Columbia University, wanted to obtain sand grains from a white layer that separated orange rocks below from brown ones on top.
Unfortunately, this band of white rock was about two hundred feet up a sandstone cliff that was in places almost too sheer on which to stand. In others, it was so highly weathered that one misstep could lead to a long tumble down. To get traction in these places, we had to climb step-by-step using footholds that we carved with our rock hammers. Not being a climber, and moderately scared of heights, I had made progress by only looking at my feet, hammer, and hands, knowing that even a momentary glance down the cliff could summon a rush of vertigo that would freeze me in place. On previous occasions, this panic usually brought the assistance of a team of patient colleagues who formed a human bucket brigade to coax me down to the beach below.
An hour or two of Paul’s cajoling propelled me to the layer. Up close, the white band was about as tall as a human. For about an hour we chiseled rocks, placing small specimens in labeled bags for analysis back home. Our reward came when we looked at the vista of the Bay of Fundy in front of and below us. It was a glorious early summer day: the tides were high, the wind low, and the bay so smooth it looked like reflective glass. The splendor of the bay reveals its history. The shape of the coastline reflects the long-term action of glaciers, faults, and erosion. The pasture and human settlements form a recent veneer on this ancient landscape. Layer after layer of history reveals itself when you know how to look.
It was the vista inside the rocks that drew Paul’s attention. The white band as well as the composition of the rocks surrounding it brought us here, because inside lie clues to events that shaped our existence.
Moving continents and changing oxygen levels gave the world a decidedly modern configuration by 200 million years ago, except for one major thing: for millions of years, the largest animals were not mammals, as they are today, but dinosaurs and their “reptilian” cousins—mosasaurs, plesiosaurs, crocodiles, and pterosaurs. Land, water, and air were populated by an entirely different world of creatures, all of them successful by every yardstick we can apply: there were numerous species that thrived for millions of years across wide stretches of the globe. Then they disappeared.
The cliff in Nova Scotia from afar (left) and up close at the white band (right). Geologists for scale. (Illustration Credit 7.1)
LOST WORLDS
In 1787, William Smith was hired to assess the financial value of the land within an estate in Somerset, England. He was never to find monetary rewards; Smith’s gold was mined from something else altogether.
Smith set off to survey the rocks that lay exposed along streams, on hills, and inside coal mines. Working in one of the pits of the older mines on the estate, he noticed that the rocks that border the mine were set in layers that he could easily recognize by their colors and textures. On closer inspection, he discovered each layer was made of a particular kind of rock with a distinctive collection of fossils inside. Comparing these layers with others nearby, Smith had a rush of insight: the rock layers in the mine were similar to others at the surface elsewhere on the estate. As he looked closely at the layers, he saw he could use the fossils to match them in different regions, almost like a huge jigsaw puzzle. Natural philosophers, even Leonardo da Vinci, noted that it is possible to do this kind of comparison of rocks, fossils, and layers locally. Now, armed with this simple insight, William Smith had the key to map the geology of Earth: rocks and fossils arranged in layers.
William Smith (top), John Phillips (bottom), Smith’s map of England (right). (Illustration Credit 7.2)
Smith widened his hunt, first looking at the area around Bath, then ultimately broadening his aspirations to cover all of Britain. This new task required money, and with neither an academic post nor the auspices of any scientific society Smith was strapped for cash. He convinced about one hundred patrons to fund his effort and set off to visit every rock exposure he could. He had expert help: his nephew John Phillips had been his ward since the death of both of his parents, when Phillips was seven years old, and he accompanied his uncle on his excursions. By the age of fifteen Phillips had gained a phenomenal eye for fossils.
Today we use aerial photographs and GPS-driven survey equipment to construct geological maps, relying on comparing rocks fortuitously exposed at the surface and in deeper levels of bedrock brought up by drill cores inside Earth. This is big science, often heavily financed by oil companies, mineral interests, and governments. Geological maps are the seed corn of research on Earth: everything we do on expedition starts here. In 1815, Smith accomplished this feat largely on his own using tools of his own design. When finished, the map was a triumph. Standing seven feet high, it revealed the relative position of major layers and fossil eras throughout Britain.
Unfortunately for Smith, however, George Bellas Greenough was a leading light in the London Geological Society at the time. Without Greenough’s support, Smith’s map could not gain the kind of professional traction needed to sell enough copies to pay his debts. Not only did Smith fail to get Greenough’s endorsement, but Greenough set off to produce his own map. And, piling the frustrations on his rival, Greenough made sure his map was cheaper than Smith’s.
Smith’s map was such a sales disast
er that he ended up spending eleven weeks in debtors’ prison, returning to find his property seized. He had hoped to keep the fossil collection he made with his nephew but had to sell it to pay his debts. The situation went from bad to worse; about this time his wife went insane and had to be institutionalized.
Despite these setbacks, Smith’s legacies are many. He confirmed that the fossils in rock layers change from the deepest ones, the oldest, to the highest and youngest ones. He revealed how fossils can be used as markers to trace the same layer across a wide area. And, importantly, he gave his nephew John Phillips an eye for fossils and geological layers.
If his uncle was an antiestablishment symbol troubled by an unfortunate marriage, then Phillips was the opposite: an established Oxford don who lived with his sister for all of his adult life. Phillips devoted himself to his uncle’s layers: his uncle recognized them, but Phillips was determined to find their meaning.
Work with his uncle gave Phillips the keen eye and fastidious technique that allowed him to assemble a prodigious and well-curated collection of shells, bones, and fossil impressions. Starting with his uncle’s map, he traced every known fossil from every layer and asked what happens at the transition between each layer.
Phillips saw three eras of time, each with its own world of fossils inside. The differences between these lost worlds were defined by a sharp boundary where creatures simply disappeared, only to be quickly replaced by new forms of life. Phillips saw these as three major divisions of geological time and named three geological eras based on them: Paleozoic, Mesozoic, and Cenozoic. He published his findings in 1855, and if you want to know how significant his work is, just go to any museum today. You will find his three great eras plastered on the time charts adjacent to the dinosaurs, sharks, and trilobites.
