The Universe Within (18 page)

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.

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.

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.

J
ust 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.