The Universe Within (14 page)

Galileo envisioned that the gravitational pull defining the orbits of celestial bodies also has an effect on animal and plant organs. Bodies are pulled to Earth to a degree that is proportional to their
mass. Heavier creatures, being pulled relatively more, need to change their shape to support themselves. This relationship even explains
Darlington’s rooftop experiment with frogs. Lighter animals accelerate less during a fall than do big ones for these same reasons. The force of gravity can mean the difference between life and death for large creatures like us.

Gravity is not a significant factor for creatures that dwell in
van Leeuwenhoek’s microscopic world. Look no further than at a fly or an ant on a wall. For the fly, the gravitational pull of Earth on its
body is negligible; the forces that are really important are the ones that bind
molecules together. A fly can stick to a wall because these sticky, and tiny,
molecular forces are proportionally more significant than gravity for a light animal. Imagine a hippo on that wall: gravitational pull would far exceed the pull of the molecules on its feet. No amount of molecular Velcro would work to keep the hippo stuck to the side of a room.

We, relatively big creatures that we are, barely think of these
molecular forces when we go through our day. We swim in water and feel that the water is more viscous than air. But if we were a small creature, say a bug no longer than a quarter of an inch, these forces would dominate our existence, and swimming in water would feel like swimming in Jell-O. The surface of the water would take on new meaning. At our weight, we can dive right through to the bottom of a pool. Since a number of these molecular forces are at work on the surface of fluids, a bug can walk on it.

In 1968,
F. W. Went wrote a classic paper, published in
American Scientist
, that explored the consequences of
size for humans. His starting point was the seemingly absurd question: Could an ant begin a workday like a person? Understand that Went was not a crank; the discoverer of a critical plant hormone, member of the august National Academy of Sciences, on the faculty of Caltech, and, later, director of the Missouri Botanical Garden, he was an eminent Renaissance man of science. In the obvious “no” to Went’s ant question lie a number of profound biological truths. Went reasoned that an ant shower, of course, would be out of the question. The droplets of water, being bound by those molecular forces, would hit the ant’s body like cannonballs. A morning cigarette (the essay was written before the Surgeon General’s report had a major impact on our behavior) would also be impossible. The smallest effective size for a controlled fire is about that of the ant’s body. Saying good-bye to the spouse and kids would be different too. The ability to hear deep tones—slow vibrations in air—is possible only at larger size. Also dependent on size is the ability to hold a job in the first place. The development of brain capacity for thinking, forethought, and memory requires a certain body girth. The ant story makes one point abundantly clear: many of our abilities, such as talking, using tools, designing machines, controlling fire, and so on, are possible only because of our size. Size defines the opportunities of our species.

Our ancestors gained new possibilities when they made the
shift from
van Leeuwenhoek’s
microscopic world to the Galilean one over a billion years ago: they left a world dominated by intermolecular forces and entered one more influenced by gravity. This great moment of our past is written inside our cells, deep within the rocks, and in the ways many of us die.

Impressions of disks, ribbons, and fronds in slabs of 600-million-year-old rocks are a pretty unremarkable bunch of fossils. But looks are deceiving. These fossils reflect revolutionaries, a new kind of individual that the world had not yet seen. These are the first creatures with bodies composed of many cells.

The advent of bodies changed the planet forever. A
single cell is restricted in size because molecules can only diffuse over short distances, a measurement set by the laws of
physics. This limitation affects how an
animal can feed, respire, even reproduce. Small animals can transport
oxygen across their bodies by simple
diffusion. Once animals get large, they need new mechanisms to move nutrients and wastes about. How do they deal with this? Large animals have specialized systems to circulate blood, carry wastes, and capture and pump oxygen to their far-flung cells. These kinds of specialized organs are game changers in the world of size. Hearts, gills, and lungs are all inventions necessary in large animals. This complex machinery brings the opportunity to get even bigger and realize a world of new capabilities, as our ant would have experienced when starting its hypothetical workday.

Bodies may make a dramatic appearance in the fossil record, but if the
genomes of living creatures hold any clues, changes were under way for a long time. The first 2.5 billion years of the history of the planet were entirely devoid of big creatures; then, by about 1 billion years ago, there were not one but several
different
species with bodies populating the ancient seas: plant bodies, fungal bodies, and animal bodies, among others. The origin of bodies wasn’t some magical event. The molecular tool kit that makes bodies and their organ systems possible—the proteins, large lipids, and other large molecules that allow cells to stick together and communicate—is not unique to creatures with bodies. Antecedents are present in small
single-celled creatures that use versions of these same molecules to feed, to move about, even to communicate with one another. The biological mechanisms needed to build big creatures existed on the planet for billions of years before those creatures ever hit the scene.

What opened the floodgates and turned this potential for big creatures into a tangible reality? Insights, yet again, come from
iron-ore-containing rocks.

Standing a wiry five feet six inches tall, Preston Ercelle
Cloud Jr. (1912–1991) was one of the most imposing figures in all of
paleontology in the postwar decades. Graduating from high school with a lust for travel and the outdoors, he entered the navy for three years, where he became the bantamweight boxing champion of the Pacific Scouting Force. Cloud put himself through school during the Great Depression, eventually rising through the academic ranks to become chief paleontologist for the U.S. Geological Survey. He was a stickler for detail in his fieldwork and commanded respect from his staff. When mapping rocks, he would often crawl on them, putting his eyes inches from the layers. During one of these sessions, he was slithering through juniper thickets in Texas and came face-to-face with a large rattlesnake. As his field colleague at the time said, “Pres was not easily bluffed: after a few minutes of staring at each other, the rattlesnake crawled away.”

Cloud had a talent for using close-up encounters with rock layers to see the global picture. In his eyes, the planet was one
big interlinked system: the history of life and the workings of climate, oceans, and continents formed a single unified narrative. If the search for
iron led to the discovery of early living creatures, then the iron itself led to understanding their links to the planet.

Iron-rich layers begin to appear in
rocks about 2 billion years old on every continent. No matter whether on Australia,
North America, or
Africa, they generally form the same series of precisely layered reddish-brown bands. As anyone who has left wet tools in the garage knows, the color is a clue to the iron’s chemistry. Oxygen in the air turns iron reddish brown as it
rusts.

This kind of rust is absent from the
oldest rocks in the planet. When Earth formed over 4.5 billion years ago, the only major
source of atmospheric gases was Earth itself.
Volcanoes spew all kinds of molecules but precious little oxygen. We’d have an easier time breathing on the top of Mount Everest than on this
ancient Earth. The bands of rust in mor recent rocks reveal the change: a global increase of oxygen in the atmosphere.

The oxygen in the atmosphere exists in a balance between the entities that produce it and those that consume it. Like a bathtub faucet running with a partially open drain, the level in the tub is the outcome of the rates of inflow and outflow.

Clues to the inflow of oxygen in the ancient atmosphere come from ancient life itself. Most of the
single-celled creatures found in the oxygen-poor world had one very important thing in common. Judging by their closest living relatives, they used
photosynthesis to make energy from the chemicals around them. Photosynthetic creatures use energy from the
sun to make usable energy for their bodies. They do not use oxygen; they produce it. The only possible source for the oxygen that changed Earth
is one that we see today—photosynthetic creatures. Today that bunch of organisms includes microbes and plants, but billions of years ago there was one main suspect. We see it inside the
rocks that
Tyler and
Barghoorn peered at under their microscopes. With cell walls and distinctive colonies that take a range of forms—small clumps to toadstool-shaped masses—they look much like modern
algae. Algae, quietly producing oxygen for hundreds of millions of years, gave breath to life on Earth.

If algae were the
source of oxygen in the ancient atmosphere, what happened to the sinks that served to remove it? Some molecules in the atmosphere can remove free oxygen from the air by binding with it to make new compounds.
Volcanoes under the sea, for example, belch gases that are derived from the melting of the crust at the ocean bottom. These gases have a special feature: the molecules inside bind to oxygen, removing it from the air.
There are a number of ideas to explain the rise of
oxygen in the atmosphere. One hypothesis is that around 2 billion years ago, when oxygen became prominent, there was great geographic change: a reduction in the amount of oceans and the number of undersea volcanoes emitting gases that would remove oxygen from the air. With algae pumping oxygen and fewer mechanisms to consume it, the oxygen in the atmosphere increased.

Cloud put the different observations—bands of
iron, algae pumping oxygen, and the origin of big creatures—all together. The inspiration for his synthesis lies in the structure of the
oxygen atom itself. Oxygen is an atom that is greedy for
electrons because it lacks two of them in its outer shell. The powerhouses of our
cells—
mitochondria—along with aerobic
bacteria make use of this fact in their
energy processing. Some
metabolic reactions, such as respiration, have elaborate cascades that transfer electrons from one molecule to the next, where, at each electron-transfer step, energy is either stored in new forms or released. The more free oxygen there is about, the more fuel there is for living creatures to use.

Cloud also knew that being big costs energy. The
proteins, such as collagen, that form much of our bodies take a relatively large amount of energy to build and maintain. The growth and maintenance of a body require a new kind of higher-level energetics.