The Universe Within (7 page)

Tiny grains tell the story of the solar system: it got its start over 4.6 billion years ago, and by at least 4.1 billion years ago liquid water, so essential for life, was already on Earth.

We may live on the “blue planet”—unique in the known universe for its abundance of liquid water—but our bodies’ ocean lies on the inside. Adult
humans are about 57 percent water by weight. Our body dries out with every passing year; newborns are about 75 percent water, not much different from an average potato. Most of the body’s water is not in the fluid of our blood, but remains locked inside the cells of our muscles, brains, and hearts.
Metabolism of food and oxygen depends on water, as do the growth and communication of our cells. Even
reproduction, with the motility of sperm and egg, is based in a fluid medium. Virtually every
chemical reaction in our bodies depends in some way on the presence of water.

We are tied to water for more than our present lives: our bodies contain the history of water itself. The first 2.7 billion years of our history was entirely in water, and the imprint is in every organ system of our bodies. The fundamental organization of our
head is based on a series of swellings that develop into the bones of our jaws, ears, and throats as well as the muscles, nerves, and arteries that supply all of them. Equivalent structures are seen in everything with a head, including fish and sharks. In these creatures, the bones develop into the structures that support and supply the gills. In a sense, the muscles, nerves, and bones that we use to talk, chew, and hear correspond to the
gill bones of our fishy ancestors. This deep tie to gills is also seen in fossils, where we can follow the transformation of gill bones to structures deep within our own heads, including our ear bones.

While most of our past lay inside the water, the most recent 300 million years has been defined by our separation from it. Our
kidneys have developed specializations to help balance the water and salts inside the body in the face of life
on dry land. Our
reproduction doesn’t depend as much on water as it did for our ancestors: sperm and egg are fertilized inside the body, and the developing fetus is shielded from the outside world by membranes and vessels that protect it and attach it to the mother. Our hands and legs, structures so adept at supporting life on land, are modified fish fins. Our terrestrial existence comes about through repurposed organs that fish use to live in water.

The human kidney, like that of other mammals, is a magnificent adaptation to life on land; kidneys help kangaroo rats and antelope live in dry deserts, surviving only on the water locked inside the molecules of their food. Yet deep within this most unique of terrestrial organs lie roots of its aquatic origins. All
jawless fish—ones we share a
common ancestor with over 500 million years ago—have a very primitive kind of kidney: tissues that run the length of the body, take fluid wastes from the bloodstream, and dump them directly into the body cavity,
ultimately allowing for excretion from an opening at the tail.
Bony fish, which share a
common ancestor with us 450 million years ago, have a more specialized arrangement in which these clumps of tissue connect to a plumbing system that carries wastes outside the body. The most recent of these kidneys, the system that mammals use, doesn’t run the length of the body but sits at the level of the lower back.

During our time in the womb, we form three different kinds of kidneys, one after the other. The first kidneys are clumps of tissue that line the body and open to the body cavity, much like those seen in jawless fish. The second, like those of bony fish, run the length of the back to a common plumbing system. The adult kidney, which appears at the end of the first trimester, replaces both of these. In our first three months, we track our fishy past.

Life’s connection with
water is no accident; the
water molecule itself has special properties. With one oxygen and two hydrogen
atoms, it looks something like a Mickey Mouse head: small hydrogen atoms form the ears atop a head made by a large
oxygen atom. This whole molecule is polarized, with a negative charge at one end, where the oxygen resides, and positive charges at the opposite end, corresponding to the hydrogens. This arrangement makes water the ideal medium in which to dissolve a large variety of substances. Salts, proteins, amino acids—so many compounds can be incorporated into water that it provides the matrix for the
chemical reactions on which life depends. No longer dependent on the vagaries of the water outside our bodies for our metabolic processes, we maintain that stable watery environment inside us.

Water has another property, one seen in a kitchen: it can exist as a liquid, solid, and gas within a relatively narrow range of temperatures and pressures. We have so many different kinds of interactions with water because it occurs on the planet as solid
ice, gas in the air, and the fluids that are the substrate for living processes. Over 97 percent of the planet’s
water lies in the oceans, with the rest in the clouds, ice, and freshwater, and each of these forms is vital to our existence and that of the planet.

Just as water is the matrix for the chemical processes that run inside our bodies, so too is it for the metabolism of the planet. Water raining from the sky and from melting ice erodes rock on land and, as it flows from high to low elevations, returns minerals to the sea. This gradual weathering provides the counterpoint to the uplift of mountains and plateaus over
geological time. Molecules in the air, many of which impact climate and atmosphere, are continually recycled between rock and sea by the action of water. Water provides the links that define a livable Earth.

The
water inside our bodies and in the oceans also tells of its
origins. Being two parts hydrogen and one part
oxygen, water can be thought of as a two-to-one ratio of atomic nuclei derived from the
big bang to those derived from
fusion reactions inside stars. While their constituent atoms have a history that extends across the universe,
water molecules themselves are linked to the solar system. The
chemistry of the water in Earth’s oceans, particularly the mix between different kinds of hydrogen atoms, is distinctive and can be compared to the ice
in comets,
asteroids, and other
planets. Probes sampling water in the ice of the comet
Hale-Bopp, which passed by Earth in 1997, revealed differences between Hale-Bopp’s
water and that of Earth. This discovery was a huge disappointment to many because the reigning dogma in the 1990s was that comets were the likely source of Earth’s water. Fans of the cometary hypothesis were in for a treat in 2011, when newer probes sent to other comets, such as
Hartley 2, revealed water with very oceanlike proportions of atoms. The story of water is more complex than simply comets, because the more we look across the solar system, the more water we find. With powerful
telescopes and ever-newer
satellites, we have seen water turn up on the moon and within asteroids. There are even hints of water in the most unlikely places imaginable. Mercury is the closest planet to the
sun; its surface reaches temperatures of 800 degrees Fahrenheit, hot enough to melt lead. NASA’s
MESSENGER satellite, sent to Mercury in 2004, captured photos of structures that have the distinctive reflective properties of ice deep within craters at the poles of the planet. Water may survive there because the craters of Mercury, shielded from the sun and on a planet with no atmosphere, are likely very cold. With so much water across the solar system, it seems likely that some water arrived here from space, but it is also possible that some came from the rocks of the forming Earth itself. When rock is super
heated, as was the likely condition 4.5 billion years ago, it can vaporize and release
water molecules trapped inside its molecular structure. Whether originating from the ice of comets or vaporized from the rocky debris of the early solar system, or both, each glass of water we drink is derived from sources at least as old as the solar system itself. And, as
zircons tell us, water has been here on Earth in liquid form since at least 4 billion years ago.

Our history has been shaped by water, our existence made possible by it, and our future likely defined by our relationship to it.
Events far and wide have conspired to define our
watery existence and with it, the fundamental structure of our bodies.

Exhausted following a three-day meeting in California, I slumped on a sofa in a hotel lobby waiting for an airport shuttle bus. Seated across from me was an eminent colleague, his face partially hidden by the computer he had open on his lap. His facial expressions drew my attention. He was staring at his laptop, alternately laughing to himself and shaking his head in disbelief. I felt guilty watching him, so I tried staring at my bags to avert my gaze. Noticing my failed attempts to be discreet, he kindly beckoned me over to look at his computer. On the screen was a cliff face with a surface I had seen many times before. The way the layers crisscrossed is characteristic of rocks that formed in ancient dunes. I was familiar with this pattern, having seen it during
fossil-hunting trips to
Canada and
Africa. I had even found fossils in this kind of rock. The rock beckoned;
paleontologists dream of these kinds of geological exposures. But these photos were not from Earth. My colleague was part of the scientific team analyzing images returned from one of the
Mars rovers,
Spirit
, and the computer images had just beamed back to Earth the day before.

In the 1988 movie
Twins
, the character played by
Arnold Schwarzenegger is a virtual superman who goes looking for his long-lost brother. He ultimately finds his twin in a character played by
Danny DeVito, a short, ungifted brother with a criminal past. They were born of the same mother, but an accident of fate left one brother with the gifts, the other with considerably less. Arnold’s character, after meeting his brother, learns much about himself. So too can we learn when we look to neighbors in the solar system—
Venus, Mars, and
Jupiter—for insights into
our planet and even the makings of our bodies. We have Arnold in our past and Danny in our future.