The Universe Within (12 page)

Similar clock DNA not only leads us to our deep connection to other species but also reveals a fundamental machine inside the
cells of our body. The same kind of genetic clock is within each of them, from
Richter’s patch and the skin at the tip of your
finger to those of the liver and brain. If you have a sleep disorder that has a genetic basis, the doctor can diagnose it from a scratch of skin, drop of blood, or swab of your cheek. Every cell ticks a daily rhythm with a molecular
clock that has parts at least as ancient as animals themselves.

What sets the molecular clock inside our cells and aligns it to our days? A travel alarm clock might keep a twenty-four-hour rhythm, but it has no way of knowing what
time zone it is in; it needs something to set it to where it is on the planet. Why do we experience
jet lag or, in my case, rise at 2:00 a.m. in the Arctic summer?

Our cellular clock is tuned to the outside world by a number of triggers, the most important of which is
light. Most of the light that enters our eyes ends up as a signal to the parts of our brain that interpret visual information. Some of these signals, though, get sent to a different part of the brain—to
Richter’s patch of cells. The path from Richter’s patch travels to a little pea-sized gland at the base of your brain called the
pineal. To some, including the great French philosopher
René Descartes, the pineal is the seat of the soul. In some lizards and fish it actually forms a kind of third eye that records light information directly. In us, it is like a relay center for information. It emits the molecule
melatonin, which triggers responses all over the body.

This reaction—from light to brain to the
pineal gland to melatonin and its targets across the body—tunes our bodies to the light of the day and the darkness of night. When we travel to a different time zone, this pathway eventually resets us to a new regime of light and dark.

Shine bright light on somebody’s eyes in the middle of the day and what happens? Usually nothing at all beyond the usual adjustments of his or her visual apparatus. Shine bright light in people’s eyes at dusk, and you can affect sleep. People hit with
light at dusk tend to become tired later than normal. The opposite is also often true: shine bright light on people at dawn, and their
sleep cycle will be shifted earlier than normal. Our sleep cycle is dependent on light shined on us when our
brains expect it to be dark. In a world of gadgets that blare
artificial light into our eyes at all hours of the day, we are resetting our
clocks with each text or e-mail sent in the middle of the night. We live in an age of disconnect between the ancient rhythms inside us and our modern life.

Much of our health depends on clocks: shift workers who sleep during the day and work at night have higher rates of heart disease and some cancers, notably of the breast. Researchers
studying mice discovered that the
error-correction machinery of the DNA of skin cells functions on a clock: it’s most active in the evening. The DNA that gets copied in the morning is likely to carry the most errors. The UV radiation of the
sun causes cancers in the skin when it induces errors in copying DNA. Putting these facts together leads to the conclusion that in mice
UV light hitting the skin in the morning is more carcinogenic than evening and afternoon exposure. Humans also have these clocks, but ours are reversed relative to mice: our DNA error-correction apparatus is most active in the morning. This means tanning at the end of the day is more carcinogenic than doing so in the morning. Even our
metabolism is affected by the clocks inside our cells: some kinds of
obesity can be correlated to a lack of sleep.

Given that mechanisms of DNA function and cell division are dependent on internal clocks, it should come as no surprise that a number of medicines are most effective at certain times of the day, when our brain anticipates the level of light. Our susceptibility to disease, and our treatment of it, carries the deep signal of a planetary cataclysm that happened over 4.5 billion years ago.

Dotting the landscape of southern Indiana are cemeteries that house the
graves of Europeans who settled in the region in the late eighteenth century. This was a hearty crowd whose harsh existence is recorded on their tombstones. Few lived past the age of forty, and judging by the carved dates, the cemeteries were busy places some years. By an accident of geography, these settlers found a near-perfect material for their headstones. The fine grains and hardness of each stone preserve etchings from the early nineteenth century as sharply today as when they were originally carved.

We are so accustomed to looking at the front of tombstones that it is easy to overlook other edges that have stories to tell.
The sides of the pioneers’
grave markers are not even; they are composed of a series of ridges with sharp edges and small depressions. The stones were quarried from a pit near
Hindostan, Indiana, from an exposure that reveals how the rock was formed. Hundreds of millions of years ago, this part of Indiana lay under the sea. Year after year, sediment settled from the tidal waters, leaving small ripples in the mud. There is a rhythm to these marks, recording the variations of the
tides throughout the year. As Earth spun and the moon circled it, the water rose and fell, only to be recorded as ridges in the sediment. The sides of the grave markers reveal the tidal rhythm from a moment when Earth rotated faster and the
days were shorter than today. Time is sculpted everywhere on these tombstones, by the work of man and of the planet. The bodies in the graves and the rocks that mark them are united by the history they share with colliding and rotating celestial spheres.

A
s the young sun pulled matter into its orbit over 4.6 billion years ago, lumps of
rock and ice smashed together and combined. The cataclysm that gave us the moon was only one of many. Judging from the ages of the
craters on the moon and
meteorites here on Earth, collisions were the order of the day until about 3.9 billion years ago. Then this violent start gave way to a long period of relative quiescence.

In this quiet Earth lay a scientific puzzle that confounded scientists. The very top, or youngest, layers held
fossils—the different shells and bones that can be seen in any natural history museum today. Lying below were ancient layers of rock with no evidence of anything living inside: no fossil bones, markings
of animals, or plant spores; no evidence of any living thing whatsoever.

This lifeless layer of basement rock wasn’t just a sliver; the barren layers were miles deep. All of human history—and the entire known history of life on the planet itself—were confined to a thin veneer of crust. If the entire history of Earth were scaled to a year, with its formation on New Year’s Day and the present being December 31, Earth was utterly lifeless until mid-November. Translate this relative timescale into years, and we find that about 4 billion years of the history of our planet was devoid of living things. To
Charles Darwin, the abrupt appearance of life was an “inexplicable mystery.”

The solution to Darwin’s mystery, along with clues to how our modern world emerged, came from a source no one could have ever predicted.

The steelworks of Gary, Indiana, stand like hulking
fossil skeletons of a thriving bygone age. In the 1950s, humming mills fed a burgeoning automotive industry, with plants sprinkled across the Midwest. The need for iron was great, and geologists such as
Stanley Tyler worked to feed it by studying the
iron-ore-containing rocks in the region.

Because the rocks that contain iron tend to be among the
oldest rocks around, ore geologists like Tyler focus mostly on the geological basement. As Tyler knew, these rocks were great for industry, lousy for
paleontology.

One afternoon in the mid-1950s, Tyler was studying samples he collected from a deep test pit in northern Michigan. Rock chips from different layers were brought back to his lab in Madison, Wisconsin, where each was ground thin and placed on a slide to observe the fine structure of minerals and grains. Sitting with a checklist and a
microscope, Tyler performed the usual scoring and counting of the color, grain size, and mineral content
that are the necessary but rote measurements of much geological work.

When Tyler looked at one of the slides under the scope, he saw something familiar yet completely out of place:
coal. He knew that coal reflects ancient plant detritus and that most of the coals known at that time were from layers no older than 350 million years, when plant life was abundant. But Tyler also knew the age of his sliver of rock from Michigan. It was almost 2 billion years old. Something was entirely wrong.

Not believing his own eyes, Tyler discreetly passed the rocks to experts. Shuffling from one specialist to the next, the samples eventually ended up in the hands of Elso Barghoorn, a curator of early
plants at the Harvard
Museum of Comparative Zoology. Scanning the slides under a microscope, Barghoorn immediately confirmed Tyler’s hunch. Tyler had found the earliest life yet known on the planet—coal-forming
algae and other microorganisms.