by Neil Shubin
The lessons of the moon rocks are clear—the minerals on the moon formed at the same orbital distance from the sun as Earth and then suffered some kind of blast. What do these facts tell us of the origin of the moon?
The current theory for the formation of the moon envisions something like a cosmic demolition derby. In these automotive mosh pits, common at fairgrounds in the 1970s, cars intentionally smashed into one another, with the last car running being the winner. Along the way, cars would slam into each other with wild abandon. The most violent of these collisions would eject the light outer layers of the cars, hubcaps and bumpers, leaving the inner ones hopelessly entangled.
This type of collision offers an insight into how the Earth-moon system came about. Over 4.5 billion years ago, a large, perhaps Mars-sized, asteroid is thought to have collided with the forming Earth. Much like the twisted mélange of car parts in a demolition derby crash, the collision ejected lighter parts of each body while the heavier pieces fused. The lighter debris, consisting of dust and smaller particles, now depleted of volatile elements, began to orbit Earth as a disk. Over time, this debris disk coalesced as the moon. The cores of the two bodies did not propel into space but liquefied under the great heat of the impact, only later to cool and solidify as the new core of Earth. In addition, the impact so whacked Earth that it left a 23.5-degree tilt in its axis of rotation.
Initially, there were two large bodies in the same orbit of the sun. Then they collided, forming what we know as Earth and moon today. Ever since that impact, the two bodies have been locked in an orbital dance—Earth and moon exert gravitational pull on each other, while the laws of physics and momentum tie the speed of the spinning of Earth to the rotation of the moon. The impact on our lives is as straightforward as it is profound: the length of days and of months, like the workings of the seasons, derive from the Earth-moon system. Every clock and calendar, like the cells of our bodies, holds artifacts of a cataclysm that took place over 4.5 billion years ago.
The big whack. The origin of the moon.
KEEPING TIME
The Romans had an effective way of controlling troublesome officials in the far-flung regions of their empire. Instead of gerrymandering districts to stay in power—to help friends and get rid of foes—Caesar and his cronies found the ultimate way to retain control. They gerrymandered the calendar. Have a political friend in one region? Add a few extra days to his term. Want to get rid of a foe in another place? Lop days of his rule off the year. This was wonderfully effective; however, over time, not only did the decentralized calendar make ruling difficult, but the year became a patchwork of political kludges, fixes, and compromises.
The nature of Earth’s rotations in space makes it ripe for these kinds of abuses. We all learn this material in school, but most of us forget the meaning of the planet’s rotations by the time we are in college. A recent survey of Harvard undergraduates asked the simple question: What causes the seasons? Over 90 percent of them got the answer totally wrong. The answer has nothing to do with the amount of light that hits Earth during winter and summer, nor with Earth rocking back and forth, nor with the planet getting closer to the sun over the course of the year.
As we’ve known since the days of Copernicus and his contemporaries, the moon rotates around Earth, while Earth retains its constant 23.5-degree tilt as it rotates around the sun. The angle that sunlight hits the planet changes at different parts of the orbit. Direct light generates the long days and heat of summer; tilted and less direct light gives us shorter and colder winter days. The seasons aren’t generated by Earth rocking back and forth; they derive from the planet having a constant tilt as it rotates around the sun.
Because of the different orbits that affect our lives—ours around the sun and the moon around us—there are choices to make when constructing a calendar. Of course, the length of a year is based on the rotation of Earth around the sun. If we know the longest and shortest days, we can carve up the year into months based on the seasons. Another way to do this is to base the calendar on the position of the moon as it goes from full to partial to new every twenty-nine days. The problem is that you can’t synchronize a lunar calendar with a seasonal, or solar, one. The number of lunar cycles does not correspond easily to the number of seasonal ones.
So what do we do? We add fudge factors. Julius Caesar’s calendar had a leap year every three years to keep the months in line with the seasons. The problem with this calendar for the Catholic Church was the extent to which the date of Easter wandered. To rectify this situation, Pope Gregory XIII initiated a new calendar in 1582. Italy, Spain, and a few other countries launched it immediately following the papal bull, resetting October 4, 1582, to October 15, 1582, losing eleven days. Other countries followed to different degrees. Britain and the colonies, for example, only accepted it in 1752. One of the most important issues to iron out, naturally, was when to collect taxes.
Years, months, and days can, at least in theory, be based on celestial realities, but minutes and seconds are mostly conventions. Our calendar has seven days because of the biblical story of a six-day creation, followed by a day of rest. Minutes and seconds are in units of 60 due to a matter of convenience. The ancient Babylonians had a number system based on 60. It turns out that 60 is a wonderful number because it is divisible by 1, 2, 3, 4, 5, and 6.
Humans are a timekeeping species, and much of our history can be traced to the ways we parse the moments of our lives. These intervals are based as much on astronomical cycles as on our needs, desires, and the ways we interact with one another. When the necessities of shelter, hunting, and survival were highly dependent on days and seasons, humans used timepieces derived from the sun, moon, and stars. Other early timepieces relied on gravity, with hourglasses that used sand or water clocks such as those first seen in Egypt in 4000 B.C. Our need to keep time has itself evolved; an ever-increasing necessity to fragment time corresponds to the demands of our society, commerce, and travel. The concept of moments parsed into seconds would have been as alien to our cave-dwelling ancestors as seeing a jet plane.
There are clocks in our world that do not rely on convention, political choice, or economic necessity. The DNA in our bodies can serve as a kind of timepiece. Averaged over long periods of time, changes to some parts of the DNA sequence happen at a relatively constant rate. This means that if you compare the DNA structure of two species, you can estimate how long ago they shared an ancestor, because the more different the strands of DNA from two species are, the more time they have changed separately. As we’ve seen with zircons, atoms in rocks also tell time. Knowing the ratio of different versions of the elements uranium, argon, and lead can tell us how long ago the minerals in the rock crystallized.
The different clocks in bodies and in rocks don’t tick independently; they are part of the same planetary and solar metronome. Comparisons of the DNA inside humans, animals, and bacteria speak of a common ancestor of all three that lived over 3 billion years ago. This is roughly the age of the earliest fossil-containing rocks. The broad match of dates from rocks and DNA is all the more remarkable given how the rocks have been heated and heaved over the same billions of years that DNA has mutated, evolved, and been swapped among species. Agreement between these different kinds of natural clocks leads to confidence in our hypotheses. On the other hand, discordance between the clocks in DNA and those in rocks can also be the source of new predictions. Whale origins are a case in point. With some of the largest species on the planet, blowholes in the middle of their heads, ears specialized for a form of sonar, and odd limbs, backs, and tails, whales are among the most extreme animals on Earth. Yet, as observers have known for centuries, their closest relatives are mammals: they have hair, mammary glands, and innumerable other mammalian affinities. But which mammals are their closest relatives, and when did whales enter the seas? Comparison of the DNA of whales with that of other mammals revealed that whales likely diverged from odd-toed ungulates such as hippos and deer. The differences in the
genes and proteins implied that the split happened nearly 55 million years ago. But this created a whole new puzzle for paleontologists. Not only were there no fossils that showed transitional organs in the shift; there was nothing that ancient with whalelike features in the fossil record. The gap served as a challenge. Vigorous paleontological exploration brought confirmation: the discovery of whale skeletons with ankle bones similar to those of hippos and their relatives inside rocks over 50 million years old. And it all happened by relating the different clocks in rocks and DNA.
Rocks and bodies contain more than clocks: they also hold calendars. Slice a coral, and you will find that the walls of the skeleton are layered with light and dark bands. As they grow, corals add layers of mineral to their skeletons, almost like slapping plaster on a wall. The ways that the mineral forms depend on sunlight, so the variation in the layers reflects the waxing and waning of each passing day. Mineral growth is fastest in summer, when the days are long, and slowest in winter, when the days are short. Consequently, bands deposited in summer months will be thicker than those at other times of the year. Count the number of layers embedded in each cycle of thick and thin layers, and what do you find? Three hundred sixty-five of them. Coral skeletons can be an almanac of days of the year.
The beauty of corals lies not only in the reefs that reveal the splendor of the underwater world but in the insights they give us into our past. Crack rocks along the sides of roads in Iowa, Texas, even north into Canada, and you will see coral reefs that once thrived in ancient seas hundreds of millions of years ago. The city of Chicago is built upon an ancient coral reef. And reefs like these tell the story of how time itself has changed. Go to fossil reefs 400 million years old, and you will find four hundred layers inside the corals—suggesting that each year was actually four hundred days long and contained a whopping thirty-five more days than our current year. What accounts for this discrepancy? Since the duration of a year is fixed by Earth’s rotation about the sun, the days must have been shorter 400 million years ago than they are today. To make the algebra work, each day had to have been twenty-two hours in length. In the eons since those corals were formed, two hours have been added to every day.
Like a slowing top, Earth spins slower and slower with each passing moment, making days longer now than in the past. As the planet rotates, the water in the oceans moves about and serves to brake the spin of the planet. That is why today is two milliseconds longer than yesterday.
Fossil corals are silent witnesses to the lengthening of days. Clocks and calendars abound in the natural world, sometimes in the most surprising places.
IT IS IN YOUR HEAD
In the rush of pitching my tent, I inadvertently left a hummock of tundra under the floor. With a mound in the center and slick nylon surfaces inside, my sleeping bag slid to one corner each time I drifted to sleep. After a frustrating few hours of writhing like a pupa in a cocoon, I became determined to find a flat surface, and in a fit of fatigue and desperation I jury-rigged one by contorting myself over heaps of clothes, books, and field gear. It was a good thing we expended a lot of energy that first day setting up camp; my exhaustion led me to a reasonable facsimile of sleep.
I arose to the bright morning sun and dressed quickly, not wanting to hold the team back. Today would be our first day in Greenland looking for fossils, and the excitement made me surprisingly alert despite the fitful rest.
I made my way to the kitchen tent, my first goal being to get the coffee going. Our field gear was packed so tightly for the trip north that simply finding the breakfast containers was no small task. After about ten minutes of fumbling with the packing lists and crates, I broke out some cooking supplies and got the java brewing.
Life was good. It was a clear, bright Arctic summer morning. The dry air made images incredibly sharp; features in the distance looked as if they were right next door even though they were miles away. Warming my fingers against the coffee mug, and relishing the stillness, I walked in my mind through the different hills I was going to hike that day.
After a few cups and about twenty minutes of savoring the calm, I realized something was wrong. The world was still, a bit too much so. With each passing minute of silence, I began to feel more alone.
A glance at the clock revealed the cause for my solitude: it was 2:00 a.m. Yet here I sat, fully dressed, primed for a whole new day, and bristling with energy. I felt like a total chump, albeit a well-caffeinated one. Returning to sleep was an impossibility, so I broke out a novel I was saving for a snowy day and struggled to read for the next few hours until my companions arose.
It was the light, of course. The walls of my tent did not block it out, leaving the inside illuminated at all hours. My brain, acclimated to the southern world, was completely in tune with the equation “light equals day and dark equals night.” Because that simple relationship was lacking in the twenty-four-hour daylight of Arctic summer, my brain’s usual cues were utterly useless. My sleeping colleagues, old hands at fieldwork, prepared by bringing eyeshades, while all I had was a flashlight.
Those first few days were a real jangle. I felt off-kilter, as if the insides of my body were struggling to keep up with a whole new planet. Think of a major case of jet lag, but without any night whatsoever, the only reference point comes from a clock. The longer I dwelled in the landscape, though, the more my brain became attuned to it. The sun traces a large ellipse through the sky, casting different shadows throughout the day. Almost without thinking, the brain begins to make a sundial out of any standing object. Of course, in the high Arctic we lack trees; any large rock or tent ends up doing the job.
From jet travel we all know that our sleeping and wakefulness are matched to the sun. Virtually every part of us—every organ, tissue, and cell inside—is set to a rhythm of day and night. Kidneys slow down at night. That’s a wonderful trait if you want to minimize trips outside bed—something very useful when inside a sleeping bag in the Arctic. Body temperatures vary over the course of the day, with the coolest ones happening at 3:00 a.m. Liver function is time dependent as well: the human liver works slowest in the morning hours, meaning the cheapest dates would be at breakfast.
Our bodies respond to more than days; they also are tied to seasons. The changes from winter to summer bring new patterns of light, temperature, and rainfall. Animals are tied to these in the ways they feed and reproduce, and humans are no different. Even our moods relate to the season. By some estimates 1.4 percent of Floridians suffer from seasonal affective disorder, compared with 14 percent of New Hampshire residents.
Drunks see time flying by, with the party just getting going as everybody leaves. Cannabis brings an eternity to a twenty-minute episode of The Three Stooges. Intense concentration or emotions make us lose track of time. Even the proverbial “watched pot” that never boils is a statement about how our perception of time is sometimes at odds with the clock itself.
In 1963, a young French geologist had a plan to change the way we think about time. By the age of twenty-three, Michel Siffre had visited some of the largest unexplored patches of Earth. These were underground, and by mapping the world below, Siffre revealed vast caverns and glaciers inside the Alps. The subterranean landscape is a beautiful and dark world, and in this void Siffre was inspired to ask a whole new question.
What happens when people completely disconnect from the clock? Each of us is a slave to it: we chop our days into little moments and plan our lives around them. Not only do we live in a world defined by natural time—the dark of night and the light of day, the warmth of summer and the cold of winter—but we have inserted man-made inventions into this equation. Beepers, buzzers, and alarms tether us to each passing moment. What happens when we completely cut the cord that binds us to these stimuli?
Siffre intended to be his own lab rat and concocted a plan to live for two months in a cavern two hundred feet belowground, completely removed from normal human existence. He would bring food, a sleeping cot, and artificial light, but—and thi
s is the important point—no timepiece or anything that could even indirectly give a clue as to time. Siffre’s only connection to the outside world was a telephone with which he called his friends on the surface to inform them of the times he spent awake and asleep. The plan was for a sixty-day disconnect from the normal light-dark cycles of our world and from our clocks that are based on them.
A meticulous note taker, Siffre dutifully recorded each passing day on a calendar with his bodily functions and mental states. His diaries record his daily movements, his body temperature, his mood, and his libido.
On the thirty-seventh day of his records, with twenty-three days yet to go, Siffre was on the phone with a colleague from above. Pierre, one of his chums, asked, “How much time in advance do you want to be warned that your experiment is about to end?”
“At least two days to gather up my things.”
“Start getting ready,” Pierre responded. The experiment was over. By relying solely on his mental clock, Siffre had lost twenty-three days.
What happened?
One answer lay inside Siffre’s diaries. Having recorded when he woke and went to sleep, he called his friends when he was able so that they could log the real time for him. But lacking a watch or alarm clock, he had no idea how long each interval of sleep really was. What he perceived as brief ten-minute catnaps were in reality eight-hour slumbers.
His misperception of time ran deep. At one point in the experiment, Siffre called his friends to see if he could mark off two minutes simply by counting. Most of us can pace this off roughly, within ten seconds or so. Siffre began counting from 1 to 120, in an attempt to march off the seconds in two minutes. This simple task took him five minutes.
