National Geographic, September 2001
Whenever I feel prematurely old and creaky, I look at a rock that sits on a corner of my desk. It is a dark gray hunk of granite-like rock called gneiss, flecked with bits of feldspar. I picked it up off the ground along the Acasta River in Canada’s Northwest Territories, and it’s pretty much like any other piece of gneiss except for one thing: It comes from a formation that dates back more than four billion years—the oldest rock yet found on Earth.
Its age is so vast that it’s almost impossible to comprehend. From our planet’s infancy the atoms that make it up have held together, even as continents have been torn apart and rearranged. If you think of a year as equaling one yard of twine, you’d need enough twine to stretch between the Earth and the moon more than four and a half times to equal the age of the Acasta rock.
How can we possible know this? Nature doesn’t print birth certificates or hammer a year on its creations as if they were coins. Scientists have learned to tell the age of bones, rocks, planets, and stars by using clocks that tick away in the very atoms that form them.
And with these natural chronometers—which they can read with staggering resolution—they can understand the forces that have shaped the continents, life itself, human civilization, the galaxy. No longer can human history match the scale of natural history. If the age of the universe about 13 billion years, were equal to one summer day, then the past 100,000 years—which saw the rise of modern humans, the dawn of agriculture, and all of written history—would fit into the flash of a firefly at sunset.
Scientists can choose among different kinds of natural clocks depending on the scale of time they work with. For the period reaching back 40,000 years or so, they rely on radioactive carbon. By measuring the amount of radioactive carbon in a sample from something that was once alive, they can determine how long it has been dead. For example, archaeologists know that one of the oldest parts of Stonehenge, a ditch that encircles the famous stones, was dug with antlers found at the site. By measuring the carbon in those antlers, they have determined that the digging took place 5,000 years ago.
Where does radioactive carbon come from? All atoms—carbon atoms and atoms of every other element—contain subatomic particles in their nuclei, including positively charged protons and (except for hydrogen) neutrally charged neutrons. Generally atoms of the same element have the same number of protons and neutrons. Carbon, for example, usually has six protons and six neutrons, which added together give this form of carbon its name: carbon 12.
When atoms of the same element have different numbers of neutrons in their nuclei, the atoms are called isotopes. Carbon 12 is one carbon isotope; another is carbon 14, which has eight neutrons and is radioactive. Carbon 14 is formed when particles from space slam into nitrogen atoms in the atmosphere.
Radioactive isotopes decay at a predictable rate, and carbon 14 is no exception. If you bottled up a pound of it, half the bottle would decay in 5,730 years. After another 5,730 years, only a quarter of it would be left. (Physicists call these 5,730-year periods the half-life of carbon 14.) Plants and animals that are alive and absorbing carbon dioxide from the air have constant levels of both carbon 12 and carbon 14. But as soon as they die, the supply of carbon 14, which decays back to nitrogen 14 at a known rate, begins to dwindle. By comparing the carbon 14 level to the total amount of carbon in the material, scientists can calculate how long ago the plant or animal died.
Fossils older than 40,000 years have so little carbon 14 left in them that scientists have had to search for other ways to determine their age. A geologist named Gifford Miller from the University of Colorado showed me around a site at Lake Victoria in southern Australia where he used two new dating techniques to get around the limits of carbon 14.
Lake Victoria is bounded by an enormous crescent of high dunes, piled up over tens of thousands of years. Under swarms of pink-breasted galah cockatoos, Miller and I hiked the rippled sands. Signs of Australia’s history, unburied by scouring winds, were everywhere. We saw rusted shell casings left from Royal Australian Air Force training runs in the 1940s. From deeper layers of the dunes—and farther back in time—came piles of mussel shells that had been collected from the lake by Aborigines. Spearpoints lay nearby along with the bones of kangaroo and emu the Aborigines hunted.
Descending into a gully, we walked down toward the water and further back through time. “Here is the extinction layer, I think,” Miller said, stamping a layer of clay. Below it paleontologists have found the skeletons of hippo-size marsupials, kangaroos ten feet tall, marsupial lions—a collection of giants.
There’s a debate in Australia over how those giants became extinct. Did humans wipe them out, or was it a climate change? The first step in solving the mystery is to decipher the age of the fossils, but there’s not enough carbon 14 left in them to measure their age accurately. So Miller has become a connoisseur of new clocks.
“There it is. Genyornis,” he said, picking up a fragment the size of his fingernail. Genyornis is the name of one of the vanished monsters: a 400-pound flightless bird. Miller held a piece of an eggshell from one of them—the color of putty, with small dimples on its surface.
He has amassed a collection of thousands of similar shell fragments from many sites in southeastern Australia. It turns out they’re everywhere, and once you know what to look for, it’s easy to spot them against the sand. “It’s amazing,” he says. “Who’d think you could just go around picking up eggshells?”
Miller and his colleagues have determined the age of Genyornis shells with two kinds of clocks. The first measures age by determining how long it has been since a mineral—such as quartz in the sand where the shells are buried—was exposed to sunlight.
Radioactive atoms surrounding and inside such buried quartz release particles that can knock electrons out of their normal positions, orbiting the nucleus in an atom. The released electrons sometimes get stuck in a defect in the crystal structure of the quartz. These crystal traps gradually fill up with electrons in a regular, clocklike way. If you know the rate of the trapping and can count the trapped electrons, you can figure out how long it’s been since the quartz saw the light of day—a method called optically stimulated luminescence.
Miller’s challenge was to find shells in sand containing quartz that had not been exposed to light since the moment it was first buried. If the sand was exposed at any point, sunlight would have given the trapped electrons enough energy to break out and return to their original places. In only a few seconds sunlight can clear out all the trapped electrons in a grain of quartz, setting the clock back to zero.
To date quartz crystals with trapped electrons, he enlisted an expert in this kind of dating, Nigel A. Spooner, a physicist from the Australian National University. Spooner hammered hollow stainless steel cylinders into the sand that held Genyornis shells. He quickly capped the cylinders, wrapped them in black plastic, and brought them to his lab. There, under dim red darkroom lights, he put grains of quartz in a machine that fired a beam of photons at them, releasing the trapped electrons. As the electrons settled back into their atoms they shed some energy as light. By measuring that light, Spooner could count the electrons that had been trapped and figure out the age of the shells.
Miller himself perfected another method of dating the shells by examining the proteins preserved within them—amino acid racemization. The building blocks of proteins, amino acids can take either a left-handed or right-handed form. For reasons still unknown, nature overwhelmingly prefers left-handed amino acids. Once an amino acid is formed, however, it can spontaneously flip over to become right-handed. The rate at which amino acids flip isn’t as regular as radioactive decay because it depends on temperature: Heat speeds up the reaction and cold slows it down. But Miller has been able to account for these variables by estimating climate changes in Australia over the past 100,000 years.
Both clocks point to the same age for the Genyornis shells: The bird became extinct about 50,000 years ago. Miller thinks his result rules out the environmental causes for the extinction. Climatic records from his sites show that 50,000 years ago water, and therefore vegetation, was abundant. Australian scientists have found a clue to the truth a hundred miles away on the dunes surrounding Lake Mungo, where they have counted trapped electrons to date a human skeleton and quartz from surrounding sediments back to 60,000 years—the oldest sings of humanity in Australia. The presence of humans at the time of the extinction—along with evidence that environmental factors were favorable for survival—implies that humans were the agents of Genyornis’ destruction.
“If humans hadn’t been there, the extinction would not have happened,” says Miller. “The real struggle is to say how people did it.” He suggest that by hunting prey and altering their habitat by fire, humans wiped out the giant fauna, and did so in a geologic blink of an eye.
The ages that Miller tosses around in conversation—50,000 years, 60,000— would have been absurd to a European in 1700. Scholars had painstakingly studied the chronology of the Bible to calculate how much time had passed since the days of Eden, adding up the ages of Adam and his descendants. In the 1650s Archbishop James Ussher came up with the date that would become the standard for over 200 years: God created the Earth in 4004 B.C.—on October 22, to be precise.
Scientists began to dismantle the idea of a young Earth in the late 1700s, when they discovered that the planet’s rocks are organized into a system of layers. The layers were formed by forces that we can still see at work today: the steady grinding down of mountain and the gentle rain of sediment to the bottom of rivers. But these forces work slowly, and for them to have actually created today’s landscapes, the Earth would have to be billions of years old. It would take until the 20th century for scientists to determine exactly how old the Earth is. Shortly after physicists discovered radioactivity and realized it could be used to fix dates to rocks, they realized they could also use it to find the age of the planet itself.
Some of Earth’s radioactive atoms were blasted out of neighboring stars in supernova explosions. They were swept up in a primordial disk revolving around the young sun and eventually helped form the solar system, coalescing into planets, comets, and meteoroids.
Because they’ve been with Earth from the start, these radioactive atoms can tell us how old the planet is. Some of them are uranium isotopes that decay into lead: uranium 235 into lead 207, with a half-life of 704 million years, and uranium 238 into lead 206, with a half-life of 4.47 billion years.
In the 1950s Clair Patterson, an American geochemist, compared the amounts of uranium and lead in rocks from Earth and in meteorites that had struck Earth. All his samples pointed back to a common origin at the dawn of the solar system. The age of the Earth, Patterson calculated, was 4.55 billion years.
As the Earth cooled down and developed a crust, the first rocks formed—the Canadian Acasta rock that sits on my desk among them. Discovering the age of the earliest rocks turns out to be much more difficult than calculating the age of the Earth itself. Once a rock forms, its uranium starts slowly turning to lead—but if underground water adds lead or uranium (or takes them away), researchers will end up with the wrong age.
Fortunately, nature has created the perfect rock clock for geologists. When magma cools, rugged little crystals known as zircons form. Made of zirconium, silicon, and oxygen, zircons also lock a few uranium atoms into their lattices. Once formed, a zircon shuts out just about any contaminant and can survive for billions or years. Over those billions of years, the trapped uranium steadily decays to lead. “Zircons are God’s gift to geochemistry,” says Ian Williams of the Australian National University. They can survive even after the rock where they originally formed erodes away. In Western Australia geologists have found a zircon crystal 4.4 billion years old trapped inside a rock that dates back only 3.1 billion years.
Zircons allow scientists to put dates on the history of the Earth, but zircons are not easy to find. I learned just how difficult and intense the search can be as I stood one chilly May morning at the harbor at Admiral’s Beach on the south-east coast of Newfoundland. there on the beach was a 15-foot wooden boat that would carry me and three scientists—Sam Bowring, a geologist from the Massachusetts Institute of Technology, Paul Myrow, a geologist from Colorado College, and Ed Landing, the state paleontologist from the New York State Museum—to deserted Great Colinet Isalnd, three miles away.
We made our way along the west side of the island toward its Southern end, motoring past merciless sea cliffs that exploded the waves into spray. As we lurched through the water, Landing identified the layers of rock exposed on the cliffs. They were from the late Precambrian, ranging roughly from 600 to 550 million years old. It was around that time that animal life proliferated.
The first distinct chemical signs of life that scientists have detected on Earth are actually much older than that—found in Greenland in the planet’s oldest sedimentary rock. We know their age because the rock is enclosed in slightly younger zircon-bearing rock that indicates a date of 3.9 billion years. But for well over 3 billion years after those first imprints, life left only microscopic marks in the fossil record. Then, not long before the Cambrian, strange multicellular fossils appear—giant fronds, ornamented disks, and other oddities collectively known as Ediacaran.
Paleontologists aren’t sure which, if any, of these creatures are the forerunners of later animals. What they do know is that in the early Cambrian the earliest fossils of most of the major groups of animals turn up. By dating fossil-bearing rocks from around the world, Bowring and his colleagues have shown that the burst of evolution known as the Cambrian explosion began around 530 million years ago. Short of the origin of life itself, that episode represents evolution’s supreme scientific challenge.
Bowring was on Great Colinet Island to test a theory championed in 1998 by Paul Hoffman, a geologist from Harvard, about what triggered the Cambrian explosion. The theory suggests that evolution was given a hard push when the planet fell into an ice age that got out of control. Glaciers kept growing until they covered the entire Earth, and life died back to almost nothing. After a few million years volcanic eruptions had released enough carbon dioxide to create a greenhouse effect that raised the planet’s temperature. The glaciers melted, and the rising ocean created vast shallow seas that life could recolonize, giving evolution a tremendous jolt.
This “snowball Earth,” as Hoffman and others call it, may have lasted ten million years. And if it did exist, it should have left its mark on Great Colinet Island. “This is one fo the few places in the world where you can find glacial deposits that you could hope to date, because they’re interlayered with volcanic rocks,” Myrow explained. Using the zircons in the volcanic rocks, the geologists were hoping to bracket the ice age—to find ash layers as close to the bottom and the top of the glacier-delivered rocks as possible. This could possibly tell them not just how old the ice age is but also how long it lasted.
We split up to hunt for zircons. I went with Myrow and Bowring. We climbed to the flat grassy top of the island. We hiked through bogs, past nets of flecked gull eggs, over mats of dwarf spruce trees. The struggle was worth it. Scrambling down among boulders battered by waves, Myrow spotted two layers of good volcanic rock, one of them 6 meters below the bottom of the glacial deposits and one of them 1.2 meters into them.
“Oh baby, it’s all ash,” he shouted. Bowring pointed to the rocks he wanted, and the two of them hammered away, with water pouring down the cliff onto their heads.
Bowring stuffed the samples into canvas bags and helped me load some of them into my backpack. The three of us started the long march back to the boat, to put their zircons on a plane, get them back to Bowring’s lab in Massachusetts, put dates on them, and try to figure out if it was “snowball Earth” that triggered the Cambrian explosion.
Blair Hedges, a biologist at Pennsylvania State University, is investigating the origins of animals with a different kind of clock. I stood with him in front of a bank of humming freezers while he inspected an ice-encrusted tray of little tubes, each filled with tissue. Hedges is creating a refrigerated zoo, collecting tissues from animals scattered among the 35 major taxonomic groups known as phyla. In his trays he had tissue from scorpions, centipedes, peanut worms, octopuses, mollusks, jellyfishes, sponges—”we’ve got about three-quarters of all the phyla right here,” he said.
Those cells contain clocks of their own that can tell time for hundreds of millions of years. From generation to generation certain genes of a species mutate at relatively steady rates. If you compare the genes of two species, say humans and chimpanzees—and you know the rates at which their genes have been mutating—you can estimate how long it has been since their ancestors diverged from a common ancestor.
This kind of molecular clock, as it’s known, has come into its own in the past ten years. In 1996 Hedges caused a stir by using molecular clocks to date the dawn of mammals. When paleontologists look at the record of mammal fossils, they see a burst of diversity just after dinosaurs became extinct 65 million years ago. It was this burst, they theorize, that produced most of the orders alive today—from hoofed mammals to bats to us primates. But when Hedges and his colleagues look beyond the fossil records at the genes of mammals, they see their roots extending back more than 100 million years.
Hedges is now investigating what molecular clocks have to say about the Cambrian explosion, which researchers such as Sam Bowring have determined took place 530 to 520 million years ago. Again, Hedges’s results are far different from what fossil records show. He and his colleagues have compared genes from three animal phyla, and their molecular clocks point to an origin over a billion years ago—once more a doubling of evolutionary history.
The conflict between fossils and genes will take a long time to sort out. Critics of molecular clocks suspect that evolution can make them speed up or slow down. But Hedges counters that he and his colleagues can guard against this sort of variability, and when they do, their dates still hold up. As for the lack of fossils to support his dates, Hedges argues that the earliest forms didn’t leave fossils behind, or at least any that have yet been discovered. Only around the start of the Cambrian did they get big enough for us to find.
Telling time is important not just to the history of life but to the history of the universe itself. Clocks that pin down the formation of the solar system can be found in meteorites that have fallen to Earth after wandering around the sun for billions of years. But for more ancient time telling, scientists cannot use any clock to be found on Earth. They have to look at the sky.
The sky was cloudy on the evening I met George Djorgovski, an astronomer from Caltech working in Hawaii, and rain was falling as we walked quickly across a dark lawn. “Can you believe we can look at stars in this weather?” he asked. We entered a small building and slipped into a room filled with bright fluorescent light and eight giant computer screens. Even if the sky was clear, we couldn’t have seen the stars through the drawn blinds.
Djorgovski sat in front of three computer screens pushed next to each other. The computers are hooked up to data cables that run 48 miles from this room to the 13,800-foot-high summit of Mauna Kea—and to two of the finest telescopes in the world, at the W. M. Keck Observatory.
As the sun set, Djorgovski sent coordinates to technicians at the top of Mauna Kea, and the telescope hew as using swung across the sky. A disembodied voice from one of the computer sin the room said, “Exposure complete,” and a white field filled with black spots appeared. One giant blob dominated the center of the picture—a ferociously bright object known as a quasar, with the intensity of trillions of suns. “That’s our guy,” said Djorgovski.
He touched the image of the quasar with one finger. “Just think,” he said. “As the Earth formed, the light from this had already traveled two-thirds of its way here.”
Quasars and galaxies are hurtling away from us as the universe expands. As they speed off, the light they emit lowers in frequency and shifts toward the red end of the spectrum—much as a train whistle drops in pitch as it passes by. This process is known as redshift, and by measuring it—and thus the rate at which the galaxies are flying apart—it’s possible to figure how long it has been since they were all contained together in one point of infinite density at the moment of the creation of the cosmos. In other words, astronomers can use that rate along with other cosmological data to tell how old the universe is. Today’s estimate for the expansion rate indicates that the universe is 13 billion years old.
Knowing the age of the universe is as important to astronomers as knowing the age of the Earth is to geologists. It lets them start putting together its history. How, for example, did the universe get from a uniform big band to the state it’s in today, with galaxies separated by vast stretches of relatively empty space? Did giant clusters of matter break down into galaxies, or did groups of stars join together?
Djorgovski’s quasar has a redshift that indicates it formed less than a billion years after the universe began. “What we’re after is the first galaxies,” he says. Djorgovski and others have been puzzled by evidence that these youngest galaxies are already rich with elements like carbon and oxygen—elements that can only be produced in mature stars. “We find galaxies in a good state of assembly after only a few hundred million years. How did they form so quickly?” says Djorgovski. Well into the 21st century, astronomers will be wrestling with the puzzle of how so many galaxies evolved so fast after the dawn of the universe.
By ten o’clock Djorgovski was waiting for the next observation. Sitting there with him, looking at the signs of young galaxies, I thought about what it means for something to be old. if you are 12 years old, or 50, or 90, that only means that a certain network of atoms has come together for that time. Many of the individual atoms that make up that network will stay in your body only a short time before being replaced by new ones. And all of those atoms have been wandering through the air and ground and ocean for billions of years, and before then they were made in stars out of other atoms, which in turn reach back to the dawn of galaxies, to the first second of the universe when all matter came into being.
“Hey, George, how’s your little girl?” asked Teresa Chelminiak, an observatory assistant working at the computer next to Djorgovski.
“Let me show you,” Djorgovski said. With one kick, he propelled his chair over to another screen. “Here’s the other thing these machines are good for,” he said. He got on the Web and pulled up his home page. Slowly, strip by strip, the spectrum of a baby galaxy was hidden behind a photograph of Djorgovski’s own baby. She was uncomplicated in her happiness. Her carbon 14 had no anomaly. Her redshift was zero. Once again, the clock was rest.
Copyright 2001 National Geographic Magazine. Reprinted with permission.