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Playing the Dating Game
Step into the world of writers and photographers as they tell you about the best, worst, and quirkiest places and adventures they encountered in the field.

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By Carl Zimmer Photographs by Robert Clark

A fossil, the Grand Canyon, the universe. Armed with simple principles and new techniques that find clues even in atoms, scientists can now date all manner of matter.

Get a taste of what awaits you in print from this compelling excerpt.

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 (0.9 meters) 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 possibly 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.

Get the whole story in the pages of National Geographic magazine.

Humans have an obsessive need to date things. Why is age so important? Speak your mind.

In More to Explore the National Geographic magazine team shares some of its best sources and other information. Special thanks to the Research Division.

In the magazine research business (something National Geographic takes very seriously), superlatives are killers. One learns to pick them out of a new manuscript immediately and to focus on them intently: the biggest this, smallest that, the most destructive, costliest, densest, tallest. Our article “How Old Is It?” describes methods for answering questions of age and is chock full of superlatives: the oldest rock on Earth, the first evidence of complex animal life, the oldest fossil evidence of modern humans. Inherently tricky to confirm (actually, they rarely hold up), such facts are further complicated by the nature of science itself—new information constantly changes the answer.

In our June 1983 and January 1994 articles on the universe, for instance, the best estimate for the age of the cosmos was 15 billion years. When we covered the subject again in October 1999, a refinement of the Hubble constant and new data on the amounts of matter and energy in the universe suggested a younger 12 billion years. New evidence has further refined the value of the Hubble constant, and for this month’s article a good estimate of the age of the universe is 13 billion years. All of these ages were meticulously reviewed, discussed, argued, and confidently published as the best available. Will the answer ever be final?

Another date that is exceedingly difficult to pin down is when the Cambrian explosion took place. That time, when animal structure became increasingly complex and creatures started making shells, is determined from the global geologic record. At the time of our June 1989 article on mass extinctions, evidence placed the event at around 570 million years ago. In our October 1993 article on the Cambrian, the time was revised to a slightly more recent 545 million years ago, with a certain amount of qualification (“more than half a billion years ago”). For the current article, it becomes more complicated. The accepted definition for the beginning of the Cambrian period is 543 million years ago. However, the fossil record, calibrated with new isotopic dating techniques, now suggests that it took tens of millions of years for complex animals to proliferate. Thus the Cambrian explosion, that real burst of life-forms, is now best timed at 530 to 520 million years ago. Stay tuned.

—Barbara W. McConnell

Museum Primer
London’s Natural History Museum introduces the subject of paleontology and provides links to botany, entomology, zoology, and mineralogy sites.

Snowball Earth
A Scientific American article summarizing the theory of an iced-over Earth.

Dating Techniques
A McMaster University site with explanations of dating techniques and links to projects using those techniques.

Carbon 14
An explanation from Scientific American on calibrating carbon 14 results with calendar dates.


Dalrymple, G. Brent. The Age of the Earth. Stanford University Press, 1991.

Flannery, Tim. The Future Eaters. George Braziller, 1995.

Freedman, Wendy L., and others. “Final Results From the Hubble Space Telescope Key Project to Measure the Hubble Constant.” Astrophysical Journal, vol. 553, no. 1 (May 20, 2001), 47-72.

Halliday, Alex. “Radioactivity, the Discovery of Time and the Earliest History of the Earth.” Contemporary Physics, vol. 38, no. 2 (1997), 103-114.

Knoll, A. H. “Learning to Tell Neoproterozoic Time.” Precambrian Research, vol. 100 (2000), 3-20.

Lunine, Jonathan I. Earth: Evolution of a Habitable World. Cambridge University Press, 1999.

Morris, Simon Conway. The Crucible of Creation: The Burgess Shale and the Rise of Animals. Oxford University Press, 1998.

Mojzsis, S. J., et al. “Evidence for Life on Earth Before 3,800 Million Years Ago.” Nature, vol. 384 (November 7, 1996), 55-59.


Garrett, Wilbur E. “Where Did We Come From?” National Geographic (October 1988), 434-437.

Keyser, Andre W. “The Dawn of Humans: New Finds in South Africa, ” National Geographic (May 2000), 76-83.

Gore, Rick. “The Dawn of Humans: People Like Us, ” National Geographic (July 2000), 90-117.

Berger, Lee R. “In The Footsteps of Eve: The Mystery of Human Origins, ” National Geographic Books (2000), 1-326.

Westenberg, Kerri. “The Rise of Life on Earth: From Fins to Feet, ” National Geographic (May 1999), 114-127.

Berger, Lee. “The Dawn of Humans: Redrawing Our Family Tree? ” National Geographic (August 1998), 90-99.

Monastersky, Richard. “The Rise of Life on Earth, ” National Geographic (March 1998), 54-81.

Gore, Rick. “Expanding Worlds: The Dawn of Humans, ” National Geographic (May 1997), 84-109.

Gore, Rick. “The First Europeans: The Dawn of Humans, ” National Geographic (July 1997), 96-113.

Hoffman, Hillel J. “The Rise of Life on Earth—When Life Nearly Came to an End, ” National Geographic (September 2000), 100-113.

Gore, Rick. “Extinctions, ” National Geographic (June 1989), 663-699.


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