Image: RYAN REID
These days hardly a week goes by without important discoveries concerning the history of life on Earth making headlines. Indeed, just last month researchers described a fossil that pushes the origins of key mammal features back some 45 million years. And last week scientists announced that new dates for an extinction event that claimed most of Australia's large animals show that humans, not the climate, wiped them out. Although visual inspection of the rocks, fossils and archaeological remains used to reconstruct our planet's past provides critical information, only by ascertaining their ages can researchers put this data into a meaningful context.
The first step toward accurately measuring geologic time came at the turn of the 20th century, when French physicist Henry Becquerel discovered the natural radioactive decay of uranium. Shortly thereafter, building on related work by Ernest Rutherford, American chemist Bertram Borden Boltwood determined that he could use the predictable decay of radioactive elements such as uranium into other elements to keep track of time. Although Boltwood's resulting estimates for things like the age of Earth¿which he placed at around 2.2 billion years¿have since been significantly revised, he indicated correctly that our planet was far older than people had imagined possible.
In the decades that followed, scientists made important new discoveries about the structure and behavior of atoms, and they refined their existing dating techniques. More recently, they have developed a number of new methods. Some use radioactive isotopes; others take advantage of different phenomena, such as thermoluminescence and electron spin resonance. Still others, like amino acid racemization, show promise but have not yet taken wing.
Now, nearly 100 years after Boltwood's groundbreaking work, it is estimated that Earth formed at least twice as long ago as he had claimed. The following summaries offer a quick introduction to some of the dating techniques researchers have been using to explore and reconstruct our planet's past, from 4.5 billion years ago to the present. ¿Kate Wong
Image: created by RETO STOCKLI, NAZMI EL SALEOUS and MARIT JENTOFT-NILSEN, NASA GSFC
The premise behind techniques involving the use of radioactive isotopes is straightforward. Each isotope has what is known as a half-life¿that is, a period of time in which half of the atoms in a population decay into stable daughter elements. This half-life differs dramatically from isotope to isotope. As a result, different isotopes are better suited to dating different items. In the case of carbon 14, for example, the half life is only 5,730 years. Carbon 14 can thus reliably date items only up to around 40,000 years old.
Other radioactive isotopes can be used to accurately date objects far older. The decay of argon 40 to argon 39, for instance, played a vital role in underscoring the significance of two ancient human skulls unearthed in the Republic of Georgia last summer. These remains, Carl C. Swisher III of the Berkeley Geochronology Center and his colleagues reported, are more than 1.7 million years old, and as such represent the first humans to leave Africa to colonize the rest of the world. Argon dating can also be used to date materials as young as 10,000 years and as old as billions of years.
Uranium and lead isotopes take us back farther still. Indeed, findings presented earlier this year suggest that infant Earth may have been ready to support life far earlier than previously thought. Uranium-lead dates for a single zircon crystal found in the oldest sedimentary rock yet known suggest that by 4.4 billion years ago our planet already had already cooled enough to have a crust. The first life-forms may have been just around the corner.
THERMOLUMINESCENCE AND ELECTRON SPIN RESONANCE
Image: C. C. WONG/Mandarin Collection
Many crystals, including diamond, quartz and feldspar, accumulate and trap electric charges at a known rate over time. Heating the crystals, it turns out, liberates these electrons, emitting a measurable amount of light. Researchers can thus determine the amount of time that has passed since the buried crystal was last exposed to heat.
In the case of thermoluminescence, resetting the crystal clock means heating it to around 500 degrees Celsius. Because of that condition, scientists say, the technique is well suited to dating meteoritic impacts, fire-treated stones used by early humans, cooking hearths and old ceramics.
Somewhat similar to thermoluminescence, electron spin resonance (ESR) dates crystals, too (although these are found in shells and enamel.) Unlike thermoluminescence, however, this method counts the number of "unpaired spins" of electrons trapped in the crystal, instead of freeing them.
ESR can be used to evaluate materials up to one million years old and has become an indispensable tool for paleoanthropologists, who often use it to date the teeth of animal remains found among the precious human fossils.
AMINO ACID RACEMIZATION
Over time, the amino acids that make up proteins slowly convert from their so-called left-handed state to their right-handed form¿a phenomenon known as racemization. When temperature and environment are constant, conversion occurs at a constant rate.
In theory, this should allow researchers to date protein up to 100,000 years old. So far, however, the technique has proved problematic¿perhaps because it is difficult to know whether conditions have been constant. (Some researchers have suggested, though, that levels of amino acid racemization can be good indicators of ancient DNA preservation.)