Image: RYAN REID DATING TECHNIQUES have come a long way since researchers first started attempting to reconstruct our planet's past.

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 PLANET EARTH may have been ready to support life by 4.4 billion years ago, according to a zircon crystal retrieved from ancient sedimentary rocks in Australia.

RADIOACTIVE ISOTOPES

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.

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