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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.

Other radioactive isotopes can be used to accurately date objects far older.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 forma phenomenon known as racemization.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.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.

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.

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.

ANCIENT ARTWORK, such as this Chinese horse, can be evaluated using thermoluminescence.