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.)