RETURN TO THE HOMINID JOURNEY....
If single people think dating is tough, they should try paleoanthropology. To decipher the tangled tale of human evolution, scientists need accurate dates for the fossil evidence left behind by our distant ancestors. They have an especially tough time pinning down ages for remains between about 40,000 years old and half a million years old -- the time period when Homo sapiens probably arose. To meet this challenge, scientists over the past few decades have developed a variety of dating techniques. Most of these methods date human remains indirectly by determining the age of the material with which the remains were found.
Two of the techniques rely on the fact that radioactive elements constantly bombard the atoms in minerals with energy. This background radiation knocks an atom's electrons into different orbits. Most of the electrons soon fall back to their original state, but some become permanently trapped in physical flaws in the mineral's crystal structure. Over time, more and more excited electrons become caught. Scientists can thus estimate an object's age by measuring the number of trapped electrons.
In electron spin resonance, scientists count the electrons by measuring the amount of energy trapped within a crystal. Electron spin resonance has become popular for directly dating fossil teeth because tooth enamel traps electrons. In another method, called thermoluminescence, scientists heat an object in order to knock trapped electrons out of their excited state. Each falling electron releases a certain amount of light, which the scientist can measure to count how many electrons were in the fossil. Thermoluminescence is especially good at dating flints or pottery fragments.
Scientists use the third technique, known as uranium-series dating, for remains found in limestone deposits, such as those in caves and around hot springs. Uranium-series dating works because uranium atoms naturally decay into lighter elements, including the isotope thorium-230. Uranium dissolves in the mineral-rich water often present in caves, while thorium does not. When limestone deposits, such as stalactites, form from this water, they contain uranium but essentially no thorium. Thorium only begins to appear when the uranium decays away at a known rate. Scientists can thus deduce an object's age from the amount of thorium present. -- Alexandra Witze