The uranium-thorium method is often helpful for dating finds in the 40,000- to 500,000-year-old range, too old for radiocarbon but too young for K-Ar or Ar-Ar.
These methods — some of which are still used today — provide only an approximate spot within a previously established sequence: Think of it as ordering rather than dating.
Both plants and animals exchange carbon with their environment until they die.
Afterward, the amount of the radioactive isotope carbon-14 in their remains decreases.
Sometimes only one method is possible, reducing the confidence researchers have in the results. “They’re based on ‘it’s that old because I say so,’ a popular approach by some of my older colleagues,” says Shea, laughing, “though I find I like it myself as I get more gray hair.” Kidding aside, dating a find is crucial for understanding its significance and relation to other fossils or artifacts.
Methods fall into one of two categories: relative or absolute.