When Higham eventually got the bones, his team came up with a more likely scenario: they were closer to 33,000 years old and one of the earliest examples of ceremonial burial in Western Europe.
In radiocarbon dating terms this makes the atmosphere appear older, which is reflected in the tissues of plants taking in CO2 during photosynthesis, and their products such as cottons.
"Despite La Brea's significance as one of North America's premier Late Pleistocene fossil localities, there remain large gaps in our understanding of its ecological history," said lead author Anna Holden, a graduate student at the American Museum of Natural History's Richard Gilder Graduate School and a research associate at the La Brea Tar Pits and Museum.
"Recent advances are now allowing us to reconstruct the region's paleoenvironment by analyzing a vast and previously under-studied collection from the tar pits: insects." The new study focuses on ground beetles and darkling beetles, which are still present in and around the Los Angeles Basin today.
Research has been ongoing since the 1960s to determine what the proportion of in the atmosphere has been over the past fifty thousand years.
The resulting data, in the form of a calibration curve, is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample's calendar age.