Radioactive dating of skeletal remains unearthed in a Mexican cave indicates the area was settled by humans at least 13,000 years ago.
Research conducted by a team led by Wolfgang Stinnesbeck of Heidelberg University in Germany set out to resolve contradictory previous results obtained using carbon-14 dating by testing the location using a different technique, known as uranium-thorium dating.
This method measures the degree to which naturally occurring uranium-234 – with a half-life of 245,000 years – decays into thorium-230, with a half-life of 75,000 years. The method is accurate up to a range of half a million years
Although the now-established age of the bones is well within the dating range of the carbon-14 method, earlier tests had delivered wildly varying estimates. This, the researchers explain, is because over thousands of years the cave has been repeatedly inundated with both salt and fresh water – leaching almost all the collagen, which is essential for carbon dating, from the bones and teeth.
Rather than attempt to date the skeletal remains themselves, the researchers instead dated a stalactite growing a few milimetres above the pelvis, and a rock deposit known as a speleotherm nearby.
The results gave a date range of approximately 11,000 to 13,000 years, with Stinnesbeck considering the latter to be more likely.