AMS shows modern humans and Neanderthals overlapped

Strong evidence that Neanderthals overlapped with modern humans for up to 5,400 years has been revealed using an improved accelerator mass spectrometry technique.

Researchers from the University of Oxford dated over 200 samples of bone, charcoal and shell from 40 key European archaeological sites linked to Mousterian (Neanderthal tool-making sites) or transitional sites related to either early modern humans or Neanderthals.

“Using accelerators we are able to undertake direct ion counting. We can measure the ¹⁴C abundance directly,” Professor Thomas Higham told Laboratory News.

“We carefully clean and pre-treat the sample for dating, then we combust and purify the CO₂. We then used a catalytic reduction method to produce graphite, and around 1mg of this is put into an aluminium target which goes in to the accelerator.”

“We then focus a beam of caesium onto the target, the sputtering produces negatively charged particles thereby removing ¹⁴N, the main isobar. The negatively charged particles stream in to the accelerator and we select mass 14 particles using the magnets in the AMS. Molecular ions of mass 14 are broken up using a stripper gas in the main body of the machine.

The middle of the AMS contains a thin foil or argon gas film to strip electrons and change the charge state of the particles that pass through into 3+. ¹⁴C is detected using a faraday cup after their velocity is checked in the final stages of the accelerator. Each sample is in the machine for around 20 minutes and a wheel of 50 samples is rotated three or four times.

“The measurement is much faster that for decay-based methods and the sample size requirements are much lower (1000 times lower on average). We can date samples as small as a rice grain or a lentil, single hair etc,” added Higham.

The new radiocarbon dates allowed the team to show that Neanderthals and modern humans overlapped for significant periods giving ample time for interaction and interbreeding.

“We believe we now have the first robust timeline that sheds new light on some of the key questions around the possible interactions between Neanderthals and modern humans,” said Higham. “The chronology also pinpoints the timing of the Neanderthals’ disappearance, and suggests they may have survived in dwindling populations in pockets of Europe before they became extinct.’

The timing and spatiotemporal patterning of Neanderthal disappearance