Encounters between AMH and Neanderthals probably took place at different times in different places to leave the genetic signal of some 3% Neanderthal genes in non-African AMH. Early encounters would have taken place in central Asia (perhaps 50,000 years ago) with later encounters in Europe (c. 30,000 years ago). Now new methods of radiocarbon dating at archaeological sites is providing evidence which indicates that Neanderthals and AMH overlapped for many hundreds of generations.
Tom Higham et al, The timing and spatiotemporal patterning of Neanderthal disappearance, Nature 512,306–309, doi:10.1038/nature13621
From the Abstract:
The timing of Neanderthal disappearance and the extent to which they overlapped with the earliest incoming anatomically modern humans (AMHs) in Eurasia are key questions in palaeoanthropology. Determining the spatiotemporal relationship between the two populations is crucial if we are to understand the processes, timing and reasons leading to the disappearance of Neanderthals and the likelihood of cultural and genetic exchange. Serious technical challenges, however, have hindered reliable dating of the period, as the radiocarbon method reaches its limit at ~50,000 years ago. Here we apply improved accelerator mass spectrometry 14C techniques to construct robust chronologies from 40 key Mousterian and Neanderthal archaeological sites, ranging from Russia to Spain. …….. We show that the Mousterian ended by 41,030–39,260 calibrated years BP (at 95.4% probability) across Europe. We also demonstrate that succeeding ‘transitional’ archaeological industries, one of which has been linked with Neanderthals (Châtelperronian), end at a similar time. Our data indicate that the disappearance of Neanderthals occurred at different times in different regions. Comparing the data with results obtained from the earliest dated AMH sites in Europe, associated with the Uluzzian technocomplex, allows us to quantify the temporal overlap between the two human groups. The results reveal a significant overlap of 2,600–5,400 years (at 95.4% probability).
This has important implications for models seeking to explain the cultural, technological and biological elements involved in the replacement of Neanderthals by AMHs. A mosaic of populations in Europe during the Middle to Upper Palaeolithic transition suggests that there was ample time for the transmission of cultural and symbolic behaviours, as well as possible genetic exchanges, between the two groups.
They did not look at sites on the Iberian peninsula which may contain evidence of later identifiable Neanderthals.
“According to this study, the overlap of humans and Neanderthals has been refined to a few thousand years, which is quite a long time. Archaeologists can get quite blasé about thousands of years because we deal with millennia and we forget that, in human terms, this is several generations and lot can happen,” said William Davies, an archaeologist who studies the dispersal of human species at the University of Southampton in the U.K., and who wrote an accompanying commentary.
Specimens 50,000 years or older do not contain the radiocarbon needed for radiocarbon dating and even samples as young as 30,000 years old can be prone to contamination from present-day carbon molecules. “Contamination is very significant for specimens of this time period,” said Higham. “A 40,000-year-old sample that has 1 percent modern carbon will give you a date that is 7,000 years younger than the specimen’s real age. It’s difficult to overcome removal of contamination from samples, so there has been a real urgent need to improve dating methods.”
To overcome this hurdle, the team had previously developed new sample prep methods that include the filtering of collagen and isolation of a single mammalian-specific amino acid from bone—both of which allow for more accurate radiocarbon dating.
The samples analyzed included Neanderthal bones as well as tools and other artifacts from two European stone tool cultures, the Mousterian and Châtelperronian, associated with Neanderthals.
“Until recently, the main view was for a coexistence [of Neanderthals and early modern humans] in Europe between about 30,000 to 40,000 years, with a few sites suggesting Neanderthal survived even later than 30,000 years,” Chris Stringer, an anthropologist at London’s Natural History Museum, told The Scientist in an e-mail. “This new work seems to have falsified that model with no signal of a Neanderthal presence after 39,000 to 40,000 years ago.”
Chris Stringer also comments at Discovery:
“Significant interbreeding between Neanderthals and early modern humans had probably already occurred in Asia more than 50,000 years ago, so the dating evidence now indicates that the two populations could have been in some kind of contact with each other for up to 20,000 years, first in Asia then later in Europe,” Chris Stringer, research leader in Human Origins at the Natural History Museum in London, explained.
“This may support the idea that some of the changes in Neanderthal and early modern human technology after 60,000 years ago can be attributed to a process of acculturation between these two human groups,” Stringer said.
If the interactions at the various locations each spanned 100 to 250 generations – and approximately 2,000 generations ago – it is not difficult to imagine a gradual assimilation and subsequent disappearance of Neanderthals – especially if there was a fertility difference in favour of AMH. There would be no need then to assume catastrophic extinction or genocide of the Neanderthals. An AMH population expanding much more rapidly than a co-existing, socially compatible but declining population of Neanderthals would suffice to explain the 3% or so of Neanderthal genes left in the surviving population today after a further 2,000 generations.