Neanderthals became extinct around 40,000 years ago. This timing, based on research published in Nature in 2014, is much earlier than previous estimates, and derives from improved radiocarbon dating methods analyzing 40 sites from Spain to Russia. Evidence for continued Neanderthal presence in the Iberian Peninsula 37,000 years ago was published in 2017. Various hypotheses on the causes of Neanderthal extinction implicate: violence; parasites and pathogens; competitive replacement; extinction by interbreeding with early modern human populations; natural catastrophes; climate change; inbreeding depression. It seems unlikely that any single one of these hypotheses is sufficient on its own; rather, multiple factors probably contributed to the demise of an already low population. Possible coexistence before extinction In research published in Nature in 2014, an analysis of radiocarbon dates from forty Neanderthal sites from Spain to Russia found that the Neanderthals disappeared in Europe between 41,000 and 39,000 years ago with 95% probability. The study also found with the same probability that modern humans and Neanderthals overlapped in Europe for between 2,600 and 5,400 years. Modern humans reached Europe between 45,000 and 43,000 years ago. Improved radiocarbon dating published in 2015 indicates that Neanderthals disappeared around 40,000 years ago, which overturns older carbon dating which indicated that Neanderthals may have lived as recently as 24,000 years ago, including in refugia on the south coast of the Iberian peninsula such as Gorham's Cave. Zilhão et al. (2017) argue for pushing this date forward by some 3,000 years, to 37,000 years ago. Inter-stratification of Neanderthal and modern human remains has been suggested, but is disputed. Stone tools that have been proposed to be linked to Neanderthals have been found at Byzovya (:ru:Бызовая) in the polar Urals, and dated to 31,000 to 34,000 years ago. Possible cause of extinction Violence Kwang Hyun Ho discusses the possibility that Neanderthal extinction was either precipitated or hastened by violent conflict with Homo sapiens. Violence in early hunter-gatherer societies usually occurred as a result of resource competition following natural disasters. It is therefore plausible to suggest that violence, including primitive warfare, would have transpired between the two human species. The hypothesis that early humans violently replaced Neanderthals was first proposed by French paleontologist Marcellin Boule (the first person to publish an analysis of a Neanderthal) in 1912. Parasites and pathogens Pathogens or parasites transmitted by humans may have passed to Neanderthals. Neanderthals had poor protection to illnesses they hadn't been