Monday, February 4, 2013

Earlier Neandertal demise suggested by redating

From: Science News

The story of the Neandertals may need a new ending, a controversial study suggests. Using improved radiocarbon methods, scientists redated two of the youngest known Neandertal cave sites and concluded that they are at least 10,000 years older than previous studies have found.

The findings cast doubt on the reliability of radiocarbon dates from other recent Neandertal sites, the researchers suggest online February 4 in the Proceedings of the National Academy of Sciences. This means the last Neandertals might have died out much earlier than previously thought, which could cause anthropologists to rethink how and why these hominids vanished. Researchers have long debated whether the harsh Ice Age climate, the appearance of modern humans migrating out of Africa, or some other factor drove Neandertals to extinction.

“The paper is simply excellent,” says archaeologist Olaf Jöris of the Romano-Germanic Central Museum in Mainz, Germany. The new research supports Jöris’ own review of Neandertal dates, in which he concluded that the most-recent Neandertals probably lived around 42,000 years ago. The standard view suggests that the last of these hominids occupied Europe as recently as about 28,000 years ago.

But other archaeologists are not convinced by the new work. “We shouldn’t get too carried away over results that amount to a few radiocarbon dates from two sites,” says Paul Pettitt, an archaeologist at Durham University in England.

This Neandertal jaw from a cave in southern Spain may be at least 10,000 years older than previously estimated, a new dating analysis suggests.
Over the last couple of decades, archaeologists have determined that the Iberian Peninsula was one of the last Neandertal refuges. Neandertals throughout much of Europe appear to have gone extinct around the same time that modern humans reached the continent, at least 42,000 years ago. But the favorable climate of southern Spain and Gibraltar may have helped Neandertals hang on in for another 10,000 years or so. Getting a precise chronology is crucial to understanding what factors played a role in the Neandertals’ demise and the degree to which Neandertals and humans interacted and possibly interbred, researchers say.

The majority of the youngest Neandertal ages come from radiocarbon dating. This method dates organic material by using the steady rate at which one form of carbon transforms into another after an organism dies. But bones, charcoal and other samples can appear younger than they really are if, over time, they become contaminated with more-modern organic material. In the last several years, researchers have developed new ultrafiltration methods to better remove such impurities.

Radiocarbon scientist Rachel Wood of the Australian National University in Canberra and colleagues used ultrafiltration with radiocarbon dating to reassess the ages of some young Neandertal sites. The team collected 215 animal bones from 11 sites on the Iberian Peninsula. But after ultrafiltration — which strips away most of the proteins in a sample — there wasn’t enough organic matter left in bones from nine of the sites to do radiocarbon testing.

The team did find sufficient material from two sites: At the Jarama VI site in central Spain, a rock shelter where Neandertal tools have turned up, the researchers dated three mammal bones that contain signs of butchering. At Cueva del Boquete de Zafarraya in southern Spain, they dated five mammal bones found near Neandertal fossils. Both of the sites had been dated to between about 30,000 and 40,000 years ago. But the new analyses suggest these sites are at least 10,000 years older, the researchers say.

Given that older radiocarbon techniques underestimated the ages of these sites, other Neandertal radiocarbon dates may be inaccurate, too, Wood says. But reanalyzing other sites will be difficult because ultrafiltration requires samples to be well preserved. Since the Iberian Peninsula is so warm, organic material in fossils quickly degrades, which is why so many bones in the new study couldn’t be dated. Scientists have used other dating techniques in this region, but they are not as precise as radiocarbon dating, Wood says.

Tossing out other radiocarbon dates based on this study doesn’t sit well with many archaeologists. João Zilhão of the University of Barcelona notes that Wood’s team didn’t actually redate the part of the cave at Jarama VI where Neandertals most recently lived. As a result, they can’t say when the very last members of the group lived there. And, he adds, “Zafarraya is a badly mixed site.” Two bones found next to each other may be of very different ages, making it hard to use the age of one bone as a proxy for another bone’s age.

Although Wood questions the dating of many sites in Spain and Gibraltar, she says one spot may still have good evidence for the late survival of Neandertals. At Cueva Antón in southeastern Spain, archaeologists have securely dated charcoal from a campfire to roughly 37,000 years ago. In 2010, archaeologists including Zilhão reported finding perforated and pigment-stained marine shells that must have been modified by hominids at the site. But not much has been published about the tools found there, Wood says, so it’s too early to know whether Neandertals or Homo sapiens were the artisans.

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