In an accompanying commentary in Nature, Eric Delson of the American Museum of Natural History in New York City and Katerina Harvati of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, put the early dates in context.
"Such a late survival would reinforce the importance of southern Iberia as a refuge area at a time when modern humans were expanding and diversifying culturally across mid-latitude Europe," they write.
Scientists believe Neandertals and modern humans are two different species that shared a common ancestor and overlapped for several thousand years in Europe.
(Related: "Neandertals Hunted as Well as Humans, Study Says" [January 2006].)
Modern humans arrived in western Europe at least 32,000 years ago. Competition between the species in a period of climate change is believed to have doomed the Neandertals by around 30,000 years ago.
But the two species did not overlap at Gorham Cave, allowing Neadertals there to survive for much longer, according to the Gibraltar Museum's Finlayson.
Modern humans are believed to have spread north from Africa into Europe, then west, reaching the Iberian Peninsula last.
The first evidence of modern humans in Iberia is dated to about 18,500 years ago. Before that, human populations in neighboring areas were small, according to Finlayson.
And the mountainous, rugged landscape of the peninsula "probably contributed a lot to keeping the populations isolated from each other," he said.
Without modern humans as competition, the Neandertals likely feasted freely on plants and animals that inhabited the region's woodlands, sandy plains, wetlands, and shoreline.
As the climate cooled in more northern latitudes and modern humans moved farther south and west, "these last pockets [of Iberian Neandertals] kept doing what they had always done," Finlayson said.
But about 24,000 years ago, according to analysis of sediment, a period of prolonged drought struck the region and likely doomed the last Neandertals, he says.
Paul Mellars is an archaeologist at the University of Cambridge in England and co-author of a review paper on radiocarbon dating techniques published in Nature this February. That paper revised the timeline for the spread of modern humans across Europe.
According to Mellars, the idea that the Iberian Peninsula was the last place modern humans reached and that it served as a Neandertal refuge is "perfectly plausible."
"The question is whether these dates provide evidence of it. And frankly, I don't think they do," he said.
Mellars said of the late Neandertal dates presented by Finlayson and colleagues, "The majority are around 30,000 to 32,000 years ago.
"If you've got a spread of dates, you should look at the majority of the dates you find, not just the ones that happen to be younger," he said.
"Second," he continued, "radiocarbon dateswhether on charcoal or anything elsecan be very prone to contamination by more recent carbon that is intrusive from the top [layers of material."
Contamination of even one percent of the sample, Mellars says, can skew dates 3,000 to 4,000 years younger.
"That makes me think at least some of those datesthe younger dates, 24,000, 28,000are almost certainly contaminated," he said.
Finlayson counters that his analysis ruled out contamination of the sample from the higher levels.
Delson and Harvati, the Nature commentators, say Finlayson and colleagues were correct to cite 28,000 years ago as the youngest date.
"There are just too many instances of dates younger than 28,000 years that are out of order, implying that these dates might not be reliable," they write.
"More extensive sampling of the in situ [unremoved] hearth and surroundings might resolve this issue."
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