Cave bears (Ursus spelaeus) are named after the European caves where their bones are often found. These giants were roughly a third larger than modern grizzly bears - their populations started to plummet in Europe 24,000 years ago, dying out from unknown causes roughly 20,000 years ago, back when ice dominated the Earth. Now scientists believe that giant cave bears might have been driven to extinction by the advance of humanity.
     An international team of scientists analyzing DNA in 17 newly identified fossils of cave bears has revealed that genetic decline started 50,000 years ago - "much earlier than previously suggested, at a time when no major climate change was taking place, but which does coincide with the start of human expansion," said researcher Aurora Grandal-D'Anglade at the University of Coruña in Spain. In the May issue of the journal Molecular Biology and Evolution, genetic scientists report that they compared 59 DNA sequences from cave bear mitochondria - the powerhouses within their cells - with 40 modern and fossil DNA samples from brown bears (Ursus arctos) to find out why the former went extinct while the latter did not. Other fossil evidence reveals that cave bears ceased to be abundant in Central Europe roughly 35,000 years ago. (Diversity of genes can provide indirect evidence for the number of breeding individuals, because with more bears mating more genes are thrown into the mix, and vice versa.)
     "This can be attributed to increasing human expansion and the resulting competition between humans and bears for land and shelter," Grandal-D'Anglade reported. "As humans became more effective at using caves, the number of places where cave bears could hibernate, which was essential to reproduction and everything else they did, started to decrease," said anthropologist Erik Trinkaus at Washington University in St. Louis. During the ice age 20,000 years ago, the combination of fewer caves for hibernation and significant reductions in the vegetation the animals largely depended on may have delivered "the 'coup de grace' for this species, which was already in rapid decline," Grandal-D'Anglade said. In contrast, the brown bear may have survived until today precisely because they did not depend so heavily on caves. "Brown bears rely on less specific shelters for hibernation," Grandal-D'Anglade said. "In fact, their fossil remains are not very numerous in cave deposits."

Source: Live Science (27 August 2010)