Neanderthals went extinct 40,000 years ago, and researchers are continuing to find clues about why.Mike Kemp/Getty ImagesScientists found new clues about one of the last living Neanderthals.By sequencing the DNA from one of the Neanderthal's teeth, they discovered a completely new lineage.The DNA indicates recent inbreeding that may help explain why this species went extinct.Scientists are one step closer to solving the mystery of humanity's last great extinction: why the Neanderthals died off.The Neanderthals are our closest ancient human relatives.