A genetic analysis of a 45,000-year-old thigh bone narrows down the time when modern humans and Neanderthals first interbred.A 45,000-year-old leg bone is raising questions about just how close modern-day humans are to our thick-browed Stone Age ancestors. DNA from the femur of a Siberian man is helping to pinpoint when modern humans and Neanderthals first interbred, researchers say.