How are Neanderthals related to modern humans? What evidence exists that suggests there may have been inter-mating?  For what world populations did it occur?

How are Neanderthals related to modern humans? What evidence exists that suggests there may have been inter-mating?  For what world populations did it occur?

What happened to Neanderthals as a species?  Did they co-exist with modern humans?  Why did they go extinct?