Neanderthals also known as Neandertals, Homo neanderthalensis or Homo sapiens neanderthalensis are an extinct species or subspecies of prehistoric humans who lived in parts of Europe and Asia until sometime less than 40,000 years ago. There are many theories about what caused the extinction of Neanderthals. It's hypothesized that  factors such as small population size, inbreeding, disease and climate fluctuations were possible contributors. Some research has also hypothesized that modern humans replaced Neanderthals by out competing them for resources. Others have hypothesized that modern humans inter-bread and eventually replaced Neanderthals genetically. It is likely that some combination of many of these factors over thousands of years lead to their population decline and eventual extinction.[1]