The first Neanderthal fossils found in Europe, a fragmented child’s cranium in Belgium in 1830, and an adult cranium in Gibraltar, were not immediately recognized as a divergent kind of human. Only in 1856 after a partial skeleton was found in a cave in the Neander Valley in Germany it became clear that these fossils belonged to an extinct human and our closest evolutionary relative (Hublin and Pääbo, 2006). Since then, questions about their relationship with modern humans have been fiercely debated between anthropologists. But what attracts most interest from scientists and popular media is the possibility of hybridization between Neanderthals and modern humans if, in other words, they were a genetically different specie or a single specie capable of producing offspring.
The first morphological features that later would become typical of Neanderthals, the projecting middle part of the face and a depression at the back of the skull, have been observed in fossils found in Europe as old as 400,000 years (Stringer & Hublin, 1999). These fossils belonged to Homo heidelbergensis, which in one of the various evolutionary scenarios that ties Neanderthals and modern humans is considered the ancestor of both Homo neanderthalensis and Homo sapiens (Hubmlin, 2009).
Neanderthals lived in Europe and Western Asia between 200,000 and 30,000 years ago. Fossil record shows that around 150,000 years ago (Bar-Yosef, 1998; Grün & Stringer, 2000) their range extended from Europe to Middle East and Asia spreading to Uzbekistan and Russia (Herrera et al., 2009). They were probably the only hominin group living in Europe and Western Asia for a long period of time until the arrival of modern humans. Their extinction is dated at around 30,000 years ago, small groups of Neanderthals survived even longer in isolated parts of southern Europe. In Gibraltar a fossil finding suggests that Neanderthals may have survived until 28,000-24,000 years ago (Finlayson et al., 2006).
Several different hypotheses have been formulated to explain the extinction of Neanderthals, from climate changes to intoxication from cave-associated contaminants like smoke, from cannibalism to diseases (Herrera et al., 2009). It is only certain that Neanderthals disappeared from fossil record after the arrival of modern humans, around 40,000 years in Asia, and 10,000-15,000 years in Europe (Shreeve, 1995). Fossil evidence shows the presence of modern humans in Middle East from 130,000 to 75,000 years ago, in the same areas where Neanderthals retreated between 65,000 and 47,000 years ago (Mellars, 2004).
As a result Neanderthals and modern humans coexisted for thousand of years both in Europe and in Asia, sometimes populating the same caves at different times, before Neanderthal extinction. Even though the prehistoric population density of Neanderthals and humans were probably very low and potential encounters could have been quite rare (Shea, 2007), their concurrence raises question about the...