The variation of mitochondrial DNA (mtDNA) between modern humans and Neandertal sequences lie outside the mtDNA sequence variation within modern humans. This variation has led several researchers to conclude that Neandertals did not contribute to modern human DNA and are a separate species that went extinct in Europe. It is feasible that DNA can be retrieved from specimens that died thousands of years ago, given the ideal preservation conditions and extraction protocols. However, DNA also decays as the organism decomposes. Spontaneous hydrolysis, oxidation, and nucleotide modifications are a few of the processes that cause DNA decay and likely interfere with reliably obtaining a mtDNA sequence that accurately reflects the Neandertal mtDNA sequence.