More recently, genetic techniques have been employed to study Neanderthal and human phylogeny. These techniques are hampered by problems of decay, retrieval, and contamination of the DNA obtained from fossil bones (Hofreiter, et.al, 2001) but they generally confirm that divergence of Neanderthals from the lineage of modern humans occurred before the common ancestor to all modern humans. Sequencing and comparing maternally inherited mitochondrial DNA (mtDNA) of Neanderthals and humans, Ward and Stringer (1997) found about three times as many differences between Neanderthal and human DNA as among populations of modern humans and placed this divergence at 555,000 to 690,000 years ago, some four times the age of the common ancestor of modern humans. Green et. al. (2006) find a similar date of divergence at 500,000 years ago using comparative DNA sequence alignment.