sequenced from desiccated muscle of the quagga, and was found to differ by 12 base substitutions from the mitochondrial DNA of a mountain zebra. It was concluded that these two species had a common ancestor 3-4 million years ago, which is consistent with known fossil evidence of the species. Denisovans The Denisovans of Eurasia, a hominid species related to Neanderthals and humans, was discovered as a direct result of DNA sequencing of a 41,000-year-old specimen recovered in 2008. Analysis of the mitochondrial DNA from a retrieved finger bone showed the specimen to be genetically distinct from both humans and Neanderthals. Two teeth and a toe bone were later found to belong to different individuals with the same population. Analysis suggests that both the Neanderthals and Denisovans were already present throughout Eurasia when modern humans arrived. In November 2015, scientists reported finding a fossil tooth containing DNA from Denisovans, and estimated its age at 110,000-years-old. Mitochondrial DNA analysis The mtDNA from the Denisovan finger bone differs from that of modern humans by 385 bases (nucleotides) in the mtDNA strand out of approximately 16,500, whereas the difference between modern humans and Neanderthals is around 202 bases. In contrast, the difference between chimpanzees and modern humans is approximately 1,462 mtDNA base pairs. This suggested a divergence time around one million years ago. The mtDNA from a tooth bore a high similarity to that of the finger bone, indicating they belonged to the same population. From a second tooth, an mtDNA sequence was recovered that showed an unexpectedly large number of genetic differences compared to that found in the other tooth and the finger, suggesting a high degree of mtDNA diversity. These two individuals from the same cave showed more diversity than seen among sampled Neanderthals from all of Eurasia, and were as different as modern-day humans from different continents. Nuclear genome analysis Isolation and sequencing of nuclear DNA has also been accomplished from the Denisova finger bone. This specimen showed an unusual degree of DNA preservation and low level of contamination. They were able to achieve near-complete genomic sequencing, allowing a detailed comparison with Neanderthal and modern humans. From this analysis, they concluded, in spite of the apparent divergence of their mitochondrial sequence, the Denisova population along with Neanderthal shared a common branch from the lineage leading to modern African humans. The estimated average time of divergence between Denisovan and Neanderthal sequences is