Ancient DNA refers to DNA molecules extracted from ancient biological specimens. DNA molecules are highly susceptible to degradation after an organism dies, making sample acquisition extremely difficult. However, scientists have made some attempts. In 1980, researchers from Hunan Medical University extracted DNA and RNA from the female corpse in the Marawudui Han tomb, marking the world's earliest ancient DNA research. In 1984, scientists from the University of California, Berkeley extracted mitochondrial DNA sequences from the skin of an extinct zebra donkey preserved in a museum for more than 150 years and compared them with those of its relatives—horses, donkeys, and zebras. They found that zebra donkeys were most closely related to zebras and least related to horses or donkeys. Two years later, German geneticist Peter Pretis extracted DNA from a mummy that had died thousands of years ago. Since then, a global research boom in ancient DNA has taken place, yielding many important findings.
Many studies on ancient DNA have focused on phylogenetic relationships, playing a crucial role in solving the mysteries of Neanderthal identity and Neanderthal-modern human kinship. German scientists obtained Neanderthal mitochondrial DNA in 1997 and 2000, proving that Neanderthal mitochondrial DNA was unrelated to modern human mitochondrial gene pools, supporting the theory of a single regional origin of modern Europeans. However, through the application of new sequencing technologies in the field of ancient DNA, a German research team obtained 4 billion base pairs from three female Neanderthal skeletons and mapped the Neanderthal genome draft. By comparing it with the genomes of modern humans from five regions around the world, they found that 1% to 4% of modern human DNA originated from Neanderthals, indicating that modern humans likely encountered Neanderthals in the Middle East after leaving Africa and engaged in limited genetic exchange with them. The extraction and genome sequencing of Neanderthal ancient DNA have solved the centuries-old mystery of whether Neanderthals had genetic exchanges with modern humans.