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What is aDNA?Ancient DNA (aDNA) can be defined as any DNA (deoxyribonucleic acid) recovered from bones, tissue, or other biological residues that have not been immediately, intentionally, stored in an environment optimal for DNA preservation. This includes the remains of individuals (such as murder victims) who have been dead for weeks or months before discovery, as well as those of species that have been extinct for thousands or millions of years. DNA is an organic molecule made up of four types of nucleic acid held together along a sugar phosphate chain. Put together in various combinations (or "sequences"), the four nucleic acids encode the genetic information necessary for the growth, development, maintenance, and reproduction of all known forms of life. Many of the similarities and differences between species, and between members of the same species, are reflected by similarities and differences in the nucleic acid sequences. These patterns allow species to be organized into phylogenetic (family-tree) groups and allow differentiation between individual members of each species (as used in forensics). Ancient DNA holds the potential to solve many of the mysteries of human history and prehistory. There have been many successes, but there also appear to be a number of limitations on the recovery of informative ancient DNA sequences. Such limitations involve the characteristics of the burial environment, the age of the specimen (time since death), the portion of the genome under study, and the sensitivity of our analysis methods. DNA is a relatively stable molecule that has been shown to survive for tens of thousands (to millions) of years under ideal conditions. Recovered aDNA sequences have been used to test for the genetic relatedness between early human relatives (such as Neandertals) and humans that are alive today, and research in these areas is ongoing. If we can continue to develop improved recovery methods, and recover DNA of even earlier ages and from more varied (especially climatically warmer) contexts, there is great potential for clarifying the details of many species' evolutionary history, including our own.
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