Nucleic Acids Isolated From Ancient Remains

The examination of preserved, organic archaeological remains with the techniques of molecular genetics is providing direct access to the genetic components of these tissues. Although the nucleic acids are often chemically modified and substantially sheared, i t has been possible to retrieve ancient DNA sequences. The polymerase chain-reaction method, which has been modified to overcome the inhibitory properties of lesioncontaining DNA, is capable of reconstructing ancient DNA sequences from template molecules that are shorter than the amplified products. This has made it possible to analyze both ancient or extinct mitochondria1 and genomic DNA sequences. These studies have resolved ambiguities in the phylogenies of extinct animals and have contributed to knowledge about the patterns of human population migration. The potential of ancient nucleic acid analysis to make contributions in paleopathology, molecular evolution, and population genetics depends on improved methodologies which, in turn, requires a more comprehensive understanding of postmortem chemical processes. Ancient DNA, molecular evolution, DNA damage, DNA polyOrganic archaeological specimens are routinely analyzed with techniques that are designed to probe molecular structures. The application of the techniques of molecular genetics focuses specifically on the nucleic acids preserved in ancient tissues cTable 1 ) It is the information encoded by these chemical components that may make i t possible to study aspects of inheritance, paleopathology, and molecular evolution at the nucleotide level. Decoding the nucleotide sequence information in preserved materials presents some unique challenges, since the nucleic acids are often neither pure nor intact. When soft tissues are available for study, they are usually preserved by mummification. Mummification is the desiccation or functional inactivation of tissues at a rate greater than the decomposition of the tissues. The action of bacteria, fungi, and autolytic enzymes requires an ambient, osmotically balanced aqueous environment. Dehydration arrests, to some extent, the degradation of the gross morphology of the tissue by inhibiting the degradation of its chemical constituents. The rate of this process can depend on climate (frozen or desiccated remains; Hansen et al., 1985; Allison, 19851, pH and anoxia (bog environment; Royal and Clark, 19601, or intra-corporeal osmolarity (artificial mummification with natron; Lucas, 1932).