The analysis of DNA from ancient material is a recent technological advance that has produced and promises further important scientific insights. It has also inspired the popular book Jurassic Park, by Michael Crichton, and the film of the same name. DNA is, fundamentally, a chemical that can survive after the death of the organism that carried it. It is a relatively stable chemical compared to the proteins found in the cell, although it can be fragmented and destroyed by chemical processes, such as oxidation in air, and biological processes, such as digestion by enzymes produced by bacteria. If DNA can be extracted from ancient specimens of dead material, then, in theory, it can be analyzed in the same manner as DNA from living organisms. The first DNA to be extracted from an ancient specimen was from 150-year-old tissues from the quagga, an extinct relative of the zebra. This was soon followed by extraction of DNA from a 2400-year-old Egyptian human mummy. Both studies used reasonably large amounts of dead tissue to extract the DNA and cloned fragments of the DNA in a vector, and so future studies were limited by the amount of dead tissue available from any specimen.
The major advance in ancient DNA analysis was the development of the polymerase chain reaction (PCR). This allowed specific amplification of DNA sequence from tiny amounts of starting material--theoretically one molecule of DNA. This allowed amplification from older specimens that had heavily degraded DNA with only a few intact molecules surviving. PCR also removed the problem of contamination by DNA from fungi or bacteria growing on the specimen, because PCR can be made specific for a certain sequence that would only be found in the species of interest. However, the power of PCR introduces another problem, contamination with modern DNA. For example, if DNA was amplified from the mummified human mentioned above, how is it known the DNA was from the mummy and not from a stray hair or skin cell of the researcher? The only solution to this problem is to use a series of laboratory precautions, and conduct the amplification of ancient DNA and the analyses in several independent laboratories.
Several early studies suggested that ancient DNA could be amplified by PCR from weevils and termites trapped in amber between 30 and 135 million years old. This inspired the Jurassic Park idea of extracting dinosaur DNA from mosquitoes trapped in amber. However, these reports were based on single specimens, and could not be repeated. A later study attempted to extract DNA from the large collection of insects trapped in amber at the Natural History Museum in London. They failed to produce reproducible results. Additionally, attempts to amplify DNA from younger specimens preserved in pine resin (the precursor to amber) were not successful. Occasionally, some DNA was amplified but analysis of the DNA sequence showed that it was not from the species of insect used. The fact that amber is permeable to air suggests that all the DNA had been oxidized and destroyed in the samples, and the original studies were amplifying contaminant DNA.
After these problems became clear, scientists were sceptical about any work involving ancient DNA. However, rigorous laboratory procedures and independent analysis in different laboratories have made ancient DNA studies scientifically valid, although the work has always been on much less ancient samples than the 135 million year old weevils. Neanderthals, named after the valley in Germany where the fossils were discovered, were a group of humans adapted to the cold who lived in northern Europe between 130000 and 30000 years ago. Until recently, it was not known whether Neanderthal humans were direct ancestors of modern humans or not. However, rigorous analysis of the sequence of mitochondrial DNA extracted from these fossils and comparison with modern human mitochondrial DNA sequence showed that they were not direct ancestors but cousins--members of the hominoid family that had become extinct.
As well as human evolution, evolution of other animals can be studied. DNA from mammoths has been amplified, sequenced and compared with modern elephants. The evolution of the cave bear and brown bear has been studied by analyzing both modern DNA samples and ancient DNA from fossils. Several studies have also shed light on the origins of infectious diseases. For example, DNA from the bacteria that causes tuberculosis has been amplified from a 1000-year old mummy of pre-Columbian American woman, showing that she probably died from tuberculosis and that disease was present in the New World before colonisation by Europeans. Ancient DNA studies can also contribute to the understanding of more recent history. In France, the king Louis XVII died as a 10-year old boy in Paris in 1795. However, rumours remained that he had escaped and lived in exile. One individual, who died in 1845, claimed he was the exiled Louis XVII, and his mitochondrial DNA was extracted and compared with that of known living maternal relations of Louis XVII. The DNA sequence did not match, and a later study on the mitochondrial DNA extracted from the preserved heart of the 10-year old boy who died in 1795 matched the mitochondrial DNA of the maternal relations of Louis XVII, showing that the heart was from Louis XVII.
As younger and younger specimens are used, the technology essential for reliable DNA analysis of specimens becomes more important for forensic scientists. Many of the precautions that were learned from the mistakes of the early ancient DNA investigators are now routinely used in extracting DNA from specimens from a modern crime scene.
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