关键词: 古代DNA；PCR；考古学；人类学；污染DNA；污染控制；污染识别

Abstract: With the advent of the PCR technique in molecular biology [20], DNA can now be extracted and analyzed from ancient remains [1, 51]. Ancient DNA studies hold great potential for anthropologists and archaeologists to address many important issues that cannot be well dealt with in conventional ways [5,9—14, 16—17]. However, the field is siill full of technical and interpretive challenges [28]. The most difflcult is proving that amplified DNA is authentic ancient DNA. The high risk of contamination is due to the fact that ancient DNA is highly degraded and only minute amounts are preserved,while the PCR technique is extremely sensitive and can easily pick up tiny amounts of contaminant DNA [7]. Contamination controls and detection therefore become extremely important in ancient DNA studies. This paper will discuss some practical guidelines that can be used to carry out effective contamination controls and detection in order to obtain authentic ancient DNA.
Dedicated Laboratory for Ancient DNA Studies
A dedicated laboratory is required for ancient DNA extraction and other pre-PCR work [28]. It is crucial to physically separate pre-PCR and post-PCR work [5]. The laboratory and all equipment should be dedicated to ancient DNA work. No modern DNA work should ever be carried out in the dedicated ancient DNA laboratory. Ideally, the pre-PCR laboratory should have UV-filtered ventilation system and positive pressure airflow. Sterile disposables and filtered tips should be used. Gloves, masks, boots and lab coats sliould be worn. 10 %bleach and UV light should be used to clean and irradiate the surfaces of benches and equipment to destroy contaminant DNA.
Selection and Decontamination of Ancient Remain for DNA Studies.
When selecting specimens for ancient DNA extraction, besides other criteria, the ease of decontaminating remains must be considered carefully since most remains excavated in the past had been contaminated by subsequent handling and analysis. There are several methods currently available for specimen decontamination [5, 30]. Physical methods remove the contaminated surface using sandpaper or electronic drills. Chemical decontamination uses chemicals such as 10 % bleach to damage and destroy surface contaminant DNA. Ultraviolet (UV) irradiation is another effective method for decontaminating specimens, reagents and other supplies [31]. UV can cause DNA to crosslink and preclude it from use in PCR amplification [7].
DNA Extraction from Ancient Remains
Selection of optimal DNA extraction methods and setup of blank extractions should be carried out in this step. Blank extraction should be used to monitor possible contamination of extraction reagents, commercial kits and the entire extraction process. Experiments that involve less steps or less human involvement should be considered advantageous.
PCR Amplification of Ancient DNA
The great difficulty in the amplification of ancient DNA is due to physical and chemical degradation of DNA templates [22, 51]. Ancient DNA can only be extracted in minute amounts, with small fragments and is often associated with PCR inhibitors, therefore, protocols for ancient DNA amplification must be optimized accordingly. Shorter target DNA fragments Should be sought since extracted DNA is usually less than 300 bp. The shorter the target fragment, the more templates will be potentially available for amplification. Obviously, with older remains, the difficulty to amplify longer fragments increases [22]. Thisfact can be used in the authentication of ancient DNA. Both negative and positive controls should be setup along with ancient DNA samples for PCR amplification [38]. Positive controls can be used to indicate whether PCR conditions are set up correctly and negative controls including blank extracts will show amplification products if contamination occurs. Multiple negative controls should be setup in order to more effectively monitor contamination [7]. For ancient human mtDNA,we have found that multiple quantified positive controls should also be used to indicate the sensitivity of individual PCR amplification and the level of contamination if it occurs [38].
Quantification of Ancient DNA Templates
Accurate estimation of the number of ancient DNA templates is of great assistance in determining whether amplified DNA is from authentic ancient DNA since greater numbers of ancient DNA templates result in a diminished likelihood of contamination [19, 43]. Competitive PCR can be used for the quantification of ancient DNA [39], but the estimated amount of templates may also include contaminant DNA. Another method has been proposed to examine the preservation state of ancient DNA through amino acid racemization analysis [42]. Although the analysis cannot produce any accurate estimation of ancient DNA templates, the preservation state can clearly indicate the possibility ofextracting DNA from very ancient remains such as fossils.
Electrophoresis, Sequencing and cloning PCR Products and Sequence Analysis
Once ancient DNA is amplified, it can be treated as any modem DNA sample would be and no special laboratory or equipment requirements are needed. Electrophoresis of multiple positive and negative controls should be used to quickly examine whether contamination occurs and the level of contamination if it occurs. Sequencing results can be of assistance in detecting contamination. For example, a DNA sample from one individual usually only contains one mtDNA sequence. A good indication of possible contamination is the presence of more than one type of mtDNA sequence or if the same type of mtDNA sequence is detected from many unrelated individuals.
PCR products can be cloned to determine the number and percentage of different types of sequences present in PCR products. For more ancient remains such as fossils, cloning should be carried out since it is not only good for detecting contamination but also very useful in reconstructing an authentic ancient DNA sequence. When the number of DNA templates is extremely low and DNA itself is highly degraded, incorrect nucleotides may be incorporated into the synthesis of new DNA molecules and generate incorrect DNA sequences [43], or prematurely terminated DNA fragments may jump from one template to another and produce chimeric DNA sequences(jumping PCR) [44]. These amplification errors are generally random and can be detected though cloning and repeat experiments.
Obtained ancient DNA sequences must make a phylogenetic sense [19] and or at very least should not contradict genetic rules and patterns. Otherwise, contamination should be suspected. For example, dinosaur DNA should be more similar to reptilian DNA than to mammalian DNA. In humans, one individual should only have two copies of nuclear DNA fragments. Special attention should also be given to the presence and detection of nuclear mtDNA amplification [45] and mtDNA' s heteroplasmy [46]. When these occur, they may complicate the detection of contaminant sequences.
Reproducibility Test and Ancient DNA Authentication
The reproducibility test is an integral part of ancient DNA research [43]. Replication of the entire ancient DNA processes should be undertaken to examine whether the same results can be obtained and is a requirement for the authentication of ancient DNA. Authentic ancient DNA and contaminant modern DNA have different “behavioral patterns” in the test. If DNA is authentic, the same DNA should be extracted, amplified and sequenced from different bones of the same individuals, in different laboratories and by different groups of researchers. Thus, it should be expected that different repeats generate the same DNA sequence. However, due to its random nature, contaminant DNA generally fails in reproducibility tests.
The purpose of ancient DNA authentication is the analysis of all contamination controls, laboratory procedure' s and amplified DNA sequences to demonstrate that extracted and amplified DNA is authentic ancient DNA and not contaminant modern DNA [51]. Strict contamination controls are required but they cannot guarantee contamination-free results. There are also no absolute physical, chemical and biological criteria one can use to determine DNA' s antiquity. Thus, there is no way to directly authenticate ancient DNA based only on the DNA itself.
Though we cannot directly determine whether amplified DNA is authentic or contaminant, logically, we can exclude one source and indirectly prove the other. Compared to the scarcity of ancient DNA templates, contaminant DNA is much more plentiful, making contamination with modern DNA an inevitable reality in ancient DNA studies. Therefore, one must analyze the possibility of contaminant DNA first before one can accept the result as authentic ancient DNA. Negative, positive controls and DNA sequence analyses are all capable of indicating contamination. Each individual control may not have a strong power in excluding contamination. When all controls and analyses do not indicate any contamination, statistically, there is likely no contamination.

Key words: Ancient DNA; Archaeology; Anthropology; Ancient remains; PCR; Contamination; Contamination controls and detection; Authentication