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DNA Preservation

Fossil retrieval starts with selecting an excavation site. Potential excavation sites are usually identified with mineralogies of the location and visual detection of bones in the area. However, there are more ways to discover excavation zones like using technology such as field portable x-ray fluorescence and Dense Stereo Reconstruction, which helps with identifying excavation sites and aids the excavation process.(7) Tools used includes knives, brushes and pointed trowel which assist in removal of fossil from the earth. (9)

To obtain the most amount of DNA, specimens are handled with gloves and stored in -20°C immediately after unearthed, and it is best if the lab that analyse the sample is not used for mammalian species DNA.(4) (8) Bones are milled to a powder and treated with solution before the PCR process. (3) Samples for DNA amplification may not necessarily be fossil bones. Preserved skin, salt- preserved or air-dried, can be used if well-preserved or in certain situations.(6)

DNA preservation is hard because the bone fossilisation degrades and chemically modifies DNA, usually by bacteria and fungi in the soil. The best time to extract DNA from fossils is when it is freshly out of the ground as it contains six times the DNA when compared to bones stored for. Temperate of where the fossil is located also affect the amount of DNA we can obtain from it and it is shown by decrease success rate for DNA amplification as the fossil is found it warmer regions. A drastic change of a fossil environment also affects DNA preservation. Since excavation causes an abrupt change in the fossil environment, it may lead to physiochemical change and affect DNA preservation. Moreover,   DNA preservation is also affected by other factors such as the treatment of the unearthed fossil, such as being washing, brushed and sun dried, pH, irradiation, chemical composition of bone and soil, and hydrology...etc. There are three perseveration diagenetic phases. The first phase is bacterial putrefaction, which is estimated to cause a 15-fold degradation of DNA. Diagenetic phase 2 is when bone chemically degraded, mostly by depurination. The third diagenetic phases occur after fossil is excavated and stored. This is the phase where bone DNA degrades the fastest. (3)

Methods of DNA Extraction

Once a specimen is collected from an archaeological site, DNA can be extracted through a series of processes.[1]  One of the more common methods utilizes silica and takes advantage of polymerase chain reactions in order to collect ancient DNA from bone samples.[2]

There are several challenges that add to the difficulty when attempting to extract ancient DNA from fossils and prepare it for analysis. DNA is continuously being split up, while alive these splits are repaired, however, once an organism has died the DNA will begin to deteriorate without repair. This results in samples having strands of DNA measuring around 100 base pairs in length. Contamination is another significant challenge at multiple steps throughout the process. Often other DNA, such as bacterial DNA, will be present in the original sample. To avoid contamination it is necessary to take many precautions such as separate ventilation systems and workspaces for ancient DNA extraction work.[3] The best samples to use are fresh fossils as uncareful washing can lead to mold growth.[1] DNA coming from fossils also occasionally contain a compound that inhibits DNA replication.[4] Coming to a consensus on which methods are best at mitigating challenges is also difficult due to the lack of repeatability caused by the uniqueness of specimens.[3]

Silica-based DNA extraction is a method used as a purification step to extract DNA from archaeological bone artifacts and yield DNA that can be amplified using polymerase chain reaction (PCR) techniques.[4] This process works by using silica as a means to bind DNA and separate it from other components of the fossil process that inhibit PCR amplification.  However, silica itself is also a strong PCR inhibitor, so careful measures must be taken to ensure that silica is removed from the DNA after extraction.[5]  The general process for extracting DNA using the silica-based method is outlined by the following:[2]

  1. Bone specimen is cleaned and the outer layer is scraped off
  2. Sample is collected from preferably compact section
  3. Sample is ground to fine powder and added to an extraction solution to release DNA
  4. Silica solution is added and centrifuged to facilitate DNA binding
  5. Binding solution is removed and a buffer is added to the solution to release the DNA from the silica

One of the main advantages of silica-based DNA extraction is that it is relatively quick and efficient, requiring only a basic laboratory setup and chemicals.  It is also independent of sample size, as the process can be scaled to accommodate larger or smaller quantities.  Another benefit is that the process can be executed at room temperature.  However, this method does contain some drawbacks.  Mainly, silica-based DNA extraction can only be applied to bone and teeth samples; they cannot be used on soft tissue.  While they work well with a variety of different fossils, they may be less effective in fossils that are not fresh (e.g. treated fossils for museums).  Also, contamination poses a risk for all DNA replication in general, and this method may result in misleading results if applied to contaminated material.[2]

Polymerase chain reaction is a process that can amplify segments of DNA and is often used on extracted ancient DNA. It has three main steps: denaturation, annealing, and extension. Denaturation splits the DNA into two single strands at high temperatures. Annealing involves attaching primer strands of DNA to the single strands that allow Taq polymerase to attach to the DNA. Extension occurs when Taq polymerase is added to the sample and matches base pairs to turn the two single strands into two complete double strands.[1] This process is repeated several times, and is usually repeated a higher number of times when used with ancient DNA.[6] Some issues with PCR is that it requires overlapping primer pairs for ancient DNA due to the short sequences. There can also be “jumping PCR” which causes recombination during the PCR process which can make analyzing the DNA more difficult in inhomogeneous samples.

High-throughput sequencing (HTS) is an alternative to PCR with a few changes that make it more attractive for use with ancient DNA. HTS involves attaching a generic sequence to every single strand that generic primers can bond to. This makes it so that all of the DNA present is amplified. The DNA is also sequenced as it is amplified. This is generally more costly and time intensive than PCR but due to the difficulties involved in Ancient DNA amplification it is cheaper and more efficient. This allows for the sequencing of highly damaged DNA even when it is present in low concentrations.[6]

Methods of aDNA Analysis

  • Sequencing

DNA extracted from fossil remains is primarily sequenced using Massive parallel sequencing [7], which allows simultaneous amplification and sequencing of all DNA segments in a sample, even when it is highly fragmented and of low concentration[7]. One method of massive parallel sequencing, developed by Margulies et al., employs bead-based emulsion PCR and pyrosequencing [8], and was found powerful in analyses of aDNA because it avoids potential loss of sample, substrate competition for templates and error propagation in replication [9].

  • aDNA analysis

The most common way to analyze aDNA sequence is to compare it with a known sequence from other sources, and this could be done in different ways for different purposes.

The identify of the fossil remain could be uncovered by comparing its DNA sequence with those of known species using software such as BLASTN [9]. This archaeogenetic approach is especially helpful when the morphology of the fossil is ambiguous [10]. Apart from that, species identification can also be done by finding specific genetic marker in aDNA sequence. For example, the American indigenous population is characterized by specific mitochondrial RFLPs and deletions defined by Wallace et al. [11].

aDNA comparison study can also reveal the evolutionary relationship between two species. The number of base differences between DNA of an ancient species and that of a closely-related extant species could be used to estimate the divergence time of those two species from their last common ancestor [?]. Phylogeny of some extinct species, such as Australian marsupial wolves and American ground sloths, has been constructed by this method [12]. Mitochondrial DNA in animals and chloroplast DNA in plants are usually used for this purpose because they have hundreds of copies per cell and so are more easily accessible in ancient fossils[12].

Another method to investigate relationship between two species is through DNA hybridization. Single-stranded DNA segments of both species are allowed to form complementary pair bonding with each other. The more closely related species have a more similar genetic makeup, and thus a stronger hybridization signal.  Scholz et al. conducted southern blot hybridization on Neanderthal aDNA (extracted from fossil remain W-NW and Krapina) with ancient human aDNA, contemporary human DNA and contemporary chimpanzee DNA. The results showed weak ancient human-Neanderthal hybridization, strong ancient human-modern human hybridization, and the human-chimpanzee & neanderthal-chimpanzee hybridization are of similarly weak strength. This suggests that human and neanderthals are not as closely related as within a same species, but they are more related to each other than to chimpanzees [13].

There are also some practices to decipher the aDNA to provide valuable phenotypic information of ancient species. This is always done by mapping the aDNA sequence onto the karyotype of a well-studied closely related species, which share a lot of similar phenotypic traits. For example, Green et al. compared the aDNA sequence from Neanderthal Vi-80 fossil with modern human X and Y chromosome sequence, and they found a similarity in a 2.18 and 1.62 bases per 10,000 respectively, suggesting Vi-80 sample was from a male individual [9]. Other similar studies include finding of a mutation associated with dwarfism in Arabidopsis in ancient Nubian cotton [10], and investigation on the bitter taste perception locus in Neanderthals [14].

References

  1. ^ a b c Hagelberg, Erika; Clegg, J. B. (1991-04-22). "Isolation and Characterization of DNA from Archaeological Bone". Proceedings of the Royal Society of London B: Biological Sciences. 244 (1309): 45–50. doi:10.1098/rspb.1991.0049. ISSN 0962-8452. PMID 1677195.
  2. ^ a b c Rohland, Nadin; Hofreiter, Michael (July 2007). "Ancient DNA extraction from bones and teeth". Nature Protocols. 2 (7): 1756–1762. doi:10.1038/nprot.2007.247. ISSN 1754-2189.
  3. ^ a b Handt, O.; Höss, M.; Krings, M.; Pääbo, S. (1994-06-01). "Ancient DNA: Methodological challenges". Experientia. 50 (6): 524–529. doi:10.1007/BF01921720. ISSN 0014-4754.
  4. ^ a b Höss, M; Pääbo, S (1993-08-11). "DNA extraction from Pleistocene bones by a silica-based purification method". Nucleic Acids Research. 21 (16): 3913–3914. ISSN 0305-1048. PMID 8396242.
  5. ^ Yang, Dongya Y.; Eng, Barry; Waye, John S.; Dudar, J. Christopher; Saunders, Shelley R. (1998-04-01). "Improved DNA extraction from ancient bones using silica-based spin columns". American Journal of Physical Anthropology. 105 (4): 539–543. doi:10.1002/(SICI)1096-8644(199804)105:43.0.CO;2-1. ISSN 1096-8644.
  6. ^ a b Bouwman, Abigail; Rühli, Frank (2016-09-01). "Archaeogenetics in evolutionary medicine". Journal of Molecular Medicine. 94 (9): 971–977. doi:10.1007/s00109-016-1438-8. ISSN 0946-2716.
  7. ^ a b Bouwman, Abigail; Rühli, Frank (2016-09-01). "Archaeogenetics in evolutionary medicine". Journal of Molecular Medicine. 94 (9): 971–977. doi:10.1007/s00109-016-1438-8. ISSN 0946-2716.
  8. ^ Margulies, Marcel; Egholm, Michael; Altman, William E.; Attiya, Said; Bader, Joel S.; Bemben, Lisa A.; Berka, Jan; Braverman, Michael S.; Chen, Yi-Ju (2005-09-15). "Genome sequencing in microfabricated high-density picolitre reactors". Nature. 437 (7057): 376–380. doi:10.1038/nature03959. ISSN 1476-4687. PMC 1464427. PMID 16056220.{{cite journal}}: CS1 maint: PMC format (link)
  9. ^ a b c Green, Richard E.; Krause, Johannes; Ptak, Susan E.; Briggs, Adrian W.; Ronan, Michael T.; Simons, Jan F.; Du, Lei; Egholm, Michael; Rothberg, Jonathan M. (2006-11-16). "Analysis of one million base pairs of Neanderthal DNA". Nature. 444 (7117): 330–336. doi:10.1038/nature05336. ISSN 0028-0836.
  10. ^ a b Palmer, Sarah A.; Smith, Oliver; Allaby, Robin G. (2012-01-20). "The blossoming of plant archaeogenetics". Annals of Anatomy - Anatomischer Anzeiger. Special Issue: Ancient DNA. 194 (1): 146–156. doi:10.1016/j.aanat.2011.03.012.
  11. ^ Kolman, Connie J.; Tuross, Noreen (2000-01-01). "Ancient DNA analysis of human populations". American Journal of Physical Anthropology. 111 (1): 5–23. doi:10.1002/(SICI)1096-8644(200001)111:13.0.CO;2-3. ISSN 1096-8644.
  12. ^ a b Pääbo, Svante; Poinar, Hendrik; Serre, David; Jaenicke-Despres, Viviane; Hebler, Juliane; Rohland, Nadin; Kuch, Melanie; Krause, Johannes; Vigilant, Linda (2004). "Genetic analyses from ancient DNA". Annual Review of Genetics. 38: 645–679. doi:10.1146/annurev.genet.37.110801.143214. ISSN 0066-4197. PMID 15568989.
  13. ^ Scholz, Michael; Bachmann, Lutz; Nicholson, Graeme J.; Bachmann, Jutta; Giddings, Ian; Rüschoff-Thale, Barbara; Czarnetzki, Alfred; Pusch, Carsten M. (2000-06-01). "Genomic Differentiation of Neanderthals and Anatomically Modern Man Allows a Fossil–DNA-Based Classification of Morphologically Indistinguishable Hominid Bones". The American Journal of Human Genetics. 66 (6): 1927–1932. doi:10.1086/302949.
  14. ^ Lalueza-Fox, Carles; Gigli, Elena; Rasilla, Marco de la; Fortea, Javier; Rosas, Antonio (2009-08-12). "Bitter taste perception in Neanderthals through the analysis of the TAS2R38 gene". Biology Letters: rsbl20090532. doi:10.1098/rsbl.2009.0532. ISSN 1744-9561. PMID 19675003.
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