There was a time scientists were on a race to see what the oldest organism fossil DNA was and they came up with one. Let me quickly tell you the story. Some over 120 million years ago, in the conifer-filled forest of present India, a weevil walked into a pool of sticky resin, and it was trapped and fossilized there for millions of years which scientists later found in the year 1993, in their search for the most Ancient DNA. They opened the amber and were able to find DNA samples from the insect and at the time, it was the oldest organism to have had its DNA sampled, or so we thought until we learned something else.
Finding DNA from the fossils of some organisms is very possible but it is not usually perfect and it is practically impossible in a lot of fossils that we have found at the moment. Talking about DNA replication, the 1993 weevil wasn't the first ancient DNA to be replicated, rather it was that of the Quagga which was similar to a Zebra and went extinct in the 19th century. Its DNA was tested from a sample of the Quagga's dry muscle of a museum specimen. This replication was done in the year 1984 and scientist were able to identify when the subspecies of the plain Zebra diverged from the mountain zebra which was about 4 million years ago.
With the discovery, over the years scientists began to use soft tissues that were preserved in things like Permafrost, ice, amber, or were mummified, in other to get and replicate DNA and amber seems to be the best place to check as it traps the organism as a whole and goes further to dehydrate the DNA thereby making it stable while preventing microbe activities since it possesses antimicrobial properties.
Although scientists were able to replicate the DNA of several organisms, we cannot deny the fact that the degrade and today we know that DNA has a half life, so how long does it take for a DNA to degrade to the point where it can no longer be read? With DNA half life, it marks the amount of time it will take until half of the DNA id degraded beyond use. Identifying the half life of a DNA depends of the organism and the preservation method that was used to keep the organism such as in the DNA of unicellular organisms reaching its half life in 15,000 years, birds like the MOA have an half life of 521 years, and so on.
Each DNA that decays, breaks into smaller fragment that cannot be read easily. So, to get a great part of DNA from an organism, they should be preserved in ideal environments like in Cold temperatures such as under ice, closed environment such as the inside of a bone, and so on. Scientist have also seen that there is no completely ideal environment as even in the best of environment to preserve the organism thereby protecting its DNA, an organism's DNA would degrade beyond readability when it reaches about 6.8 million years. With technological improvement, we can now read smaller strands of DNA, combining them together to give a larger strand of DNA.
You must have seen that in he beginning, I mentioned a weevil with a DNA of about 120 million years ago but then I mentioned in the post that even in the best of environment, 6.8 million years is the best that we can go. So what could have gone wrong. The fault of this reading came with scientists while using PCR (Polymerase Chain Reaction), and since it amplifies the smallest DNA information, it also has the likelihood of replicating things not needed causing the genetic code to overwhelm the sample.
This was exactly what happened in the case of the weevil and in the 2000s when the labs were properly controlled and cautions taken, the sample from the weevil was taken again to check its DNA but nothing was found and scientist noticed that the result they got in the past was from a modern fungus that got into the sample and it wasn't old at all. If we are going to get proper DNA from fossils, then it would depend on the type of fossils, and proper care to prevent contaminants so the wrong thing isn't read.
Reference.
https://www.scientificamerican.com/blog/guest-blog/jurassic-park-and
https://www.cell.com/current-biology/fulltext/S0960-9822(03)00923-
https://pubs.geoscienceworld.org/gsa/geology/article/44
https://www.researchgate.net/profile/George-Poinar/publication/14693297
https://www.nature.com/articles/363536a0
https://royalsocietypublishing.org/doi/10.1098/rstb.1991.0093
https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0170940
https://www.jstage.jst.go.jp/article/pjab1977/65/10/65_10_229/_article
https://www.the-scientist.com/half-life-of-dna-revealed-40361
https://royalsocietypublishing.org/doi/10.1098/rspb.1997.0067
https://www.tandfonline.com/doi/full/10.1080/20548923.2016.1258824
https://scielo.org.za/scielo.php?script=sci_arttext&pid=S0259-01902015000100013
https://pmc.ncbi.nlm.nih.gov/articles/PMC9956214/
https://pmc.ncbi.nlm.nih.gov/articles/PMC3497090/
https://pmc.ncbi.nlm.nih.gov/articles/PMC2694912/
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