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In the NYTimes news article No Bones About It: Scientists Recover Ancient DNA From Cave Dirt I've read:

Although DNA sticks to minerals and decayed plants in soil, scientists did not know whether it would ever be possible to fish out gene fragments that were tens of thousands of years old and buried deep among other genetic debris.

I haven't read the original article yet, but I'd like to double check, this is bare DNA molecules, stuck to minerals or other detritus in 14,000 to 550,000 year old compacted dirt? Roughly speaking at least? In layman's terms, just laying around in some caves?

Has something protected the DNA from degradation, or is DNA just more environmentally stable and robust than I thought?

There is also discussion in the Science commentary No bones? No problem: DNA left in cave soils can reveal ancient human occupants.

The NYTimes article goes on to say:

The new study involved searching for ancient DNA in four caves in Eurasia where humans were known to have lived between 14,000 and 550,000 years ago.

Dr. Meyer and his colleagues figured out which DNA in the cave sediment was prehistoric by looking for telltale signs of degradation at the ends of the molecules.

What are the "telltale signs of degradation at the ends" of DNA molecules described in the paper?

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  • $\begingroup$ need help choosing proper tags... $\endgroup$
    – uhoh
    Apr 27, 2017 at 22:33

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To be clear they are not finding long DNA strands but short fragments. DNA becomes more robust the shorter it gets. Fragments can last millions of years. The local chemical conditions have a huge influence on how long DNA lasts the oldest being hundreds of millions of years old under ideal conditions.

The ones the archeologists are finding are not even close to enough to rebuild an intact strand but it can be used for matching. Of course contamination is a big problem, anything that dies leaves DNA in the soil. With humans it becomes especially problematic becasue the archaeologists themselves are a major source of contamination.

For an indepth look at contamination protocols, some of which as still questionable, can be found here. Sorry its behind a pay wall.

Likely they are referring to deamination cytosine substitution, which creates mismatched pairs of DNA and occurs more and more as DNA ages.

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  • $\begingroup$ OK I'll get a hold of the paper soon. When you say millions of years, is that necessarily when found in marrow enclosed in an unbroken bone? Does deamination cytosine substitution refer to the type of process described here: en.wikipedia.org/wiki/Deamination#Cytosine ? $\endgroup$
    – uhoh
    Apr 28, 2017 at 1:25
  • $\begingroup$ "Fragments can last millions of years": citation needed. I'm not saying you're wrong, I just find that surprising (though I am no biochemist so that doesn't mean anything) and would like to read a reference backing it up. $\endgroup$
    – terdon
    Apr 28, 2017 at 17:06
  • $\begingroup$ sciencedirect.com/science/article/pii/0016703794901856 As they mention DNAhas been recovered from amber preserved insects many times. amber vastly retards DNA decay that's why finding a Dinosaur tail preserved amber is so exciting! In the end DNA decay is not like radiometric decay with a largely unaffected rate, local chemical conditions have a huge influence. $\endgroup$
    – John
    Apr 29, 2017 at 0:35
  • $\begingroup$ @uhoh not necessarily, although depending on the local chemical conditions it may be possible. DNA decay in heavily influenced by local chemistry it's not like nuclear half lives. $\endgroup$
    – John
    Apr 29, 2017 at 0:42
  • $\begingroup$ OK this is great - Thank you for adding Amino acid racemization in amber-entombed insects... This will make great reading! $\endgroup$
    – uhoh
    Apr 29, 2017 at 3:37

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