I recently came across an article indicating that the half life of DNA in the most ideal situations is 521 years (http://www.nature.com/news/dna-has-a-521-year-half-life-1.11555). However, since human's DNA is 99.9% identical (and 98% identical to chimps), I can't quite understand how DNA evidence is often used in old samples. There has been DNA extracted from 100,000 year old Neanderthals, and DNA evidence has been used in cases over 50 years old.
However, with a 521 year half life, after only 0.75 years, there would only be 99.9% of the DNA left. Surely if 99.9% of a human's DNA is identical after only this short amount of time wouldn't it be impossible to distinguish any sample from any other? After 50 years only 93% would remain the same, meaning it would be much less similar to a human than just any chimp I would think.
And the 100,000 year old Neanderthal would only have 1.6*10^-57 % of the original sample left. How is there possibly any sort of useful information left?
All in all, how can DNA be distinguished from other samples when they are all so similar and the half life is so low?