Neanderthals and Cro-Magnon

My guess, based on a few undergrad courses in genetics and my wife's research in insect molecular phylogenetics? It's not so much that DNA sequences become "corrupted" - nucleotide substitution isn't, to my knowledge, chemically likely in the environment of "decaying" DNA. It's that the DNA strands break into pieces.But you can still PCR the pieces. Short strands of DNA are very stable. Assembling the whole strand from a random assortment of its pieces is a computationally intesnive, but not impossible, task. (Imagine if you had many copies of the same manuscript, each copy torn into tiny random pieces. For any two pieces that could be validly stuck together, there's probably a third piece from another copy that overlaps them, and tells you how they should be put together.)So, not like computer data on a hard disk slowly degenerating into random 1's and 0's. More like the leaves of a book falling out of the spine and into random order.

That's my guess. It's the exact same technique, by the way, that is used to sequence the contemporary human genome. It turns out that you basically have to cleave genomic DNA into small pieces anyway, just to be able to sequence it sooner than a few decades. The tests are usually always the same; PCR-RFLP or other kinds of tests. Basically, very simple ways to examine a single gene across multiple individuals and establish a phylogenetic tree of their relationships and shared ancestors. Offhand, I don't know. Maybe your best bet is to open a thread to discuss one of the really technical explanation, and ask the biologists here questions that will help it make sense to you.