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Herein, I discuss the limits and circumstances of DNA preservation in old fossils. The stability of DNA in physiological solution suggests that DNA should be degraded at the latest within 50 000–100 000 years and has convincingly been found in fossil bones of this age. I argue here that particular fossilization conditions can cause exceptional DNA preservation over much longer periods, in an unexpected state. A working hypothesis is proposed stating that DNA can ‘hibernate’—that is, be preserved over long periods—only if it is hidden within molecular ‘niches’ where it is not governed by the rules of aqueous solution chemistry, and that such ‘geological’ DNA cannot be purified using procedures that assume that it behaves like ‘biological’ DNA.