How long does eDNA last?

You may wonder if your samples are a year or two old whether there will still be any DNA left to test. Usually the answer is yes. In fact, DNA is a surprisingly stable molecule. Under the right circumstances it can hang around for millions of years. Which begs a second question. Can I be sure the eDNA I'm looking at really is telling me about the environment I want it to, or is it really old and come from somewhere else?

This week we have published some research on this topic in PNAS (https://www.pnas.org/doi/10.1073/pnas.2508402123) in which we have studied sedimentary ancient DNA (sedaDNA) from the bottom of the North Sea. This was once a forested landscape, until about 7 thousand or so years ago. Amazingly, we found that the landscape was forested a good deal earlier than was previously thought, meaning that it was a suitable environment for early Mesolithic peoples a good deal earlier than previously thought. Moreover, we are able to show when eDNA has been brought in from elsewhere, and when it has been deposited locally. It is a real stride forward in understanding the taphonomy of DNA. While this may seem interesting in an arcane sort of a way, it tells us a lot about how eDNA behaves in the current environment and landscape, so is as relevant today as it was 16,000 years ago!