Over the weekend, I read a fascinating paper published in Geology, the peer-reviewed journal of the Geological Society of America. The authors of the paper report on their attempts to extract DNA from the sediments found at the bottom of the Bering Sea. They were successful in their attempts to find DNA, even in sediments that are supposed to be 1.4 million years old! This is a rather surprising discovery, since the half-life of DNA in bone is supposed to be about 521 years. In fact, the experiments done to obtain that half-life indicate that even at very low temperatures, no meaningful DNA could be extracted from bone after 1.5 million years.
So why is it surprising that the authors of this study found DNA in sediments that are supposed to be “only” 1.4 million years old? There are at least two reasons. First, wet sediment should accelerate the decay of biological molecules (including DNA) compared to the inside of a bone. Thus, DNA should become undetectable much earlier in wet sediment. Second, their results indicate that there would still be DNA found in much “older” sediments, because the “older” the sediment in their study, the more the DNA seem to resist decay.
But wait a minute. This is wet sediment. All manner of microorganisms can infiltrate wet sediment. How do we know that this DNA has really been in the sediment since it was formed? Couldn’t the DNA they detected be from microorganisms that recently started living there? That’s one of the clever aspects of this experiment. The authors looked at DNA that is found in the chloroplasts of cells that do photosynthesis! Since such organisms require light to survive, they wouldn’t have any reason to infiltrate the dark sediments. Even if they did get into those sediments for some reason, they would die in the upper layers, so the deeper (and therefore older) sediments definitely would not harbor any recent microorganisms that had such DNA.
In the end, it seems to me that they really did extract from the sediment DNA that had been there since that sediment had formed. That’s interesting enough, but there is something even more fascinating about what the authors found.