Archaeological dating 2.0
The premise of archaeological dating is that you can put objects into sequence, estimate dates for the sequence and then apply those dates to other, similar objects. It all depends on (a) getting the sequence right and (b) getting the dates right. No problem, the dates for new objects fall into the right places and confirm the system.
Such circular reasoning fell apart many times when C14 arrived. For instance rustic Western European passage graves must be derived from the magnificent Egyptian pyramids so add on a few years to allow for the idea to be spread and you get more recent dates for the passage graves. Oops, the passage graves turned out to be older.
The premise of this DNA technique is that you can put mutations into sequence, estimate rates of mutation and get dates out of them. The dates will confirm each other brilliantly. Can anyone spot the similarity?
The advantage of C14 is that it depends on a measurable quantity, the half life of the isotope*. Just like the traditional archaeological dating systems this DNA dating depends on assumptions. Like many other fields of archaeological research DNA's value will come from combining it with a better dating technique such as C14, much as C14 itself was improved by applying the better technique of dendrochronology.
* It does, in fact, depend on a couple of assumptions: freedom from contamination and the constancy of C14 concentrations in the atmosphere. The first is a problem which DNA shares, of course. The solution is clean up techniques to get as close as possible to the component of the sample which is meaningful. It was something we were looking at when I was working in C14 dating half a century ago and something which exercises my successors in forensic science to this day in regard to DNA. The second is calibration against tree ring sequences which reminds me that dendrochronology is another dating technique which has come into its own since Libby invented carbon dating over 70 years ago.
