Our keyholes for looking into the past are the cereal grains that people were growing and eating. It is astounding that these little pockets of information survive at all, let alone in quantities large enough for us to gain a picture of ancient agriculture. So how can these cereal grains remain intact in the ground more than 8000 years since they were buried?
The answer lies in forces that are largely thought of as destructive: fire and water. Some organic remains have been preserved having been submerged under water. These cases are pretty rare, and are mainly restricted to lakeshore sites surrounded by water. And of course shipwrecks are the preserve of maritime archaeologists who can find an incredible wealth of finds preserved after thousands of years.
More prevalent is preservation by fire. Some cereal grains would have dropped into open fires, escaping the hottest flames and turning not into ash but into crisp little blackened grains, like the popcorn that doesn’t pop. Even better for archaeologists is when entire storerooms of grains set ablaze, heating the grains to the relatively low temperature of 200 – 250˚C, preserving their identifiable shape but converting the juicy proteins and starch, so attractive to soil microorganisms, into more recalcitrant molecules. These molecules, known as melanoidins, are hugely complicated structures, which give coffee its smell and bread its lovely brown crust. They seem to be able to resist the action of enzymes and water in the soil that would otherwise break them down, converting them into the soil in which they lie.
There is still lots of work to be done to truly understand what happens to these grains during their thousands of years of burial. But it seems to be pretty certain that they retain most of their original carbon and nitrogen, which means that we can get to the original carbon and nitrogen isotopes of the grains when they were cultivated – as long as the heating process hasn’t changed them too much…