DNA in Charred Wheat Grains from the Iron Age Hillfort at Danebury, England

Article excerpt

The genetic history of wheat is the story of the world's temperate staple food. Archaeologically, charred grains are the common way wheat is preserved. Study of burnt spelt wheat from the British Iron Age shows DNA is present, and begins to shows the wheat's character.

DNA and ancient wheat

Genetic change, always central to studies of prehistoric agriculture, still operates as the principal criterion for designating certain plant and animal taxa as 'domesticated' (Harris & Hillman 1991) rather than wild. To this end, much bio-archaeological research into early agriculture has examined the gross morphology of plant and animal remains for phenotypic features that on the one hand seem to correspond to agricultural selection pressures, and on the other possess evidence of a direct genetic basis. However, it has frequently proved difficult to consolidate these two requirements. Features relating to larger, more accessible fruits and seeds, or to smaller, more docile animals, have often been judged as resulting from selection, but in many cases the genetic basis to the trait is uncertain. In contrast, phenotypes with a clear genetic foundation, such as the ploidy-dependent features of wheat chaff (Kislev 1984; Gordon Hillman pers. comm.), are often difficult to account for in terms of agricultural selection pressures. In other words, we have clear evidence of past phenotypic change consistent with what we assume to have been agricultural selection pressures, and we know in broad terms that many of those changes are due, directly or indirectly, to genetic events, but our existing methods constrain us from tying these two strands strongly together.

The discovery of preserved DNA in a range of archaeological materials could transform our approach. If the obstacles of survival, degradation, contamination and taphonomy can be overcome, then such DNA as survives in ancient plants and animals has the potential to provide a direct link between genotype, phenotype and the cultural context. Not only would this open the way to precision, it could also liberate our thinking from the gross 'genetic events' of 'domestication' to the perspective of a continuous evolutionary dynamic, in which the constant restructuring in human society through space and time is reflected in an equally continuous process of phylogenetic response, sometimes minuscule, sometimes substantial, in the plants and animals with which humans have been most closely associated.

We are attempting to start towards this goal with one of the most abundant archaeological resources of agricultural relevance: preserved wheat seeds. In archaeological contexts, plants are preserved principally by one of four mechanisms: the partial or complete reduction to carbon by heat; the exclusion of water in desiccating environments; the exclusion of oxygen in anoxic, often waterlogged environments; and partial mineralization by calcium phosphate, calcium carbonate or, less commonly with archaeological remains, iron sulphide (pyrites). We do not know which types of preservation might be compatible with retention of DNA, but intuitively one might expect biomolecular decay to be retarded in dry and/or anoxic settings. This conjecture is supported by reports of ancient DNA in anoxically preserved plant remains from Miocene deposits at Clarkia, Idaho (Golenberg et al. 1990; Golenberg 1991; Soltis et al. 1992), and in maize cobs preserved through various combinations of carbonization and desiccation (Rollo et al. 1991; Goloubinoff et al. 1993), the latter papers also demonstrating the phylogenetic inferences that are possible. Wheat and other Old World cereals have been encountered in all four preservation states, but the different states vary in their geographical and temporal coverages. Anoxically preserved and dry preserved cereals have sometimes been encountered in rich and impressive assemblages (e.g. Jacomet & Schlichterle 1984; Rowley-Conwy 1991), but the fullest spatial and temporal coverage is associated with carbonized remains: carbonized remains are therefore the principal source material on which an ancient DNA approach to wheat bio-archaeology must be based. …