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| Setaria italica grain variation: above, from Gansu; below, from Karnataka |
It is true that evolution never stops--a suitable point to reflect upon on Darwin's birthday (12 Feb 1809) Some authors, would therefore insist, that domestication processes never cease and are always unfolding (a key argument in the recent book by Robert Spengler, Nature's Greatest Success), but I think there are grounds for differentiating the initial domestication episode that separates cultivars from the their wild population and later diversification and regional adaptation that took place within geographical subsets of a crop. In a paper in 2009, "The nature of selection during plant domestication", we drew attention to some differences and different expectations between domestication and diversification [incidentally that paper was also published in February 12, the 200th birthday or Charles Darwin]. The expectations are that diversification may occur over shorter timescales, differ between regions, and may (but may not) involve some conscious selection, based on farmer preferences. As archaeobotanical datasets have become larger and from more regions it offers the potential to compare and contrast how crops were evolving and differentiating across regions-- at least in those traits we can observe on archaeobotanical material. As noted in a recent blog (Bigger Beans...) there is clear evidence for selection for larger seeded legume crops around the greater Mediterranean some time after the classical period. While Grasso et al. (2025) argue for intentional selection in southern Italian farmers of the Middle Ages for larger broadbeans, I suggested in a previous blog that this might not be so because of the striking parallels across lentil, chickpea, grasspea and broadbean for macro-seeded forms around a wider Mediterranean world. This coincidence suggests to me an unconscious process of parallel evolution/adaptation to something in the ecology of bean farming in Late Antiquity to the Middle Ages (perhaps heavier tillage and also less incidence of water stress).
Aspects of grain size and grain shape are easily measured, and indeed have been measured for many years. Last year, in a large compilation of grain metrical data from archaeological assemblages acorss Eurasia, together with Rita Dal Martello, Robert Spengler and other colleagues from the Max Planck archaeobotany lab, we considered Contrasting diachronic regional trends in cereal grain evolution across Eurasia: a metadata analysis of linear morphometrics from the ninth millennium BCE to today. This study considered barley, free-threshing wheat (presumablty predominantly hexaploid bread wheat), and the millets of Chinese origin, Panicum miliaceum and Setaria italica. All four of these crops have pan Eurasain distributions, with dispersals beyond their regions of origin beginning in the Neolithic and having largely reached their maximum distributions by the end of the Bronze Age. As these crops came into a region one can expect them to adapt to local conditions, both ecological and cultural. What we found that is most striking, however, is parallel trends in size/shape change in different crops in the same region, and often contrasting directions of change in different regions. While one might expect this with wheat and barley, that might be grown together in the same season, to find this across these cereals and millets is striking. The image above compares regional trends in grain length and width in free-threshing wheat, while the image below compares trends in the millets. Over the past 3000 years wheat and both millets tend to get longer in Central Asia; this is also true of both millets in East Asia. By contrast in northern Europe grains of all three species are becoming shorter over the same period. In Europe, wheats on average become shorter and fatter in north, but longer and skinner around the Mediterranean. In Central Asia wheat behaves more like southern Europe and the Near East.
This all points to different selective environments in different regions. Sometimes shared across crops. Previously smaller scale, and shorter timeline studies have suggested that as wheat moved east to China grains became shorter and plumper (Liu Xinyi's "virtue of small grain size"), perhaps to make them more millet-like for cooking in East Asian boiling traditions. It is not clear such patterns hold up. Would such an explanation also be suitable for the trends in post-Roman northern European wheat? More problematic still is that the large dataset in Dal MArtello et al. (2025) is that decreasing grain length is seen in wheat starting in central Asia around the same time as its arrival in East Asia. These regions we might expect to have different cooking traditions and different cultural selection environments? The trend reverses around 2000 years ago with grains again getting larger.
An unambiguous explanation for this is elusive. It could be that there were some environmental factors in each region acting across crops. These need not be directly on grain size either, as grain size may scale allometrically with selection on other aspects of size, like plant height. Shorter and smaller plants might be more resistent to lodging in some kinds of weather or more drought tolerant. For example the Indian shot wheat, Triticum sphaeroccum, has particularly short grains, as well as shorter plants, which seems to evolved twice in South Asia as adaptations to extremely high temperatures and evapotranspiration, even though sphaerococcoid grains reduce grain weight and overall yield (see Cheng et al. 2020).
The more general point is that crops have always been evolving and differ across regions. The foxtail millet (Setaria italica) grains at the head of this post contrast short plump grains from China (Lixian, Gansu) and skinner grains from India (Sanganakallu, Karnataka) both from the UCL archaeobotany reference collections. Explaining these differences may be hard, but what is clear that archaeobotany and systematic measurement of more assemblages can provide a window on tracking how regional populations varied across time.
