Indo-Europeans Ate Lots Of Millet

Food history can be an excellent indicator of cultural migrations and shifts in climate over time.

There is a limited area in Europe where so called C4 plants like millet can grow. And, in those areas of Eastern and Central Europe, isotope data from ancient human remains demonstrate, consistent with archaeological evidence, that there was a shift from so called C3 plants to C4 plants in people's diets between the early and middle Bronze Age. Millet made up at least 40% of the average person's diet after the shift.

This was right around the time that Indo-Europeans arrived in these areas and the millet eaten in Europe at the time appears to be native to the European Steppe where Indo-European languages originated.

C4 plants include warm climate grasses like maize, millets, sorghum and sugar cane, among others, but maize did not arrive in Europe until after 1492, and sorghum and sugar cane don't grow well in Europe.

Millet consumption is also supported by historical accounts according to the paper cited:

Pliny the Elder (Naturalis Historiae XVIII, 83-84) and Columella (De Re Rustica, 2, 9, 14–16) who report the use of millet flour for the production of bread and a sort of porridge cooked in water and salt and often accompanied with vegetables and cheese and very rarely with meat. Pliny (XVIII.XXIV) exactly noted: “millet is used to prepare a very white puls (i.e. similar to present-day polenta). Panic, when ground and freed from bran, and millet as well, makes a porridge which, especially with milk, is not to be despised even in time of plenty”. Columella (2.9.19), agreed with Pliny and wrote: “bread is made of millet, and it may be eaten without distaste before it cools”.

The researchers "performed an isotopic study to unveil the dietary habits of the Celtic population Cenomani Gauls, from the necropolis of Seminario Vescovile in Verona (Italy) dated between the 3rd to 1st century BC." But, "other isotopic results from bone collagen analysis of some individuals proceeding from the Bronze Age necropolis of Olmo di Nogara, Verona (1600-1200 cal. BC) and of Arano di Cellore, Verona (2040-1890 cal. BC) support the hypothesis of C4 plants consumption, and in particular of millet in some areas of the Verona province."

Plant remains also support this hypothesis. As the paper explains:

Paleobotanical studies have shown the presence of domesticated C4 plants (in particular broomcorn or Panicum Miliaceum L. and foxtail millet or Setaria Italica L. Beauv.) in Central and Eastern Europe as early as 5000-4000 BC for Panicum Miliaceum and Late Bronze Age for Setaria Italica; while in Asia (China) and in the Caucasian region (Georgia) this is documented as early as ca. 10000 years B.P. Based on the botanical evidence, it seems that millet was first introduced into Central and Eastern Europe from the Steppe regions during the Neolithic period. Broomcorn millet seems to have arrived in the Mediterranean zone from the north or the north-east; in northern Italy, millet appears in early Bronze Age settlements (1700-1500 BC). The earliest records of foxtail millet (Setaria Italica L. Beauv.) come from Peigang and Cishan (North China) in the 6th millennium BC. In Europe the first carbonized seeds of foxtail millet appear in the 2nd millennium BC, from Bronze Age settlements in central Europe and France. It is also reported from the late Bronze age Kastanas site (Macedonia, Greece). In the Near East the earliest evidence for foxtail millet cultivation dates back to the Iron Age (c. 600 BC) in Tille Höyük, south-east Turkey. However, these data only indicate the presence of millet, but do not reveal their role in the human diet. 

C4 plants are typical of warm climates and their abundance is highly correlated with climatic factors, such as temperature, precipitation and the degree of aridity. In general, they are not present in environments where night temperatures are lower than 8 °C. Nowadays in Europe C4 biomass is very scarce; however these plants are naturally present in small proportions especially in the SW-Europe (e.g. Spain, Portugal, SW-France and Italy). Before the onset of agriculture, herbivores collagen isotopic δ13C values from S-Italy (32.6 to 13.3 ka B.P.) suggest that C4 biomass was practically absent (calculated vegetal biomass is around −25‰ vs V-PDB). Even during the warmer periods of the Late Miocene, fossil isotopic data indicate the absence of C4 biomass in central and southern Europe. Man has introduced certain species, such as millet, with a planting strategy in the warm season (i.e. summer), possibly as a response to Holocene arid periods. . . . . 

[T]he investigated necropolis is located in an area which is still today characterized by a fertile plane (called Pianura Padana or Po valley): it offers ideal climatic and environmental conditions for the cultivation of C4 plants during the warm seasons (spring-summer). Pliny the Younger (Epistulae, IV, 6,1) praised the Po Valley region and wrote “in regione Transpadana summa abundantia”. Since the Middle and Late Bronze age (1550-1170 years BC) this plane was characterized by intensive agriculture, pastoralism and the demand of large amounts of wood for building sites. This lead to a heavy deforestation of the whole Po plain and converting it into an artificial steppe devoted to cereal crops. Furthermore, paleoclimate records of lake sediments and speleothems show trends towards a drier climate, or alternating wet and dry climates, between 2500 and 2000 BP. This might have induced a change in cropping strategies and the consequent introduction of C4 plant cultivation (probably millet). 

Unfortunately, during the archaeological excavation of Verona, archaeologists have not found seeds (or charcoal) and the ceramic grave goods (vessels and others) are still under study and initially seem not to indicate macroscopic remains of organic waste. Some previous archaeobotanical studies of Neolithic, Bronze and Iron Age sites of the NE of Italy, and particularly of the area of Verona, seem to confirm the presence of Panicum Miliaceum and of Setaria italica among the carpological remains. Another comparative paleobotanical study relating to some Iron Age sites located between the Northern and Southern Alps (Eastern Switzerland, Austria and Northern Italy), also describes the presence of millet (Panicum Miliaceum L.) and foxtail millet (Setaria Italica L. Beauv.) remains. This is also confirmed by the results of the study about carpological remains found at the site of Oppeano (Verona), dated to the second Iron Age (about 6th to 3rd century BC) and located in the same geographical context of the Seminario Vescovile necropolis. The majority of the determined cereal remains correspond to millet (Panicum miliaceum L.) and foxtail millet (Echinochloa crus-galli L. Beauv. and Setaria italica L. Beauv). Thus, the Iron Age seems to be characterized by a greater crops specialization that fits with the assumption of the pursuit of a cereal type that best fits different climates (biomass increase with more efficiency in water use). In this case of the sandy Pianura Padana (Verona), the crops of millet were preferred, because it is a plant that adapts very well to poor substrates characterized by water scarcity during summer.