Bronze Age farmers turned to a hardy crop with new relevance for modern farming

Millet reached Bronze Age Poland earlier than thought, and local farmers may have used it to cultivate land where wheat and barley struggled.

When a new crop turns up in the archaeological record, the easy reading is that new people brought it with them. In Bronze Age Poland, the first millet-eaters had been there all along. They grew up on wheat and barley, then took up the new grain as adults.

Margaux Depaermentier and Giedrė Motuzaitė Matuzevičiūtė of Vilnius University led the study. Their team read the chemistry of bones and teeth from Bronze Age graves across Poland.

Millet didn't arrive with newcomers. It arrived as people began farming ground they had not used before.

In an interview with Earth.com, Motuzaitė Matuzevičiūtė was asked what the new grain made possible. "It let them farm land where wheat and barley could not grow well or grow at all," she said.

Millet arrived earlier than known

Millet runs photosynthesis differently than wheat and barley do, and the carbon in its grain ends up heavier. Anyone eating enough millet builds that same heavy carbon into their own bones and teeth. Botanists call it a C4 crop; wheat and barley are C3.

Teeth finish forming in childhood and then stop changing. Bone rebuilds itself for as long as a person lives. The team sampled both from the same skeletons, so each person's childhood diet could be set beside what they ate near the end of life.

One woman buried at Pielgrzymowice, in southern Poland, ate no millet as a child. Her first molar, finished by age 10 at the latest, records wheat and barley alone. Her bone, rebuilt through her last years, records millet.

She died between the ages of 40 and 50, and her remains date to about 1590 BCE. That is more than a century earlier than anyone had evidence for.

The previous earliest date, about 1460 BCE in a 2021 study, came from the same cemetery. The oldest charred millet grains dug up anywhere in Poland are younger still, close to 1300 BCE.

"If you only look for charred seeds, you systematically date the arrival of a crop too late," Motuzaitė Matuzevičiūtė said.

In all, the team had usable diet data for 171 people, buried over roughly 1,200 years. Most of them, 124, ate no millet that the chemistry can detect. The other 47 ate at least some of it at some stage of life.

So millet never took over the region. Some communities took it up, some ignored it, and inside the ones that did, it reached some people and not others.

At almost every site, the earliest people buried ate no millet at all. The first to eat it at a site were usually the ones who changed diets partway through life.

"The other genuinely striking thing is that we can watch it happen within single lifetimes: six people demonstrably shifted from a C3 childhood diet to millet in adulthood," said Motuzaitė Matuzevičiūtė.

Furthermore, "eight substantially increased their millet intake, and one individual actually moved away from millet over their lifetime," she added.

Local farmers adopted millet first

Strontium answers a different question, relating to where the food grew. The element passes from rock and soil into crops, then into the people who eat them. The ratio differs from one patch of geology to the next.

People eating millet ended up with a different strontium signature than their wheat-and-barley neighbors, and with less of the element overall.

"The obvious expectation was that millet arrived with migrants - a new crop carried in by a new population," she told Earth.com. "Yet, our data contradicts that."

At Pielgrzymowice, the two earliest millet-eaters in the region were the woman and a second person buried about 120 years later. The strontium in their teeth matches the ground around the site, and both grew up eating wheat and barley.

She read the split between the two groups as new fields on poorer soil, rather than a swap inside the land these communities already farmed.

Millet crossed social divisions

At Koszyce, Bocheniec, and Pielgrzymowice, millet-eaters and non-eaters were buried in the same collective graves, and nothing in the burials separates them. Men and women ate the same way, as far as the chemistry can tell, and so did adults and children.

The elaborate graves at Dacharzów, where people did eat millet, closely resemble those at Żerniki Górne, where none of the people tested had eaten it.

"And when we looked for a social pattern in who ate it, there was none," she said.

Geography did separate the two groups. Seven of the 15 sites in the study had millet-eaters, and those are mostly up on the higher ground, while the ones without are down in the river lowlands.

Six people buried together in a single grave at Pielgrzymowice do stand out in the published DNA, and their strontium is unusual too. They were not the earliest millet-eaters at the site, and not the earliest strontium outliers either.

Climate role remains uncertain

Asked by Earth.com whether a 3,500-year-old crop switch holds any lesson for farmers now, Motuzaitė Matuzevičiūtė started with what her team cannot say. Millet spread while the climate and the population were both changing, and the data can't yet show which pushed which.

"Adoption is slow, and availability is not adoption," she said.

The crop had been in the region, eaten here and there, for at least a century before it became a staple around 1400 BCE. New grains have arrived elsewhere without changing food traditions for generations.

Two other limits are easier to name. For most people in the study, only childhood tissue survived, so a late-life switch wouldn't appear at all. And nobody has measured how much strontium a millet plant takes up compared with wheat.

Millet could aid modern farming

Farmers first domesticated millet in northern China about 8,000 years ago, and it crossed Eurasia from there. The crop fed this part of Europe for centuries.

It's now missing from most European fields and most European meals, while millions of farmers in Asia and Africa still grow it.

The United Nations named 2023 the International Year of Millets, pitching a family of grains that grow fast on poor soil with little water.

Motuzaitė Matuzevičiūtė said millet farming could expand onto drier, poorer soils that suit bread wheat and barley less well, and that this could be one way to feed a growing population.

"Given what farmers are facing, that is a crop worth a second chance," she said.

Whether a drying climate drove any of it is still an open question. The team's own fix is more radiocarbon dates from these graves, read against climate records from the same river valleys.

The full study was published in the journal Science Advances.

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