Based on the abovementioned radiocarbon-dated cereal grains from the northern Baltic, there was most likely a regional continuity of cereal cultivation in east-central Sweden during the EN and MN periods, c. 4000–2300 cal BC. Concurrently, we also see a pattern of expansion and suggest three periods for the spread of cultivated plants (Fig. 1). The first spread took place during the EN, when crops appeared around Lake Mälaren. A second spread occurred during the MN, when crops spread further into maritime locations on Åland, the outer islands of Lake Mälaren, and along the Norrland coast. The third expansion occurred during c. 1900–1250 cal BC, when cultivated plants reached eastern parts of mainland Finland.

FBC south and west of Mälaren practiced a mixed farming economy during the EN5,23. Though outside the study region, analyses of ancient DNA have recently shown that a FBC female dated to 3945–3647 cal BC (2 sigma) from southern Sweden had genetic affinities with Neolithic farmers in Central Europe18, which demonstrates that the spread of FBC in Sweden was, at least partly, due to migration, although local adoption of agriculture by Mesolithic groups has been suggested24. Unfortunately, no ancient DNA data are available for the EN in east-central Sweden, but here, as well as in southern Sweden25, the change in the material culture can be interpreted as indicating FBC farmer migration combined with interaction with local hunter-gatherer people23,26.

In the north-eastern archipelago of Mälaren, we do not find cultivated plants or domestic animals during the EN. While the material culture shows that FBC traits were present27, the overall material culture was most strongly affected by hunter-gatherer people of the northern Mesolithic and eastern Comb Ceramic traditions, and it is even possible that local people adopted ceramic manufacture from the FBC farmers28. Though cultivation is suggested by finds of cereal-type pollen29,30, no macrofossil remains of cultivated plants from the EN have been found on Åland, in Finland, or the Baltic countries31.

FBC, Comb Ceramic, and local Mesolithic peoples interacted during the EN in east-central Sweden. Interpretation of archaeological remains suggests that these groups formed a community wherein they transmitted practices requiring intense contacts, as exemplified by ceramic manufacture9,27, stone technology23, house plans24, ritual practices23, and even genes18. It has been argued that this exchange led to the material expression of the PWC in east-central Sweden from around 3500 cal BC27. Maritime subsistence and the hunter-gatherer lifestyle continued after the Mesolithic28; however, it appears that small-scale agriculture, suitable for this lifestyle, was inherited from FBC farmers, while PWC users predominantly derived genetically from hunter-gatherers16,17,18.

Former agricultural inland settlements west of Mälaren were abandoned during the Middle Neolithic A (MNA, 3300–2800 BC)27, and after this period cereal grains have been found only at coastal sites associated with the PWC (Supplementary Fig. 2). Cereal grain from Åland dated to the period and associated with the PWC shows that this group carried cultivated plants into places without any signs of earlier cultivation. Did the PWC people cultivate domesticated crops themselves, as suggested for western Sweden32 or acquire them from the FBC during the MNA?33 If we maintain that cereals and other agricultural products were exchanged with other groups, then they must have been brought from the FBC settlements. Such a scenario is unlikely because the FBC retreated from large parts of east-central Sweden and indications of exchange, such as flint imported from southern Swedish FBC to east-central Swedish PWC, greatly diminished8. That east-central Sweden was left out of the FBC network is also evident by the lack of megaliths, constructed in southern Sweden mainly during 3300–3000 cal BC (Fig. 1)34.

Rachis remains from free-threshing cereals, such as barley, are considered good indications of local cultivation because such cereals were commonly transported as threshed grains35. Barley rachis have been discovered at the MN Alvastra site (Supplementary Table 3), which speaks for local cultivation. Three barley rachis fragments have also been discovered at the PWC Sittesta site, with two of them having been found in MN stratigraphical contexts, again indicating local cultivation36. Thus far, no threshing remains have been discovered from Åland, but this could reflect the small-scale excavations and the limited number of studied samples. If we, however, hold that cereal grains found at the MNA sites (c. 3300–2800 cal BC) were acquired from the FBC, we must consider that the closest FBC sites of the time were situated in Alvastra and south and west of it (Fig. 1). The distance to Alvastra, associated both with the PWC and FBC, is 100 km from Åby, 200 km from Sittesta, and 370 km from Åland. These distances would make cereal transports, likely in canoes, challenging.

The crops of the earliest north European FBC consisted mainly of naked wheat, barley, and emmer37. Around 3750 BC, the focus shifted to emmer and barley37. This shift is discernible at the Stensborg and Alvastra sites, where emmer was the most common wheat species (Supplementary Table 3). Naked wheat was, however, still common at PWC sites and it, therefore, appears that PWC adopted crops from the FBC at the beginning of the EN.

During 2800–2300 cal BC, corresponding with the Middle Neolithic B (MNB), cereal grains have been found at coastal locations (Supplementary Fig. 3). Some argue that during the MNB, the PWC in east-central Sweden and Åland acquired cereals and other agricultural products from newcomers of the contemporary CWC33,38,39. CWC users inhabiting mainly inland locations in east-central Sweden interacted with the PWC, as demonstrated by the distribution of ceramic finds and similarities in ritual practices40. Cultivated plants at CWC sites in Finland were not discovered in the current investigation (Supplementary Results) or earlier studies41. In Finland, the keeping of domestic animals is indicated by the evidence of dairy lipids42 and mineralized goat hairs43. Charred remains and impressions of cultivated plants have been discovered at CWC sites in Estonia44 and east-central Sweden45 (Fig. 3: 12). In the eastern Baltic region, the earliest bones of domestic animals and a shift in subsistence occurred with the CWC46,47. Whether CWC produced the cereals and other agricultural products found at PWC sites is difficult to estimate because only small amounts of plant remains have ever been discovered at CWC sites. The CWC seemingly reached east-central Sweden from regions further to the east29, where there is evidence of animal husbandry, but only very few signs of plant cultivation.

Pollen indications of cultivation and animal keeping dated to the MN period have been found in east-central Sweden48. Several PWC settlements had sedentary characteristics, as shown by osteological materials from east-central Sweden and Åland49,50. These analyses demonstrate that at least parts of the population would have stayed at PWC sites during the crop growing season. Grindstones found from Åland (Fig. 2b) and eastern Sweden could have been used for processing agricultural products. Large accumulations of archaeological finds, pottery in particular, and robust hearth constructions (Fig. 2c) also point towards a substantial sedentary lifestyle, irrespective of the existence of outside camp sites. Small amounts of cattle and sheep bones from PWC sites in east-central Sweden50 and on the Åland islands38 (Supplementary Materials) further point towards contacts with agricultural groups, or to the small-scale keeping of livestock. Clearly, the Åland islands were too small to maintain a population of wild boars. It is therefore likely that pigs were brought intentionally to the islands by PWC people, though this must be confirmed by direct AMS-dating of the bones38.

Overall, arguments for a continuity in cereal grain dates, crop spectra similar to EN FBC, finds of barley rachis, diminished flint exchange between FBC and PWC users, the long distances between FBC and PWC sites, pollen from cultivated cereals in sediment cores, and fully sedentary settlements make it probable that PWC people cultivated their own crops in east-central Sweden and Åland during the MN period. On Åland, the availability and extent of cultivable soils could have been too small for plant cultivation and animal keeping prior to the PWC occupation. Land uplift around 3000 BC, during the occupation of the PWC sites Jettböle and Glamilders-Svinvallen, has revealed light tills and sandy soils, the extent of which permitted small-scale barley and wheat cultivation. Not surprisingly, our PWC sites are located close to them (Fig. 3b,c).

For the Late Neolithic (LN), cereal grains have been found north of Mälaren and along the Norrland coast (Fig. 1). In mainland Finland, the first cereal grains occur during the LN or Bronze Age, c. 1900–1250 cal BC. The earliest bones of sheep/goat from mainland Finland are earlier, dating back to 2200–1950 cal BC51. Finds of Scandinavian bronze artefacts indicate an influx from east-central Sweden, which might well be a source area for these agricultural innovations52. A similar development is found in the eastern Baltic region, where the earliest directly radiocarbon-dated cereals originate from the Bronze Age, 1392–1123 cal BC (2 sigma)31. Thus, agriculture was evident during the Bronze Age in the eastern Baltic, but at least animal keeping and probably crop cultivation were present earlier during the CWC phase.

So far, we have demonstrated that cereals were found at PWC sites in east-central Sweden and on Åland, this being the northern boundary of farming at that time. The cereals were, in all likelihood, cultivated locally by PWC, who might have preferred barley, which is suited to a harsh environment and could have given better yields, and therefore, been naturally selected over wheat species. There is evidence, however, that maritime resources were the main staples of their diet, as shown by finds of animal bones19, lipid biomarker analyses53, and studies of stable isotopes from human bones11,12. If not that important for subsistence, is it possible that cereals were significant for social behaviour, such as feasting.

It is common knowledge that communal consumption of food creates social integration and bonding, but also competition, essential for ritual feasts54. At feasts, special foods can be consumed; they differ from ordinary meals by having more participants and more food and drink55. Feasts could have been an arena for such social integration, bonding, and competition among PWC groups, but also across PWC and FBC. Feasts, in their different forms, have been discussed in numerous historical and ethnographical sources, which can also be used for identifying potential archaeological traces of feasting55. Such traces include, for example, unusually large amounts of food remains or food preparation structures accompanied by ritual activity indicators, special features, and human remains55. In particular, the large PWC cemeteries discovered on the island of Gotland show numerous features that can be related to ritual feasts. Here, the ritual significance of pigs is most clearly shown by finds of their remains within PWC burials. Up to 32 pig jaws were deposited in a single grave, nr. 7 at the cemetery of Ajvide, and they can be taken to represent the communal consumption of pigs during a feast (Fig. 5)56.

Figure 5 Pig remains in PWC burials on the island of Gotland, Sweden. (a) 32 pig jaws deposited at the feet of a seven-year-old child in grave nr. 60 at Ajvide61; photo by Göran Burenhult. (b) 19 pig jaws laid next to a burial of two adult men, and one 2.5–3-year-old child from grave nr. 7 at the cemetery of Ire62; photo by Greta Arwidsson, image from Riksantikvarieämbetet. (c) 5 pairs of pig tibia and 4 tusks, deposited in an 18–20-year-old man in grave nr. 6 at Ajvide61; photo by Göran Burenhult. Full size image

Another example of ritual sites associated with the late FBC and early PWC is the remarkable Alvastra pile dwelling (Fig. 1). Here, 100 hearths and numerous fire-making tools, tinder fungus, and numerous prestigious objects have been discovered8. At the site, bones from large domestic and wild animals, such as cattle, sheep, pig, and red deer, are common8. Alvastra has thus been interpreted as a place for cooking, feasting, cult practices, and assembly8.

Fewer graves have been discovered in east-central Sweden and on Åland, probably due to land uplift and acidic soils14. At Korsnäs, on Södertörn island (Fig. 1), large amounts of pig bones were discovered dating to the MNA and culturally linked to the PWC12. Stable isotopic analyses have indicated that these people had a maritime diet and that pigs were only consumed on special occasions – again, they can best be interpreted as the remains of ritual feasts12. Pig bones also occur at several other sites in east-central Sweden and Åland38,50, showing that pigs were available in this area.

As for cereals, thousands of their macrofossil remains have been found at Alvastra, and it is possible that they were intentionally burned together with wild plants, such as crab apples8. Our cereal finds might thus fall within the context of rare foods to be consumed on special occasions only. Ethnographical studies again provide the best explanation for how domesticated animals and plants that are not used daily were regarded as special food to only be consumed at communally enjoyed feasts55. In our case of PWC hunter-gatherers on Åland, who subsisted daily on fat-rich seals, fish, other maritime prey, and gathered plants, the cereals and the pigs were likely such rare specialities. Although not proven in our records, and thus far not tested scientifically, cereals might, however, be converted into alcoholic beverages to accompany rare solid food, thus creating another particular aspect of communal ritual feasting, one well known in ethnography.