Domestication of legumes

The distinction between cultivation and domestication of the most common edible plants is controversial. Some favor the opinion that a regime of tilling, sowing and reaping (cultivation) acts as a selective force on wild plants, selecting for mutations adapted to the new environmental conditions15,16. Others claim that mutations that are favorable for agriculture (domestication) must have been selected before the crop could be successfully cultivated17,18. The mutations typically include reduced seed dormancy, the loss of dispersal mechanisms (indehiscent pods)18, reduced seed coat thickness19 and increased seed size10. The first two mutations do not leave visible traces on the legumes, but wherever large stocks of legumes are found, plants with domestic-traits (non dormant/non dehiscent) must have been used16. Furthermore, experiments conducted on wild modern pea, chickpea and lentil prove that neither harvesting of wild stands or cultivation of wild legumes results in profitable yields17,18,20. The thickness of seed coat should be thinner and smoother in domesticated stocks to facilitate water penetration and germination19. The validity of this mutation as a trait of domestication remains controversial, because no great differences were found between the domesticated species of lentil, bitter vetch, grass pea and their wild relatives21,22. Increase in seed size is considered to be one of the major domestication-traits in crops, but analyses conducted on archaeological legumes such as pea, lentil and cowpea show that the increase in seed size does not occur at the early stage of domestication but rather later as a result of crop improvements23,24.

Cultivation versus domestication of the faba bean

Scholars that invoke a protracted process of cultivation as an unconscious cause of domestication stress three mechanisms that should increase the seed yield: the seedling depth15, ploughing25 and selection of larger seeds for seeding26. It holds that seeds buried deeper by human planting develop larger reserves because they emerge from a greater depth27. In the case of the faba bean, greater burial depth does not increase seed size, while maximum yield is obtained when the seeds are buried no deeper than 8 cm. Faba bean, as well as chickpea and lentil, has an hypogeal germination, meaning that its cotyledon remains where the seed is sown while only the shoot emerges from the soil surface; as a consequence, the seed must be buried close to the surface to sprout above the ground28,29,30.

A negative correlation was also found between tillage and yields31. Agronomists agree that tillage does not help to maximize the faba bean yield once the crop is farmed under dry climatic conditions. The relatively shallow root system of faba bean relies on the water accumulated within the first 30–40 cm from the soil surface; therefore soils that have a more stable structure prevent water from percolating to greater soil depths. Another advantage of compacted, non-tilled soils is that residues of crops remain on the surface and this prevent excessive water evaporation, which is a major constraint for plants growing in dry and semi-dry environments32. The selection of larger seeds for seedling is commonly considered a profitable way to obtain plants that have bigger seeds and produce higher yields33. Thus a relation between seed yield and seed size anticipates that seed size should have a positive influence on seed yield. Nonetheless such a correlation has not been recorded for the faba bean. On average, medium and small size grains of faba bean produce maximum seed yield compared to larger seeds29,34. The explanation for such a contradictory result is that plants originating from smaller seeds produce a greater number of pods, which are, on average, longer that those produced by plants originating from large seeds. Plants originating from small faba beans sprout, flower, form pods and mature faster, leading to the highest harvest index values. Small-sized grains produce good yield over a range of seasonal conditions, while large seeds are more sensitive to adverse seasonal conditions such as drought and low temperature29.

The agronomic studies show that common practices associated to cultivation do not lead to higher yields. Therefore it is unlikely that cultivation acted as a selective force in the process of domestication of the crop. Other mechanisms must have been adopted in order to transform the faba bean into a crop. Seed dormancy (and pods dehiscence) is a typical trait of the wild progenitors of the major legumes (i.e. Lens culinaris ssp. orientalis, Pisum humile, Cicer reticulatum, Vigna radiata subsp. sublobata and Phaseoulus vulgaris), so it is safe to assume that unknown progenitor of faba bean would also have some mechanism to delay the germination.

The problem of dormancy in wild legumes can be overcome by soaking the seeds in water (or abrade the seed coat, increase the temperature etc) to encourage the germination of the legumes; thus, repeated use of free-geminating seeds would saturate the natural gene bank with non-dormant seeds in a relatively short time15. Another option includes the inadvertent selection of domestication traits by means of cultivation of wild stands of plants; this could also have resulted in the widespread introduction of non-dormant seeds in the natural gene pool, but only after 5–6 cycles of (unprofitable) cultivation18.

Regardless of whether the selection of the original seeds had been ‘conscious’, through the selection of non dormant seeds straight from the wild stands, or ‘unconscious’, by inducing phenotopic changes in the local gene pool, the loss of seed-dormancy is the only circumstances that enables the production of legumes in large quantities.

The presence of a large quantity of seeds in Ahihud and Yiftah’el reinforce the idea that non-dormant (and non-dehiscent) stocks were used to ensure reliable harvesting, resulting in the build-up of characteristic domestication traits. The size of the archaeological faba bean cannot be used as a domestication trait, because the Δ13C shows that size depends on the amount of water received by the plant. We therefore rely on other parameters to assess domestication. The presence of storage facilities found in Ahihud and Yiftah’el , as well as the presence of other crops in great quantities, such as the 7,2 kg of lentils found in Yiftah’el35, are consistent with the notion that the surplus was kept for seeding, to ensure the continuity of legume production and the sustainability of the settlers. It is worth noting that by the Early PPNB, cereals were already domesticated36. Within this context, the domestication of the faba bean represents an important additional step that settlers took to reduce the risk of famine.