(For a more detailed description see Supplementary information)
Plant cutting generates use-polish on the tool edge that -after some hours of use- can be observed macroscopically as a sheen that is called sickle gloss. Experiments of harvesting wild cereal growing in natural stands, cultivated wild cereals, domestic cereals, reeds and grass were carried out. Between six and ten areas with gloss (650x500 µm each) were scanned with a Sensofar Plu Neox confocal microscope. The 3D surfaces were sampled, extracting areas of 200x200 µm, which were processed and later measured with the Mountain 7 software, from Digital Surf. Several parameters of texture analysis defined in ISO 25178 were selected on the basis of their discriminant capacity for the five experimental categories. Quadratic discriminant function analysis was used to build a predictive model for group membership, based on the Bayes’ theorem. The discriminant algorithms obtained from the texture analysis of experimental tools were used for the analysis of glossed tools recovered in archaeological sites.
The discriminant function analysis consistently groups the surfaces of use-wear polish resulting from cutting the three cereal types (domestic, wild cultivated and wild in natural stands), reeds and other grasses. Significant mean differences (Wilks' Lambda) were observed for all the predictor parameters and for discriminant functions, with 73% of the 3D surfaces correctly classified. Using the threshold of 45% of correct classification of samples for considering each experimental tool as correctly classified, all the experimental tools can be considered as correctly grouped. To test the real predictive capacity of the method, we blindly classified half of the samples against the other half. Eighteen experimental tools were correctly classified, while two can be considered as indeterminate, in which the threshold of 45% of the samples is not reached for any of the five categories.
The archaeological tools were classified in one of the five categories of plant cutting tools when more than 50% of the 3D surfaces were classified in one of the groups, otherwise, they were classified as indeterminate (Table 1). The classificatory threshold was raised in the archaeological tools (50%) with respect to the experimental ones (45%) in order to reduce the rate of potential errors. For ensembles of tools, we considered the proportion of 3D surfaces classified in each of the five plant cutting groups (Figs. 2 and 3). The index of the degree of maturity of harvested cereals per level/site (Fig. 4) was calculated considering exclusively the results attributed to cereal harvesting in the three stages of maturity. The proportion of unripe harvesting was multiplied by three, unripe harvesting by two and ripe harvesting by one and the addition was calculated. Thus, 300 would be the index of a site with exclusive unripe cutting and 100 in another one with exclusive ripe cutting.
Gloss texture analysis finds significant differences between the tools used for cutting the five plant categories in terms of roughness, isotropy, wavelength, complexity, depth and density of furrows and slope of surface points (Table 2). The capacity of gloss texture analysis to separate the five plant-cutting activities is explained by the different mechanical characteristics of cereals, reeds and grass. Phragmites stems are 5-15 mm wide and rigid, cereal stems are 1.5-2.5 mm wide and more flexible, while Ampelodesmos leaves are 3-9 mm wide, very supple but tenacious. The degree of humidity in the stems/leaves is higher in reeds as they grow in swampy areas, medium for the while evergreen Ampelodesmos grass and lower for cereals when they are cut in the late Spring or Summer. The capability to discriminate the three types of cereal-cutting tools is probably due to different degrees of humidity present in the stems when reaped. When the aim of cereal cultivation is maximizing the yield, cereals are harvested in a state of maturity that is as advanced as possible. Morphologically domesticated cereals are usually reaped when they are ripe and the grains are fully mature. In contrast, wild cereal stands are commonly harvested before the plant reaches maturation, that is green or semi-green, because once fully mature, the spikelets start to disarticulate, leading to grain-loss48 17. Interestingly, our method shows clear differences between the experimental tools used to harvest wild cereals from natural stands in Syria and Israel and those used to cut wild cereals cultivated in Jalès (France). Texture analysis places the use-wear created when harvesting cultivated wild cereals in an intermediate position, between wild spontaneous and domestic cereal cutting tools. To explain this discrimination, we suggest several possible explanations. 1) Synchronous planting and harvesting may lead to more homogeneous ripening even after very few generations of cultivated cereals 49. In the experimental carried out in Jalès, wild cereals were sown very densely on rich soil, and the plots developed thick stands that matured uniformly 48. 2) These wild cereal cultivation experiments demonstrated that the most productive yield is obtained when crops are in a semi-ripe stage, which lasts few days. During this short period the seeds are more developed than in an unripe or green stage. Harvesting is thus more productive, yet at the same time, grain-loss is avoided, as the plants are not mature enough for the ear to disarticulate during harvesting 48 18. Besides, cultivated fields of wild cereals can be more thoroughly surveyed than natural stands, because they are located near villages, and thus they can be more easily harvested in a more advanced moment of maturity. 3) In experimentally grown natural stands, cereals flourished together with a mix of other plants that compete with them, including perennial grasses, while in cultivated fields the dominance of cereals is expected (by clearance of the field or weeding). Thus, when harvesting spontaneously growing wild cereal stands, the diversity of plants harvested is greater than in cultivated fields, and include green perennial grasses, which could affect gloss texture, increase roughness, complexity, density of furrows and slope of surface points, while decreasing isotropy and wavelength (see Table 2).
Our method enables a high confidence in the classification of cereal harvesting tools in three states of maturity: unripe, the state in which wild cereals in natural stands were harvested; semi-ripe, the condition in which experimental wild cultivated cereals were cut at Jalès; and ripe, the state in which domestic cereals were reaped. However, for archaeological tools, it is not possible to automatically equate the degree of humidity of cereal when harvested with a specific step in the process towards agriculture (i.e. unripe cutting=harvesting wild cereals in natural stands). Ripe stems of wild cereals could be harvested for feeding the livestock or for technical activities such as basketry or for roofing houses. Similarly, domestic cereals can be cut in a semi-ripe state for making freekeh, a kind of roasted snack where the taste of the unripe grain is sought. Environmental humidity/dryness can influence the degree of moisture of cereals when harvested. Another issue is the possibility that one sickle could have been used for cutting different types of plants. In this case, texture analyses would result in an indeterminate classification of the tool or in the classification of the tool in the group of dominant use. These factors have to be considered when interpreting the archaeological data. However, when information of gloss texture analysis is considered in conjunction with the archaeobotanical information, it is possible is to evaluate how plant harvesting systems changed with the development of plant cultivation and domestication. In this work, we interpret the evidence by assuming that people in the past were trying to achieve the highest yield, which was most probably the goal during the origins of agriculture as this is a prerequisite for explaining the process of genetic selection leading to domestication. The evolution towards riper cereal harvesting from 12,800 to 7,000 cal BC both in Northern and Southern Levant strongly suggests that these changes in harvesting strategies were related to crop management shifts that took place during the transition to agriculture.
Data Availability
The datasets generated during and/or analysed during the current study are available in the Digital CSIC repository, http://hdl.handle.net/10261/225522