Commensal domestication pathways amongst plants: exploring segetal and ruderal crop origins

Abstract

Two commensal pathways to plant domestication-ruderal and segetal-have been proposed. These domestication pathways are detailed here, together with associated archaeobotanical morphometric data for multiple crops within each pathway. The ruderal pathway characterizes how plants adapted to anthropically disturbed habitats, which can be associated with foraging or farming communities, were domesticated by people. Ruderal crops discussed are squash (Cucurbita pepo), aji chili (Capsicum baccatum) and melon (Cucumis melo). The segetal pathway characterizes how weeds in agricultural contexts became crops. Segetal crops discussed are rye (Secale cereale) and kodo millet (Paspalum scrobiculatum). Metric archaeobotanical datasets are used to infer the domestication episode for crops and to calculate rates of change in domestication traits (Haldanes). Although metric archaeobotanical data limits presentation and discussion to seeds, it enables quantitative comparisons of domestication episodes and haldane rates with those of the grain and fruit tree domestication pathways, respectively. We conclude that early ruderals underwent slower domestication processes, whereas segetals and perhaps some later ruderals, underwent faster processes of domestication that probably involved conscious selection.This article is part of the theme issue 'Unravelling domestication: multi-disciplinary perspectives on human and non-human relationships in the past, present and future'.

Keywords: archaeobotany; conscious selection; evolution rate; phenotypic change; unconscious selection; weeds.

Figure 1.

Conceptualizing segetal and ruderal pathways along a spectrum from lesser (forager) to greater (farmer) habitat shaping, following alternative domestication trajectories in terms of evolutionary rate and degree of consciously directed selection.

Figure 2.

Archaeobotanical seed size data plotted against median age for Cucurbita pepo from Mexico (data: electronic supplementary material, table S1).

Figure 3.

Archaeobotanical assemblage seed size (diameter/length) data for Capsicum baccatum, indicating two alternative domestication trends (early/slow versus late/fast).

Figure 4.

Archaeobotanical seed size data from Cucumis melo. In the left graph, seed length is plotted against median age estimate, with indication of inferred domestication episode in the Lower Yangtze region of China. In the right graph, seed width is similarly plotted with arrows indicating early large, and hence domesticated, examples from early Egypt and the Indus region that support three independent regional domestication processes; also labelled are late occurrences of inferred wild-type or weedy melons from India and Japan.

Figure 5.

Archaeobotanical grain size data plotted against median age for Secale. In the left graph, an outgroup baseline from Pre-Pottery Neolithic (PPN) southwest Asia (Syria); in the right graph, measurements from European archaeological assemblages with an inferred domestication episode indicated in the first millennium BC.

Figure 6.

Archaeobotanical grain size data plotted against median age for Paspalum scrobiculatum in archaeological assemblages from India and Sri Lanka, indicating inferred domestication episode. The left graph plots grain width, the right graph plots elliptical area calculated from grain length and width.