Long and attenuated: comparative trends in the domestication of tree fruits

Abstract

This paper asks whether we can identify a recurrent domestication syndrome for tree crops (fruits, nuts) and track archaeologically the evolution of domestication of fruits from woody perennials. While archaeobotany has made major contributions to documenting the domestication process in cereals and other annual grains, long-lived perennials have received less comparative attention. Drawing on examples from across Eurasia, comparisons suggest a tendency for the larger domesticated fruits to contain seeds that are proportionally longer, thinner and with more pointed (acute to attenuated) apices. Therefore, although changes in flavour, such as increased sweetness, are not recoverable, seed metrics and shape provide an archaeological basis for tracking domestication episodes in fruits from woody perennials. Where available, metrical data suggest length increases, as well as size diversification over time, with examples drawn from the Jomon of Japan (Castanea crenata), Neolithic China (Prunus persica) and the later Neolithic of the Near East (Olea europaea, Phoenix dactylifera) to estimate rates of change. More limited data allow us to also compare Mesoamerica avocado (Persea americana) and western Pacific Canarium sp. nuts and Spondias sp. fruits. Data from modern Indian jujube (Ziziphus mauritiana) are also considered in relation to seed length:width trends in relation to fruit contents (flesh proportion, sugar content). Despite the long generation time in tree fruits, rates of change in their seeds are generally comparable to rates of phenotypic evolution in annual grain crops, suggesting that gradual evolution via unconscious selection played a key role in initial processes of tree domestication, and that this had begun in the later Neolithic once annual crops had been domesticated, in both west and east Asia.

Between domestication and civilization: the role of agriculture and arboriculture in the emergence of the first urban societies

“…In this light, given that morphological changes, as seen for cereals, are evident in the arboreal domesticates for which we have data, then early cultivation must have included the planting of stones and seeds. The patterns of change seen in stone size and shape support a protracted gradual morphological evolution for arboreal domesticates (see Fuller 2018 ), similar to that seen for the domestication of annual crops (see Fuller et al 2014 ). Such evidence corroborates an initial emphasis during the domestication process on sexual reproduction (see Goldschmidt 2013 ; Weiss 2015 ) through the planting of stones/seeds, while simultaneously implying a slow rise in the importance of perennial crops.…”

“…Morphological change in West Asian crops and Chinese rice, and soybean, have been published elsewhere (Fuller et al 2014 ; Allaby et al 2017 ) and are summarized from those sources in this paper. Datasets tracking the morphological change in some of the fruit species considered here, namely peach, date and olive, have also been published elsewhere (Fuller 2018 ).…”

“…Most significantly for some species, stone size and shape also changed, which can potentially be tracked from archaeobotanical material (e.g. Terral et al 2010 ; Liphschitz et al 2013 ; Pagnoux et al 2015 ; Dighton et al 2017 ; Fuller 2018 ).…”

“…In most fruits, increased seed length is achieved with minimal increase in overall embryo volume. Therefore, Fuller ( 2018 ) suggests that the lengthening of perennial fruit stones increases the total fruit flesh with minimal additional investment in seed volume, and thus infers some conscious element of selection for more edible matter in fruits which in turn increases seed length and the length:width ratio. Alternatively, as with cereals, if a number of stones, pips, or seeds were planted at the same time, then those with larger (more elongate) seeds that produce stronger seedlings might be selected through the removal of smaller, less well-established seedlings.…”

“…Despite these issues, the metrical data (Fuller 2018 ), combined with database evidence for both the earliest appearance and increased presence of fruit-tree crops, lead to the hypothesis that their domestication and incorporation into pre-existing patterns of agricultural land-use was a contributing factor in urbanization in Western Asia and China. While each region contained centres of domestication and subsequent urbanization (Maisels 1998 ), these processes are more spatially distinct in Mesopotamia than was likely the case in the Yellow River or Yangtze River (Fig.…”