Final Report Summary - EOA (The Evolutionary Origins of Agriculture)
This project brings together experimental ecology, molecular biology, and archaeobotany to gain a better understanding of the selective pressures driving cereal and pulse crop domestication in Western Asia.
Through experimental ecology, we established that domestication is characterized not only larger seed size, loss of dispersal mechanisms and germination inhibition, but also by other plant traits such as an increase in the size of the whole plant. Seedling growth is also faster in large-seeded crop progenitors than in other wild species. Grown individually, the total seed yield of wild crop progenitors was no greater than that of other wild species, indicating no intrinsic yield advantage to adopting large-seeded progenitor species for cultivation. When grown with other plants, however, crop progenitors produced higher yields than other wild species due to a smaller reduction in tillering (branching), suggesting that, under intensive cultivation, these species would respond well to densely packed growing conditions. We also found that, in response to high CO2 levels equivalent to the Holocene end-glacial period, crop progenitors showed a greater increase in above-ground plant size (leading to a greater number of seeds) than other wild species, and that grain yield was better correlated with final plant size than initial seed size. These observations suggest that selection during domestication may have acted primarily on the growing or mature plant rather than its seed. This opens up the possibility that domestication was driven by unconscious selection for the ability of plants to thrive under favourable conditions rather than by deliberate human selection for larger seeds or greater grain yield.
It was thought that a crop such as barley is descended from a small number of wild plants that were taken into cultivation by the first farmers. If this were correct, then the crop would display much less genetic diversity than the wild species from which it was derived, and this loss of diversity would be expected to affect all parts of the plant’s chromosomes. The discovery that domesticated barley has not undergone this genome-wide loss of diversity opens up new possibilities for the study of the events involved in domestication. Computer simulations that model the domestication of a grass species showed, however, that the selection of domestication traits occurred so close together in time that the order in which they occurred cannot be inferred from analysis of living plants. We concluded that the only way to study this would be to sequence ancient DNA preserved in archaeobotanical remains from the relevant time periods. On the other hand, by sequencing genes involved in control of seed size and root growth we showed that there had been selection for a gene involving seed size during domestication. As our ecological experiments indicate that seed size is closely related to (and potentially genetically linked with) plant size and seedling growth, this points the way to investigation of a broader range of genes responsible for other, above-ground, plant traits.
Numerical analysis of archaeobotanical samples from across Western Asia, in relation to archaeological context, has led us to question the evidence for exploitation of a broad spectrum of food plants by pre-agricultural communities prior to the domestication of crops. From our charring experiments on modern cereal grain, we have devised measures for estimating pre-charring grain mass from post-charring dimensions, and have used these to trace increases in grain size both chronologically and geographically. This suggests that overall cereal grain size may not have increased as rapidly, or as consistently, as previously thought. Morphometric analysis has also improved our ability to discriminate between (crop and progenitor) species that have proved difficult to identify on the basis of traditional archaeobotanical and morphological criteria (such as wild einkorn and rye).
Through experimental ecology, we established that domestication is characterized not only larger seed size, loss of dispersal mechanisms and germination inhibition, but also by other plant traits such as an increase in the size of the whole plant. Seedling growth is also faster in large-seeded crop progenitors than in other wild species. Grown individually, the total seed yield of wild crop progenitors was no greater than that of other wild species, indicating no intrinsic yield advantage to adopting large-seeded progenitor species for cultivation. When grown with other plants, however, crop progenitors produced higher yields than other wild species due to a smaller reduction in tillering (branching), suggesting that, under intensive cultivation, these species would respond well to densely packed growing conditions. We also found that, in response to high CO2 levels equivalent to the Holocene end-glacial period, crop progenitors showed a greater increase in above-ground plant size (leading to a greater number of seeds) than other wild species, and that grain yield was better correlated with final plant size than initial seed size. These observations suggest that selection during domestication may have acted primarily on the growing or mature plant rather than its seed. This opens up the possibility that domestication was driven by unconscious selection for the ability of plants to thrive under favourable conditions rather than by deliberate human selection for larger seeds or greater grain yield.
It was thought that a crop such as barley is descended from a small number of wild plants that were taken into cultivation by the first farmers. If this were correct, then the crop would display much less genetic diversity than the wild species from which it was derived, and this loss of diversity would be expected to affect all parts of the plant’s chromosomes. The discovery that domesticated barley has not undergone this genome-wide loss of diversity opens up new possibilities for the study of the events involved in domestication. Computer simulations that model the domestication of a grass species showed, however, that the selection of domestication traits occurred so close together in time that the order in which they occurred cannot be inferred from analysis of living plants. We concluded that the only way to study this would be to sequence ancient DNA preserved in archaeobotanical remains from the relevant time periods. On the other hand, by sequencing genes involved in control of seed size and root growth we showed that there had been selection for a gene involving seed size during domestication. As our ecological experiments indicate that seed size is closely related to (and potentially genetically linked with) plant size and seedling growth, this points the way to investigation of a broader range of genes responsible for other, above-ground, plant traits.
Numerical analysis of archaeobotanical samples from across Western Asia, in relation to archaeological context, has led us to question the evidence for exploitation of a broad spectrum of food plants by pre-agricultural communities prior to the domestication of crops. From our charring experiments on modern cereal grain, we have devised measures for estimating pre-charring grain mass from post-charring dimensions, and have used these to trace increases in grain size both chronologically and geographically. This suggests that overall cereal grain size may not have increased as rapidly, or as consistently, as previously thought. Morphometric analysis has also improved our ability to discriminate between (crop and progenitor) species that have proved difficult to identify on the basis of traditional archaeobotanical and morphological criteria (such as wild einkorn and rye).