New research from the University of Sheffield, UK, has shed light on how hunter-gatherers adopted agriculture and how crops were domesticated to depend on people.
Domesticated crops have been transformed almost beyond recognition in comparison with their wild relatives — a change that happened during the early stages of farming in the Stone Age.
For grain crops like cereals, the hallmark of domestication is the loss of natural seed dispersal — seeds no longer fall off plants but have become dependent on humans or machines to spread them.
The new research, led by Professor Colin Osborne, from the Grantham Centre for Sustainable Futures at the University of Sheffield, shows the impact of domestication on vegetable seed size.
“We know very little about how agriculture began, because it happened 10,000 years ago — that’s why a number of mysteries are unresolved. For example why hunter-gatherers first began farming, and how were crops domesticated to depend on humans,” Prof. Osborne said.
“One controversy in this area is about the extent to which ancient peoples knew they were domesticating crops. Did they know they were breeding domestication characteristics into crops, or did these characteristics just evolve as the first farmers sowed wild plants into cultivated soil, and tended and harvested them?”
Any selective breeding of vegetables by early farmers would have acted on the leaves, stems or roots that were eaten as food, but should not have directly affected seed size.
Instead, any changes in vegetable seed size must have arisen from natural selection acting on these crops in cultivated fields, or from genetic links to changes in another characteristic like plant or organ size.
In the last instance, people might have bred crops to become bigger, and larger seeds would have come along unintentionally.
Prof. Osborne and co-authors gathered seed size data from a range of crops and found strong evidence for a general enlargement of seeds due to domestication.
They discovered domesticated maize seeds are 15 times bigger than the wild form, soybean seeds are 7 times bigger.
Wheat, barley and other grain crops had more modest increases in size (60% for barley and 15% for emmer wheat) but these changes are important if they translate into yield.
“We gathered together seed size data for lots of modern crops and living examples of their wild relatives,” Prof. Osborne said.
“Across seven vegetable species we found strong evidence for a general enlargement of seeds due to domestication. This is especially stunning in crops like potato, cassava and sweet potato, where people don’t even plant seeds, let alone harvest them.”
“It is hard to think of any reason why people would breed large seeds in these crops. Instead, larger seeds in these species surely arose unintentionally.”
He added: “this finding has important implications for crop evolution, meaning that major changes in our staple crops could have arisen without deliberate foresight by early farmers, with unconscious selection more important in the genesis of our food plants than previously realized.”
The research is published in the journal Evolution Letters.
Thomas A. Kluyver et al. Unconscious selection drove seed enlargement in vegetable crops. Evolution Letters, published online May 9, 2017; doi: 10.1002/evl3.6