When did humans first domesticate plants? Well, humans living in what is now known as Turkey had domesticated wheat by 10,500 years ago. How can we be so sure of this date?
Wheat grows as spikelets, which carry the seeds, attached to a central stalk. When modern wheat is harvested, the spikelet breaks off the stalk leaving a jagged scar – it doesn't come away easily. This is a desirable trait for farmers, as otherwise the wheat seeds could simply fall off the stalk when someone brushed against them, and it's likely to have been selected for by early agriculturalists. And the remains of wheat spikelets from an archeological site in Turkey show just such scarring. Dated to 10,500 years ago, they're believed to be the earliest evidence of plant domestication – "the genetically determined physical and physiological changes a plant has undergone in response to human behaviour" (Balter, 2007).
Yet the earliest evidence for cultivation – from Egypt and elsewhere in the Near East – is rather older, at around 13,000 years ago. (And there's evidence of people using quanitities of wild wheat even earlier, perhaps 23,000 years ago.) Why the difference in dates?
Well, we need to remember that cultivating wild plants – growing them in fields – is a bit different from domesticating them. That definition of domestication implies a role for artificial selection – farmers choosing which plants to save seed from, based on some features of the plants that they viewed as desirable. In the case of wheat, this would include spikelets (& seeds) staying on the stalk, plus things like larger seeds. Farmers would also need to plant their selected plants away from the wild plants, as otherwise any attempts at domestication would be slowed right down by interbreeding between the two. (In those early Turkish fields, wild & domesticated wheat grew close together, as shown by a mixture of their seeds in the archaeological remains.)
And domestication of wheat certainly took a while, as shown by a comparison of the proportions of domestic (jagged-scar) and wild wheat spikelets from four sites in Syria & Turkey (Tanno & Willcox, 2006). At the 10,500 year-old Turkish site around 10% of the spikelets were obviously domesticated. This proportion increased to 36% at an 8,500 year-old site in Syria, and increased again to over 60% at another Syrian site, this one 7,500 years old.
So the process of domestication – of wheat, anyway – took at least a couple of thousand years. But when it was complete, it had an enormous impact on human cultural evolution in Europe and the Near East. As for the rest of the world… I'll come back to that next time.
M. Balter (2007) Seeking agriculture's ancient roots. Science 316: 1830-1835
K. Tanno & G. Willcox (2006) How fast was wild wheat domesticated? Science 311: 1886