I remembered, after my last post, that there's an excellent book that puts domestication of plants and animals into a global perspective and asks, among other things, why it was europeans who got into building large overseas empires, not people from other parts of the world. It's Guns, Germs and Steel, by Jared Diamond (1998), & I recommend it; it's a really good read & very thought-provoking.
Anyway, back to the plants…
There's that date for wheat domestication, in Turkey, of 10,500 years BP (before present). What about the rest of the world? Well, there's rice in China at 8,000 BP; sorghum in Africa, 4,000 BP; bananas, 7,000 years ago in New Guinea; chili peppers (6,000 BP) and a variety of squash (10,000 BP) from South America; maize (8-9,000 BP) and another variety of squash (10,000BP) in Mexico; and sunflowers from 5,000 years ago in what's now the eastern US (Balter, 2007). In other words, although the Americas were settled considerably later than the Old World and Asia, agriculture seems to have developed there at much the same time.
And again, there is evidence that seems to show that domestication in the New World was a relatively slow process ie that people were cultivating plants quite some time before they had any strong selective impact on their crops. For example, Balter describes work by one research team on the genetic changes involved in the domestication of maize.
Maize was developed from a wild plant called teosinte, which looks completely different from modern maize. It has only a few kernels on each stalk, compared to the 500+ on an ear of maize, and each teosinte kernel has a tough coat that makes it hard to get into the starchy endosperm inside.
There are a number of genes that have been linked with the domestication of maize. They include tb1, affecting the number of stalks in each plant; pbf, which controls storage of protein; & su1, which influences starch storage, and may be necessary for using corn to make tortillas. The research team studied ancient DNA from corn cobs found in Mexico and New Mexico, and dated from 600 to 5000 years ago. All the ancient cobs had the domesticated version of tb1 and pbf, but only some had the domesticated su1 allele. The team concluded that, because this important allele was not widespread in maize until quite a lot later, complete domestication of maize could have been a drawn-out process.
Those dates provided by Balter also beg the question: why did farming, and plant domestication, develop independently at so many different sites at pretty much the same time? Around 11,500 years ago the Earth was coming out of the last major glacial period – was the development of agriculture triggered by global climate change?
References:
M. Balter (2007) Seeking agriculture's ancient roots. Science 316: 1830-1835