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It wasn’t until the work of this remarkable scientist, John Doebley, that we began to understand the genetic changes that gave rise to modern corn from ancient teosinte.  Perhaps the most remarkable outcome of his genetic sluething, which started with the genetic analysis of teosinte-corn hybrids, one tiny ear of which is shown here in the middle.  Next to it is a fossilized maize cob recovered from the Ocampo Caves in Mexico and dated to almost 4000 years before the present.  What you can easily see is that already by thousands of years ago, the cob looks more like the multi-rowed modern ear than it does like the teosinte rachis.  Doebley’s molecular work and later his work with the evolutionary geneticist Svante Paabo, showed that the changes that converted teosinte to corn were very few -- perhaps no more than half a dozen major genes.  These investigators have traced these critical genetic events to the Balsas River Valley in Mexico and dated them on the order of 6-10 thousand years ago.  What is even more remarkable is that once this handful of mutations had been collected together, this suite of genetic modifications stayed together and spread very rapidly, so that the same group of genetic changes is detectable even in the Southwestern US, where they had already arrived 3000 years ago. 

http://teosinte.wisc.edu/

www.accessscience.com/ studycenter.aspx?main=1..