Wednesday, May 20, 2009
Children of the Corn
Here to the north of Austin, we live in an odd patchwork of new neighborhood, business parks and shopping malls interspersed with open fields. Cattle graze in the field next to our supermarket, and corn grows cross the street from our bank.
Seeing the orderly fields of corn, I'd never realized that corn represents an intriguing mystery in regards to plant evolution and the history of humanity's interaction with the plants we live off of. First domesticated in Central America around 7000 years ago, corn as we find it today is a domestic-only plant in that it is virtually incapable of reproducing in the wild.
One of the characteristics of corn that makes it such a useful crop is the incredibly high return of kernels harvested to kernels planted. Biologically, one of the reasons for this abundance is that unlike other grasses which have been domesticated as agricultural grains, the corn cob forms halfway down on the plan, closer to sources of water and nutrients, and thus the plant is able to put more energy into seed growth. In other cereals, the seeds are at the very top of the stalk, at the plant's farthest extremety.
Another great feature of corn is that the cob is covered by a husk, which largely protects the grain from pests. It pretty much requires a creature with opposable thumbs to get the husk off, which means you loose less of the grain prior to harvest. Plus, the kernels are well-rooted into the cob, as compared to grains like wheat where the ripe seeds can easily fall from the ear of grain.
However, all of this -- particularly the firmness of the kernels in the cob and the husk covering it -- means that if there are no humans to harvest the corn, very, very little of it will succeed in naturally reseeding. If a cornfield were abandoned before harvest and you returned in five years to see if any wild corn was left growing, you would probably find few to no corn plants.
This means that corn as we find it today must be biologically fairly different from the corn ancestor which Central Americans first found in the wild and domesticated. The predominant theory out there is that corn is descended from the grass called teosinte which is found in Mexico even today, but the differences between the two plants are extensive, though there is enough genetic similarity to make it pretty clear they are related. Teosinte grains is far out on the extremities of a banching stalk, the grains are covered by hard outer covering (like the chaff of wheat), the grains are not strongly rooted in a cob-like structure, and they are not covered by a husk that remains closed.
The National Science Foundation has a nice comparison here:
The prevailing theory at the moment is apparently that teosinte underwent a series of major mutations during a very short period of time which resulted in the corn we see today. I find that a bit unsatisfying, since series of major, conventient, stable mutations are hard to come by. Thus I was interested to find this article about Prof. Mary Eubanks of Duke University, who has been working on the theory that corn as we know it today is the result of multiple hybridizations between teosinte and another wild grass called tripsacum. She's developed a hybrid of tripsacum and modern corn which exhibits many of the properties of the ancient ears of corn dating back 5000+ years that have been found in caves in Mexico. Apparently she has pretty decent genetic evidence for this as well by now.
While I'm not remotely an expert, I must admit to finding the hybridization explanation somewhat more convincing on the face of it than the sudden large mutation explanation. And I had never realized that corn was so interesting.