According to Ancients Manipulated Corn Genes, the grass-like teosinte plant become the domesticated maize plant when early farmers bred for useful traits — useful to humans, at any rate:
The ancestral plant of corn, teosinte, was first domesticated some 6,000 to 9,000 years ago in the Balsas River Valley of southern Mexico, the researchers said in this week’s issue of Science magazine. At first, teosinte was a grassy-like plant with many stems bearing small cobs with kernels sheathed in hard shells.By cultivating plants with desirable characteristics, farmers caused teosinte to morph into an increasingly useful crop. The researchers said by 5,500 years ago the size of the kernels was larger. By 4,400 years ago, all of the gene variants found in modern corn were present in crops grown in Mexico.
The plant and its grain were so changed by the directed cultivation that it evolved into a form that could not grow in the wild and was dependent on farmers to survive from generation to generation, the study found.
The genes:
One gene changed the architecture of corn from a plant with many branches to one with a single stalk with a male tassel at the top and female cobs growing along the side.Another genetic change softened the outer hull on the kernel. Before the change, the plant depended on animals to spread its seeds. After animals ate the corn, the tough outer shells would allow the kernels to pass unharmed through the gut.
With a softer hull, the kernels would not survive passage through the gut of an animal. As a result, the plant became dependent on farmers to spread its seeds.
Another genetic change caused the kernels to stick more tightly to the cob. And still another change modified the starch of the grain.
This final change, the authors wrote, made the corn more suitable for making tortillas, and, thus, may have been an early variant encouraged by the farmers.