The development of genetically modified plants has allowed people to change certain traits in crops, but humans have been influencing the genetic makeup of crops for thousands of years. Maize took more than 7,000 years to descend from a tropical, small-eared plant to the modern crop grown on farms today.
Changes in plants happen through plant breeding, in which humans artificially select and grow the plants most beneficial to them. The maize everyone knows today exists because of thousands of years of selective breeding, which left modern corn with four or five changes in gene expression, altering certain
key characteristics.
'Modern maize would never have existed without humans,' said Irwin Goldman, a professor of horticulture at UW-Madison.
Plant breeding is as old as agriculture itself. The major gene changes in maize began about 7,000 years ago and humans bred it to have large ears with soft kernels, making it nutritious and easy to eat. In fact, maize barely resembles its progenitor, a tropical plant called teosinte that has several branches with small, practically inedible ears.
Maize looks so much different from its precursor that it was not until about 25 years ago that scientists officially accepted the teosinte as its ancestor, said Bill Tracy, a professor of agronomy at UW-Madison.
The changes to the plant's exterior have made maize look more like a distant cousin to teosinte than a direct descendant. One of the most significant changes was the loss of the hard outer shell of the kernel, called a fruit case, which allowed humans to use it as a food source.
According to a study published in the August issue of Nature, appearance of the fruit case is controlled by a single gene. Genes dictate which amino acids (which add up to proteins, the 'building blocks' of life) are produced'the final order and type of a chain of amino acids determines the final trait. The Nature study said that only one amino acid change within this gene's instructions might have caused the disappearance of the fruit case.
Humans have also bred maize over time to adapt it to the cooler climate and longer days of the Midwest, where it is now the principle crop of several states.
Although the changes were drastic, they occurred over a long period of time. A plant's genetic structure can change, but only after a lot of effort and concerted breeding will the desired changes remain.
'It was literally selection over thousands of years to get the big-eared modern day corn,' Tracy said.
Even several hundred years ago, corn looked different than it does today. When European settlers arrived in New England, the corn they found was small and robust'an adaptation that resulted from having to cope with the cold climate'while the corn in the warmer south was more full-bodied.
'When Europeans moved into the Midwest they brought both kinds of corn and by accident, they crossed,' Tracy said.
Most of the corn that is now grown in the United States is the end result of thousands of years of selective breeding and accidental crossbreeding. Without any modern methods of genetic modification, humans took the humble teosinte plant and transformed it into robust, delicious ears of corn.
