GMOs (bio-engineering) are relatively new to agriculture and were released in the 90s before extensive long term testing was completed. It began in the commodities (corn, soybeans, etc) and has expanded into other crops (sugar beets, papaya, some vegetables and fruits ). The early modifications were designed to make the plants tolerant to Glyphosate (Round-up) or to produce Bt -Bacillus Thuringienses (an organic insecticide ). It is estimated GMOs can be found in over 70% of processed food items.
A French study from the University of Caen stated GMOs produced tumors in lab animals. Much of the world’s scientific entities have found the methodology of the study to be faulty and consider it to be unreliable. Many believe GMOs are harmful, but so far no valid study has confirmed those claims. What studies have shown is that stress can be harmful and eating fresh fruits and vegetables are healthy for you.
In Europe they are using modified tobacco plants to produce vaccines vs lab animals. A GMO bovine gives non allergic milk. Disease resistance is being enhanced.
Like most human endeavors GMOs will probably have pros and cons. Some advocate the removal of GMOs from the food supply. The business push is to have more.
Airborne pollen can travel and contaminate non gmo crops.
Personally I have an issue with GMOs due to environmental impacts. By depending on one chemical to control weeds resistance develops and the weeds cannot be controlled with that chemistry. The same will happen with insects and Bt.
“Chemical Free Agriculture ” is a fallacy. Plants and animals have a chemical composition. Some people are sensitive to certain chemicals (conventional or organic ) and need to exercise caution. For most people the amount of allowed chemicals permitted to grow our food is safe. Most of our food problems are from food poisoning (E Coli and Salmonella).
An interesting perspective can be found at www.marklvnas.org2013/01/lecture to oxford farming conference 3 january 2013.