Can you be more specific as to which foods you mean? I'm not anti-GMO, just curious.
*Any* food that results from agriculture or ranching is genetically modified.
Examples of foods that are so modified from their wild ancestors that they are almost unrecognizable are corn, tomatoes, and cows. Corn, for example, was derived from teosinte, a wild grass of Central America. Corn and teosinte barely resemble each other; it took in-depth genetic analysis to demonstrate that teosinte is, indeed, the wild ancestor of corn. This article discusses genetic variation in corn, and also mentions my hero, Barbara McClintock.
Cows were bred from aurochs, which no longer exist. However, we have an idea of what they looked like from paintings. Compare the cave paintings in this article to the variety of living cattle breeds.
As far as I can tell, the tomato is a result of breeding of several wild tomato species. Wikipedia has a bit more information, but not a lot. By comparing the many breeds of tomatoes against their wild counterparts, you can see that they express many genes not seen in the wild (the different colors and sizes all result from mutated genes).
These are just a few examples. Any time you look at a domestic food and see differences between it and the wild relatives, you see evidence of genetic manipulation of the species. White chickens, ducks, and turkeys are almost non-existent in the wild...