So, have any fruits and vegetables hardly changed at all over centuries or millennia? I ask this as I am curious to find out if there's anything I eat that is similar to what ancient civilizations such as the Romans would have eaten. What about even later ones, such as Victorian England? How quickly do these plants change?
Well, apples have the unusual property (bug or feature, you decide) of not breeding true. That means that if you eat a Granny Smith and plant its seeds, and they happen to germinate and grow into little apple trees, they almost certainly won't bear more Granny Smith apples, and chances are, they'll bear fruit that's too sour or astringent to eat.
All the Granny Smiths you eat are the product of grafts from one tree that bore fruit that somebody tasted and said, "Hey, that's pretty good!"
Which means that if you go up to a farmer's market in upstate New York and bite into a Baldwin apple, you're tasting a fruit that's essentially identical to the first Baldwins grown in Lowell, Massachusetts in 1740 (barring the effects of watering, fertilizing, soil nutrients and all that jazz).
If you want to have some more fun with heirloom apples, check out Rowan Jacobsen's book.
The oldest variety he describes is the Roxbury Russet:
... hard and crunchy, sweet and sour, and wonderfully aromatic, like a pear splashed with lemon juice. Quite popular in the early colonies for pies, crisps, cider, and fresh eating, Roxbury Russet is rumoured to be the variety of apple tree that ate Roger Williams, founder of Rhode Island. Williams died in 1683 and was buried on his property in an unmarked grave. In 1860, an attempt was made to exhume the body and transfer it to some classier digs, but the workmen found no sign of Roger. In his place was the root from a nearby apple tree, which had formed the shape of a life-sized torso, legs, knees, and feet.
Instantly this reminded me of the evolution of maize (corn). Around the 1930s, a man named George Beadle found a relationship between teosinte and maize and did research to proved that the primitive strain of maize and teosinte only differ in 5 genes.
It has been theorized that ancient farmers selected kernels from the largest stalks to replant, selectively breeding to its present form. The dates of the beginning of this process are much debated though, some coming up with 10,000 years ago and others estimated the number to only be several hundred years ago.
Most fruits and vegetables were originally cultivated from their natural predecessors before history began, so you'll have better luck asking such a question in an appropriate subreddit, such as /r/askanthropology.