I'm 64 and much of the "modern adulteration" has supposedly occurred in my lifetime. In my opinion, many of the genetic manipulations have REDUCED the sweetness and juiciness of fruit. When I was a kid, you literally took a shower in juice when eating a plum or peach and apples/pears were much juicier too. Many recent gene adjustments were to make fruit stand up to long distance shipment without damage, not to increase the sugar content. Watermelon was also juicier many years ago. I mean, today's tomatoes are closer to cardboard than the ones I grew up on!

This link looks very interesting:

http://www.naturalhub.com/natural_food_guide_fruit_common.htm

These notes are a look at the fruits that are commercially available that we Westerners eat, but from a hunter-gatherer evolutionary perspective. It is a 'guided tour', not of the diversity of fruits in the natural environment we evolved in, but rather the fruits that are now commercially available to eat, and how it is we came to be eating only these fruits.

Evolution has forced us to become vitamin C junkies - unlike most animals, we can't synthesize it ourselves, we have to obtain it from the food we eat. Fruit and vegetables, and to a lesser extent, organ meats, are the prime source.

Only some of the wild fruits that were all around our ancestors have been domesticated. So the number of species available to us now is less - at first glance. But because commerce provides us with fruit from all the continents of the world, our actual daily possible selection range is probably as good as was available to our ancestors. And the fruits we now have available have much fewer unpleasant tannins and glycosides than some of the wild fruits. The fleshy part is larger, and the seediness in some cases reduced or eliminated.

Also an interesting paper summary here about what fruit traits may be related to domestication:

http://eco.confex.com/eco/2008/techprogram/P10938.HTM

This paper might be useful as well:

http://www.hort.purdue.edu/newcrop/origins%20of%20fruits.pdf

I'm pretty sure that quinces haven't been mucked around with, and you can do lots of things with them, and they are full of pectin, which is very good for you, from what I understand. Rhubarb, while technically not a fruit, is also un-mucked around with very easy to grow. Crab-apples and rose hips are also a couple of possibilities, as is staghorn sumac, if you live where it grows. Mountain ash berries and barberries also come to mind, as well as elderberries, or even wild grapes. I don't know if these are the kinds of things you had in mind, though.

While I'm sure that the fructose content was less, we have to admit that when a particular berry, for example, came into season the foragers would have picked as many as they could find and would have had that berry temporarily constitute a much larger portion of their diet. While I don't personally see much value in apples, I think it's misguided to think of them as being "bags of sugar" and thus outside of our design. A forager may not have come across a massive red delicious, but it wouldn't take all that much wild fruit to reach an equivalent fructose content.

There would have been whole days that consisted of only fruit consumption. Or days where they ate a substantial amount of honey. Clearly, those would have been fattening days even for foragers, but they still occurred. I'm going to continue to believe that fructose is the cause of nearly all adiposity and continue to opt out of fructose consumption for the time being personally, but certainly not because it's "unnatural."

Pomegranates are mentioned quite a bit in the Old Testament. Cranberries- the Native Americans and pilgrams both used them for (different) medicinal Apples appear in greek and Norse mythology. Cherries in Chinese mythology. Rowanberries in irish and scandinavian lore.
Maybe these are useful lines to research along?

Wild berries such as wild strawberries, blackberries, bilberries, wild raspberries can't have changed in centuries - and all are sweet and delicious.

Apples and plums I am sure have changed - the wild apples (crab apples) are practically inedible without cooking, as are wild plums such as sloes and bullaces. However, it is possible that wild plums or apples from other countries / continents may have been much sweeter and therefore palatable (I'm in the UK - perhaps the colder summers here led to wild fruit trees producing sour fruit?)

apples in adam and eve garden of eden story...