20 generations are nothing on an evolutionary timescale. How sure are you that all of your 1048576 potential ancestors ate only rice and beans? You'd be surprised how many of them didn't.

This woman: http://www.thepaleomom.com/2012/07/why-arent-humans-adapted-to-grains.html explains it really well. Read and you'll understand. :)

With the exception of a small percentage of people that have severe allergies and intolerances to grain based proteins (note I said proteins and not carbs), most people benefit from paleo because it helps with obesity, which is caused by the tremendous intake of processed foods, highly sedentary lifestyles and appetites that are out of control.

You would find that your grandparents, great-grandparents and so forth had very few issues with managing weight despite having bread and other grains as dietary staples.

In respect to considering the role of epigenetics in your progeny see: http://www.naturalhistorymag.com/features/142195/beyond-dna-epigenetics

Adding another thought... is there evidence that intolerances are the norm? Wheat, grains, soy, dairy, legumes, etc. The assumption is that paleolithic man didn't eat these because he was intolerant to them. Who's to say that present-day intolerances aren't modern in origin?

Ok, lactose intolerance is an ancient intolerance that's actually getting bred out over time. It's something that suggests that evolution is faster than we'd like to admit. Who's to say that intolerances to legumes, grains, soy aren't getting bred in?

Another example of paleo normalizing dysfunction?

If you are healthy and young, if your ancestors ate agricultural products for many generations, if you eat organic, traditionally prepared grains and beans, similar to what your ancestors ate, then probably you will be able to tolerate that well.

I'd like to remind though that your ancestors probably had weak bones, bad teeth and smaller complexion than people who ate meat oriented diet. Please don't accept as an insult - but people of agricultural empires were sicker and weaker than barbarians, that's a historical fact.

I'm currently reading about a major selection event. In the summer of 1830 the indigenous tribes along the Columbia River were struck by "cold fever". By accounts from the time this appears to have been malaria, because those struck by it were responsive to quinine and bark extracts. The population was literally decimated, with entire tribes and villages suffering 100% mortality. Tens of thousands died. Yet while the English & American fur traders in the area were affected by it, the fever was not generally fatal to them. At this point in time the affected indigenous tribes have recovered. It appears that a selective adaptation has taken place, within 5-10 generations.

Contrast this with lactose and gluten tolerance. While selective forces may have been strong in ancient Egypt, the much earlier central Anatolians were already adapted to grain eating. Considering that couscous and other grain products are now staple foods in Egypt, and that the population is much larger than it was 4000 years ago, it appears that similar selection occurred there as well.

Is this evolution? Maybe so, maybe not. These tolerances are often reversable. But they are also heritable.

Of course we are still evolving. Personally, I think of paleo as a process to get to the root of your current biological state.

Would 20 generations get you there? No, probably not. Regardless, you and your siblings have different gene expressions, so what's true for you might not be for him/her.

But, eliminate all neolithic foods and get your body right. then slow add back in other foods and see how you react. I have been surprised to know that I apparently react poorly to potatoes (And I have Irish Ancestry) but seem to have no reaction to other nightshades.

Use paleo as a template for eliminating troubling foods from your diet and to heal your body. Then experiment.

You don't need to argue evolution with your ability to digest rice and beans. Most everybody can (despite what paleo says). Just because some ancestor a few hundred thousand years ago didn't eat X, doesn't mean they couldn't digest X, doesn't mean you can't digest X, doesn't mean you shouldn't eat X.

X = rice, beans, wheat, etc...

I have not yet reintroduced rice or oats but I actually have reintroduced black beans and have no trouble with them at all. I think that eliminating grains eliminates processed foods which is the major reason that people feel so great on a paleo diet. I doubt that rice and oats are all that bad in their true form but most people dont eat that... They eat bread and crackers and chip. This is just my speculation... I plan to reintroduce rice and oats in a few weeks and see how I feel. I also think that most people rely too heavily on them. Eating a portion of grains a day is probably fine but most people today eat them as 50% of their calories.

In the history of all mankind it was seen...blah, blah, blah.

N=1

Yeah, that's still pretty much the answer to nearly every question posted on this board.

Evolution takes a long time. We do make adaptations to our current environment: indeed, one of the reasons people get so obese nowadays is to survive under the current onslaught of artificiality.
One could therefore make the argument that in the modern era, we are devolving- adapting to an artificial environment that we will soon no longer be able to maintain.
In terms of things like rice and beans- look, with evolutionary theory we can make educated guesses- hypothesis that can be tested. So we say from a very abstract level that rice and beans aren't good for us because they have lectins or whatever. Then, you ought to unleash the biologists, chemists, etc... So, these folks go look at the composition of white rice and see it is pretty clean- mainly starch and probably alright if you can handle starch. Matt Lalonde has already got into the legume thing. I don't think he recommends soy, but he's looked into other beans and it appears most of the lectins get destroyed in the cooking process. A similar thought process goes into eating dairy fat- it is animal fat. If you are particularly sensitive to milk proteins, you just purifiy it more (ghee), but despite it's neolithic origins, it is animal fat and therefore something we can handle pretty well.

So, instead of doing historical research, we need scientific research so we can refine our protocols. We cannot observe the past, nor can we go back into it and rerun while changing only one variable in order to find out if our assumptions are correct. We can, however use our knowledge of the past in order to come up with ideas to test. This is the way forward.