Clearly ancient women were not popping folic acid pills in anticipation of a deformity-free child. I would also expect that spina bifida is a disease of civilization. If it was an ancient disease, I think the mother's body would miscarry it rather than waste precious time and energy on a child that will only be a burden - sorry for the un-PC-ness, but the world was a harsher place back then and women presumably didn't coddle their pillow-angels and talk to other village women about how this magical little helpless human transformed their life, rather than go out and find some food or make some tools. I won't even get into what was probably the fate of the few mal-formed babes that were born during rough times.
But do you know hard hard it is to get the RDA's 600 ug for TTC women naturally, every single day?! I tried it for a while, and it sucks, except on liver days. You feel like you're eating nothing but spinach and asparagus (two of the densest plant sources). And, ahem, sorry for this, but I don't like having green poop.
Is folic acid being depleted in our food supply, like I've heard that Mg is? Did ancient plant and animal products naturally have way more, so that today it's practically necessary to supplement?
What about seasonality?
What about plant toxins?
Is 400-600 ug/day only necessary on a SAD diet, similar to how vitamin C works?
Is folic acid deficiency really the issue? Did a single study show a very weak correlation between it and birth defects, and now doctors are just covering their asses? Another big pharm gag?
Okay I will try to answer this.
Spina bifida occurs in all parts of the world at varying levels. Evidence shows that it has always happened throughout human history. Hunter-gatherers babies suffered just as people do now however it may have been less than more malnourished later agricultural populations. http://mcclungmuseum.utk.edu/research/renotes/rn-02txt.htm Spina bifida is preserved in the bones of paleolithic babies.
Adequate levels, 400 micro grams (600 if at high risk) of folic acid before and during pregnancy greatly reduces the chances of spina bifida happening. Although scientists are not yet quite sure why. This intake of folic acid was probably never eaten everyday by our ancestors.
The discovery of folic acid and its higher level supplementation is a modern fix for a basic fault in human design. Embryo development is an incredibly complex process with many things that can go wrong at each stage. In prehistoric times most babies born with spina bifida would have died very young. Evolution does not make bodies perfect, only "good enough".
Now we have a safe effective way to prevent many (not all) cases of spina bifida happening and the babies in question to be born healthy, even if it is a modern fix.
Due to the fact that our distant hunter-gatherer ancestors commonly suffered from spina bifida I do not think it is due to depleted food or modern diets.
Rather than trying to get that much folic acid from the diet (which is as you say very hard) I would suggest just taking a folic acid supplement while trying for a baby and when pregnant. Folic acid supplements do no harm and have a potentially huge benefit. This is especially important if you avoid processed foods which contain the most added folic acid. Folic acid supplements are very cheap and make no big profits for anyone.
This is a case to be glad of modern science and medicine.
Edit: I didn't say folic acid was harmless however much you take, just that a total intake of 400ug is safe to take while trying to get pregnant and while pregnant. Forticication of processed foods with vitamins is always a blunt instrument, individual intake can vary alot. Men don't need as much, 200mcg from diet and don't need supplements. But it is hard to get enough folic acid if you avoid processed food while pregnant. Working out how much you get from your diet and making up the difference with a supplement (total of 400mcg) before and during pregnancy will not give you cancer. You probably don't need to supplement any other time.