Back in the hunter and gatherer days, humans survived completely on what we could find/kill. This made our diet primarily Meat, Vegetables, Fruit and nuts. Food would spoil a few days after it was picked or killed, forcing us to spend all of our time looking for food. The winter months would be trying times as the plants died, animals were scarce and if your tribe couldn't find food, your tribe died out.
Potatoes, Grains and legumes (peanuts, beans) could not be eaten by the paleolithic people because in their raw form they have so many toxins in them. Eventually, it was discovered that these foods could be ridden of enough toxins to make them edible, by merely heating them up. Note that this process does not remove all toxins, just enough to not kill you.
This discovery revolutionized man as we know it, and is arguably one of the largest contributors to the survival of the human race. These foods could be stored for long periods of times, were very high in energy and were fairly easy to grow. It also freed up time for these people to do things like build, invent, etc.
So, we as humans discovered a way to "hack" this food to make it edible. But in this age of refrigeration, why are we still using this hack? We are no longer in danger of starving or not being able to find food, so why continue to put these anti-nutrients and toxins in out bodies?
Your Answer: Money! In today's capitalist world, anyone will do anything to save a buck, including eat foods that our bodies have not evolved to handle. But that discussion is for another post.
So, what about these food make them so bad for us? Well the main problem is the amount of toxins. These toxin, renamed "anti-nutrients", so as not to alarm the general public, are produced by the plants to deter animals from eating there precious seeds. By making their seeds unappetizing, plants can ensure that their seeds will not be eaten, thus ensuring the plants survival.
Grains and legumes also contain a lot of enzyme blockers. To keep it simple, these blockers keep your body from digesting and using these nutrients. Again, designed by the plant to protect it's survival, these are included in order to keep seeds from being digested. This allows animals to eat foods with the seeds in them, and poop them out unharmed and ready to grow. So why would we want to eat these things?
Another seed protecting mechanism produced by plants are lectins. In the interest of keeping this description simple, lectins are basically natural pesticides toxic to many animals, including humans.
With so many drawbacks to eating grains, potatoes and legumes, I have decided to strip my diet of them completely. Looking at the facts, our survival no longer depends on grains, so I have phased them out of my diet and so should you.