Once we were all hunter gatherers. Omnivorous animals that can and do eat almost anything which contains energy. For countless millions of years we ate unprocessed meat, vegetables, and fruit. But we didn't eat grass. Natural grass is so energy poor that it's not worth it. Without the special digestive equipment that grazers have, it's no use to us. We can eat the seeds of grasses, but they're very small and not worth the bother.
Then we domesticated grasses. There are many theories about how this could have happened. But they all add up to the carbohydrate-rich seeds being somehow selectively bred to become a viable, and more importantly, a farmable human food source.
Suddenly we had a plentiful, cheap source of vast amounts of edible energy in the form of the easily digested carbohydrates in the big new grass seeds.
Our systems weren't designed to consume such things, because they had never before existed in the history of the world.
When we eat these new things, two things happen.
They cause massive blood sugar rises. This means that they satisfy hunger very quickly. This programs us to crave them when we are hungry in the same way that a smoker craves a cigarette when his nicotine levels are low.
There is a massive release of insulin to cope with the fast rise in sugar. This drives the blood sugar into the fat cells, to be stored for future use. In fact it overcompensates. By the time the shock and response have died down we have lots of new fat, and we are low in blood sugar. We are both tired and hungry. The thing that cures hunger fastest is more grass seed.
So our new foods put us on a cycle of eating and feeling hungry, sugar shocks and insulin release, an addiction to the new food. Drug addiction itself is only a shadow of this mechanism. This was the real thing.
This cycle had bad effects long term as well. The effects of insulin become tolerated over time. Eventually the insulin response to the sugar shock stops working. This is known as Type II (adult-onset) diabetes.
And it must have killed us in our millions.
What else happens as a result?
Well, our new grasses may not be the best food, but they are food, and we can eat them. And a creature with a newly abundant food supply suffers a population explosion.
So the new farmers, malnourished and unhealthy as they were, took over the world. And anywhere that the new grasses would grow, the farmers wiped out by virtue of their sheer numbers any other people who for whatever reason carried on living in the way they were evolved to live.
And once the process was over, any part of the world where the farmers could get to and the new grass would grow was covered in masses of short lived ill overcrowded farmers eating pretty much only grass.
Not for nothing has the invention of agriculture been called the worst mistake in the history of humanity.