Here's a thread: http://paleohacks.com/questions/33218/best-summary-of-evidence-of-harms-of-gluten-other-nads
And here's a link: http://www.marksdailyapple.com/why-grains-are-unhealthy/
But here's some thoughts to get you started:
Well, what kind of grains did your grandparents eat?
Where did they come from?
How were they grown?
Were they fresh?
How were they cooked / processed?
How often did they eat it?
What else did they eat?
Now, what kind of grains does our generation eat?
Where does the grain come from?
How is it grown?
Is it fresh?
How do we cook it / process it?
How often do we eat it?
What else do we eat?
Some more questions. Think about these in conjunction with the above. Were they healthy? How were their teeth? Their eyes? Their bones?
A bit of creative thinking will get you the answer. It's up to you to figure it out now.