I'm sure @tarazkp might start disputing the measurements

I wouldn't dispute the measurement, I would dispute the relevance. Learning a skill and using it well are two different things. Give two individuals the same resources and they will use them differently. I could be wrong but I would suggest that if someone like Da vinci had a pool of resource as deep as the internet, he wouldn't be playing Farmville. Smart doesn't translate to intelligence.

Yes, food/hygiene (maybe too) are large factors in our capacity growth but it is also possible that we have reached a point that if we aren't careful, the environmental changes and substances we have created could push us backwards. If we look at the increase in opioid abuse and death in middle class america, it is a pretty good indicator that we aren't 'healthy' even though healthier than before in other areas. Depression is on the rise for a reason and it isn't because we are smarter.

The dumbing down I talk about isn't about brain capabilities, it is about cultural restriction (as you mentioned).