I don't have the numbers. You would need to go to the history books to find it but you can see the trend in action to a certain extent for certain occupations. A prime example is farming. In aggregate, total employment in farming has declined significantly. The people that are left are doing higher level work, not tilling the field with a donkey and plow, they're driving a tractor or doing soil analysis etc. The field laborers moved to cities and now they do light manufacturing or similar jobs with maybe a few weeks of training.