Running barefoot on hard surfaces like sidewalk or a road is actually quite easy, as long as you're not a heel striker. Forefoot and midfoot strikers, (which you are forced to be if you're running barefoot), typically hit the ground with the same leg stiffness whether they are running on pavement or soft dirt. The adjustment is made by the body to ensure a consistent foot strike.
Although running surface stiffness has been associated with overuse injuries, evidence to support this suggestion has been circumstantial (Dixon et al., 2000).
You may think that our ancestors ran only in the soft grass but this isn't logical. There are many hard and rocky natural surfaces.
Smooth pavement is actually quite enjoyable to run on, second only to packed dirt.