Everytime I visit an American historic site like Monticello or Independence Hall, I notice that the doorframes are significantly shorter than they are today. Although I don't have the figures it seems that, on average, humans have become taller over time -- at least a few inches on average over the past couple of centuries.
If we extrapolated height over time, I guess it would follow that humans will continually become taller and evolve in other ways, but I don't understand WHY this would happen. It's not like our environment necessitates our bodies adapt in search of food or safety -- which is, in essence, why animals evolve anyway. What are the physical limitations of our own evolution? If, over the past thousands of years, we have become taller/lankier and even less hairy, what will the human race look like in the year 2200? 2500?