Forgive me for being an uneducated hick, but is it possible that evolution on a macro scale takes a similar growth curve to an individual life form, going from the simple to the complex, until a maturity point is reached?
I would define a maturity point as when further changes become detrimental, simply by being random changes in something that has already become a complex system. Mutation would therefore no longer be as strong a positive force for change as it might once have been to a lower life form, because almost any random change, at this point, is going to be bad. Evolution of life on Earth must be taking some kind of predictable curve, and it seems to me that we are at a leveling off point for random genetic mutation.
The way I see it, human evolution is now external, taking an obvious form in how humans interact with with their environment through technological advances, and taking a more subtle path in the growth of human interaction with each other.