Progress seems to be exponential now, more or less, or at least accelerating, and that's with constant human brains.

Except that, functionally speaking, human computing capabilities are nowhere near constant, due to:

* Culture. Humans download almost all their thinking software from the pool of human culture. That's the main thing that separates a modern human from a Neanderthal man. New thinking tools are arising all the time now, and we download them from the internet - thus souping up our brains with new and improved software.

* Machines. Human brains are functionally augmented by machines. In addition to vastly expanding our sensory and motor channels, they intelligently preprocess our sensory inputs and post-process our motor outputs. The machines compensate for our weaknesses - having abilities at serial deterministic calculations that we are poor at - and having huge, reliable memories.

The net result of these two effects is an enormous increase in human capability.