An interesting article on Scientific American magazine discusses the physics of thought and limits of the intelligence. In summary:
Human intelligence is probably reached its limits according to several lines of research, and it may not be evolved anymore, and, becoming a smarter specie than today’s human being may not be possible because most of the tweaks that can make us smarter hit the limits dictated by laws of physics
Brain size: there is a tradeoff between required energy and brain cells. As the brain gets bigger more wirings are needed so the brain gets more energy consumer.
Brain Wirings: Thinner wirings in the brain leads to noisier signals and communication, similar to the effects in the computer transistor chips, because of hitting the thermodynamic limits.
HOWEVER, human probably gets more intelligent. By transferring our knowledge to the internet, we are expanding our minds beyond our body…