During the course of human evolution, there has been a general increase in the

During the Course of Evolution in humans, the cranial capacity of the brain is significant.  

During early evolution from the time of Apes, the Bipedalism plays an important role. After the evolution of bipedalism, the significant changes in the cranial capacity were observed. The evolutionary trends show a gradual increase in the cranial capacity from Australopithecus to Homo sapiens species.

the answer is lactic acid from anaerobic respiration. the acid in lactic acid is what creates sore and burning muscles. if you think about how acid burns that will give you a clue for questions like this.