Humans are a remarkable species, especially because of the remarkable properties of their brain. Since the split from the chimpanzee lineage, the human brain has increased three-fold in size and has acquired abilities for vocal learning, language and intense cooperation. To better understand the molecular basis of these changes is of great biological and biomedical interest. However, all the about 16 million fixed genetic changes that occurred during human evolution are fully correlated with all molecular, cellular, anatomical and behavioral changes that occurred during this time. Hence, as humans and chimpanzees cannot be crossed or genetically manipulated, no direct evidence for linking particular genetic and molecular changes to human brain evolution can be obtained.

The evolution of our own species fascinates many of us and is relevant to understanding our biological and biomedical conditions  As our ecological niche is apparently dependent on special cognitive features, the evolution of our brain is of particular relevance. The human brain has a long evolutionary history, but of special interest is the period after we split from our closest living relatives, the common chimpanzee (Pan troglodytes) and the bonobo (Pan paniscus). Genetic and phenotypic  changes that occurred after this split and that are common to all currently living humans can be defined as ‘human-specific'  

To define human-specific changes, comparisons need to include humans, chimpanzees and at least one primate outgroup, such as the orangutan or the rhesus macaque. Fortunately, the increasing availability of genome sequences from humans, chimpanzees and other primates, as well as even our closest extinct relatives, the Neanderthals and Denisovans allows us to precisely identify most of the about 16 million genetic changes that occurred in the human lineage, i.e. after the split from the chimpanzee lineage and before the most recent common ancestor of currently living humans More on this

https://en.wikibooks.org/wiki/Relationships/The_Evolution_of_the_Human_Brain