The original gene - SRGAP2A - is known to have a role in brain development. It is expressed very early on in embryogenesis and all through adulthood, in the cerebellum and the neocortex which happens to be the part of the brain that controls abilities specific to humans, like language and conscious thought.
There are in fact four versions of the SRGAP2 gene in humans - SRGAP2A, B, C and D - all found on the same chromosome and the result of three subsequent duplications of the same original gene. It is thought that SRGAP2A - the original gene - was duplicated about 3.4 million years ago, giving rise to SRGAP2B. SRGAP2B was duplicated a second time about 2.5 million years ago and produced SRGAP2C. And this duplicate was duplicated yet again, about 1 million years ago and resulted in SRGAP2D. So far, it seems that there has been no further duplication. Two of these variants - the original one and SRGAP2C - have remained active, while the other two appear to have no particular function. In fact, they have suffered so many mutations over the years that they are believed to have become genetic junk.
The human-specific SRGAP2C is also expressed very early on in foetal development and in the adult brain where it seems to promote quite the opposite of what SRGAP2A does, i.e. it delays neuronal maturation thus giving neurones more time to migrate and spread out dendrites.