There are many theories on what selective pressures may have led to the dramatic increase in the size of early human brains, which are much larger than any other ape's.
A new study posits that DNA sequences in the human genome–responsible for multiple traits that make the human species distinct from other species–suggest that the genes for large brain size emerged from random mutations.
As the article about the study in Popular Science puts it, "we got lucky."
"[R]andom mutations could have contributed to the 49 short DNA sequences in our genome called human accelerated regions (HARs). [Lead author Katie] Pollard and her team were the first to find these segments back in 2006 when comparing the genomes of humans to chimpanzees. HARs work as gene enhancers, controlling which genes are turned up or down during embryonic development, especially for brain formation.
"HARs in humans are very similar in each individual but vary when compared to accelerated regions in other vertebrates like chimps, frogs, and chickens. Since the initial discovery, research has found a connection between HARs and multiple traits that make our species distinct. And while Pollard has spent a lot of time understanding how HARs helped humans evolve, the current study focuses on whyHARs emerged in the first place.
"The team collected data from 241 mammalian genomes (in concert with the larger Zoonomia project) and identified 312 accelerated regions across all of them. Most of the accelerated regions identified acted as neurodevelopmental enhancers, indicating a connection to brain development. But when comparing human and chimp DNA sequences, 30 percent of HARs were in areas of the genome where the DNA was folded differently. This suggests the structural variations in the human genome likely came from a random mutation during reproduction."
I'm sure this grossly simplifies this, but it seems to me that this theory means that instead of natural selection gradually acting on normal variation in brain size, human large brain size occurred by discontinuous random mutation, which was then selected for.
This raises the intriguing possibility that in the far distant past, there was suddenly one ape born with a huge brain, who had to live his or her life as the only being on Earth with human-level intelligence.