A new theory on how humans evolved their large brains

There are several hypotheses on the selective forces that may have contributed to the early humans' much bigger brains than those of any other animal.

According to a recent study, DNA sequences in the human genome, which are responsible for many characteristics that set the human species apart from other animals, indicate that the genes for high brain size were likely the result of random mutations.

According to the Popular Science article on the study, "we got lucky."

The 49 short DNA sequences in our genome known as human accelerated regions (HARs) may have been caused by random mutations. [Principal author Katie] When comparing the chimpanzee and human genomes in 2006, Pollard and her team made the initial discovery of these segments. As gene enhancers, HARs regulate which genes are switched on or off throughout embryonic development, particularly for the creation of the brain.

Although they vary when compared to accelerated regions in other vertebrates like chimpanzees, frogs, and chickens, HARs in humans are very similar in each individual. Since their original discovery, scientists have discovered a link between HARs and a number of characteristics that set our species apart. And while Pollard has spent a lot of work figuring out how HARs assisted humans in evolving, the present study concentrates on why HARs initially appeared.

In conjunction with the wider Zoonomia research, the scientists gathered information from 241 animal genomes and discovered 312 accelerated areas in all of them. The majority of the sites with faster growth operated as neurodevelopmental enhancers, suggesting a link to brain development. However, 30% of HARs were found in regions of the genome where DNA was folded differently when comparing the DNA sequences of humans and chimpanzees. This implies that a random mutation occurred during reproduction and caused the structural variances in the human genome.

It appears to me that this hypothesis states that our high brain size developed by discontinuous random mutation, which was then chosen for, rather than natural selection gradually operating on normal variance in brain size.

This opens the interesting prospect that, in the dim past, there was one ape who was abruptly born with a large brain and had to live as the only person on Earth with human-level intelligence.