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.
