The top graph shows how Earth’s climate has fluctuated over the past 3 million years.
Notice how much the fluctuations increased between 800,000 and 200,000 years ago.
These challenges, along with an increase in body size, led to an increase in brain size.
Human brain size evolved most rapidly during a time of dramatic climate change.
A chimpanzee’s temporal cortex has less white matter, reflecting fewer connections between nerve cells.