How Has the Human Brain Changed since the Stone Age?

Studies have consistently shown that brain size isn't an indicator of intelligence. In fact, your brain isn't even as big as it would have been if you had been born some 40,000 years ago. According to research, the modern human brain is approximately 10 percent smaller than it once was -- a change that marks the reversal of cranial expansion, which began roughly 4 million years ago. Back then, our brains contained about 1.5 cups (355 ml) of gray matter. That number began to grow with evolution, and by about 130,000 years ago, our cranial capacity had quadrupled to 6 cups (1.4 l). That's when things stopped and even reversed, so that now the average human brain holds about 5.7 cups (1.3 l). A number of theories have been tossed out to explain the change, with perhaps the most interesting being that as we grew together as a people, we didn't have to know as much individually. In other words, cooperation created a collective intelligence that cost us cranial space but gained us civilization as we know it.

Does gray matter really matter?:

• The sperm whale has the biggest brain in the animal kingdom, weighing on average 18 pounds (8 kg) and growing to 500 cubic inches (8,193 cubic cm) in size.
• The human brain requires 20 percent of the body's oxygen and blood.
• A leech's body is divided into 32 segments, each of which is controlled by its own ganglia -- the equivalent of having a brain in each segment.