Human Brain Size Not Most Important Factor in Thinking Capabilities
The human brain stands out from that of all other species by facilitating cognitive tasks that other species are not able to achieve. The human brain weighs about three pounds — a gigantic size relative to the body. Scientists have long believed that the large size of the human brain and our powerful minds are closely connected. Based on fossils found of ancient ancestors, there was a huge increase in the size of human brains that began about three million years ago. With that increase, our ancestors began to leave behind signs that their minds were growing increasingly sophisticated.
In a paper in the December issue of the science journal Trends in Cognitive Sciences, two Harvard neuroscientists, Randy L. Buckner and Fenna M. Krienen, have reported their findings to help explain how the simple size increase in the evolution of the human brain led to significant changes in the faculties of our ancestors, including the ability to use stone tools and create cave paintings. They call their idea the “tether hypothesis,” and it was developed after the researchers made detailed maps of the various connections of the human brain with the use of fMRI scanners. The differences they found between the maps of human brains with those of other species were striking.
The outer layers of all mammals’ brains are divided into numerous regions referred to as cortices. These areas are where neuron processes take place, receiving signals obtained through the body’s senses, and also sending out signals. When human brains increased in size, the sensory and motor cortices hardly expanded. The regions in between — the association cortices — greatly increased. These areas are what allow humans to excel at tasks like making decisions and considering ourselves. The findings suggest that the complex human thought that we are capable of today may have resulted from a simple increase in the growth of neurons, instead of resulting from an increase in the overall structure and size.