The complexities of the human brain are what make humans such a unique species. But how our brains end up so different from those of other mammals?
Scientists have long linked a bigger brain size to higher intelligence, but scientists are still not sure why.
Two neuroscientists have proposed a new theory in a paper published in Trends in Cognitive Science in December.
Their theory proposes a possible reason why a bigger brain makes humans smarter. The scientists believe that as the human brain grew and expanded, neuron connections were pulled apart and the extra space allowed for a greater number and more complex neuron connections to form.
How big the human brain is
On a scale relating brain size and body size of different species, the human brain is about five times the size it should be if we followed the trend set by other mammals.
You can see how much the human brain stands out from the brains of other mammals in the image below:
You can see how quickly the human brain grew as hominids evolved over time in the chart below.
Humans and chimpanzees are closely related, and in the early stages of development, chimpanzees and humans have a similar brain to body size. But the human brain continues growing much longer than a chimpanzee's brain. Human brains also have many more neurons than other animal brains. A whale brain is more than double the size of a human brain, but actually has fewer neurons than a human brain, the researchers write.
The Tether Theory
The scientists, Randy Buckner and Fenna Krienen from Harvard University, have proposed a simple explanation — which they've named the Tether Theory — for how these larger brains made humans smarter.
Simply having a large brain with lots of neurons does not explain modern human intelligence. Our recent ancestors had similar brain sizes but lacked the ability to build civilizations and understand modern technologies. The scientists think the processes that the brain went through while evolving to be larger is the most important part.
Early human ancestors and most other mammals' brains are wired with straightforward circuits that pick up information from the surrounding environment through the senses and relays that information to motor neurons so the body can move and respond to the surrounding environment. One example in humans is when we touch something hot and automatically jump back. This is called the sensory-motor pathway. In these smaller brains, the neurons were physically close together.
But the researchers have proposed that as the human brain expanded, some of these simple sensory-motor pathways were pulled apart and came untethered, making room for more complex circuits to develop within the connections.
These complex circuits form in the association cortex, the area of the brain that separates humans from other animals; the neuron connections that form there are responsible for higher mental processes like thought and memory. In the image to the left you can see how many times more expanded the association cortex of the brain is in humans compared to a macaque and a chimpanzee.
These circuits, unlike the sensory motor pathway, don't form in a straight line where the signal travels from one neuron to its immediate neighbor. Instead, circuits in the association cortex link neurons that are both close together and far apart, almost like a tangled clump of wires.
You can see the difference in the image below. The simple sensory-motor pathway is on the right, and the more complex association pathway is on the left.
The neurons and circuits in the association cortex don't rely on input from the outside environment. That's why as humans we are able to think introspectively and learn about ourselves — a trait called meta-cognition.
The scientists hope to test their theory by comparing the human brain to the brains of other animals that haven't been completely mapped yet, like the chimpanzee.