Nearly 3 million years ago, our ancestors had brains about as big as modern chimps. Since then the brain that would become human grew steadily, tripling in size. But this extra cranium capacity may not have resulted in smarter hominids.
As far as tool-making is concerned, there is little evidence of improvement over much of the period that the brain was growing.
"Archaeology has found that brain size grew gradually, but cleverness took steps," said William Calvin, a neurobiologist from the University of Washington.
The most dramatic of these steps is referred to by some as the Mind’s Big Bang. It occurred between 50,000 and 70,000 years ago. This burst of creativity resulted in bone tools, including sewing needles and throwing sticks. There was also a flourishing of portable art, such as necklaces and pendants, as well as cave paintings.
"There was nothing like this before," Calvin said here Friday at the annual meeting of the American Association for the Advancement of Science.
It is hard to explain the Mind’s Big Bang with a jump in skull size, seeing as Homo sapiens with modern-sized brains had already been around for 100,000 years or more before the tool and art revolution occurred.
"The big brain was perhaps necessary for the creative explosion at 70,000 years ago, but it sure wasn’t sufficient by itself," Calvin said.
So what was a larger brain good for? What was the evolutionary advantage that propelled our family tree to make more room between the ears?
Calvin postulates that a big brain may have made our ancestors better hunters by improving their throwing accuracy. Or perhaps it allowed for the development of a rudimentary language of three-word sentences.
The social psychologist Robin Dunbar has even suggested that the higher memory capacity in a bigger brain could have helped early hominids identify freeloaders who were not pulling their weight for the community.
But none of these subtle advances, according to Calvin, led to the emergence of behaviorally modern humans.
"If you can’t speak sentences of more than two to three words at a time without them all blending together like a summer drink, you likely cannot think complicated thoughts either," he said.
Increasing sentence length or doing multistage planning requires an understanding of structure. Moreover, it is structural creativity that led to advances in tools and art.
Spread of innovation
This structure may have developed in early human language and thought through trial and error.
"We invent new levels on the fly," Calvin said.
A lot of this invention might be nonsensical, but occasionally an innovative adult might have tried out a new word or syntax, and a child heard it and began incorporating it into his or her language.
"Then long-sentence language can spread like a contagious disease, as more kids hear structured sentences and grow up to become super adults," Calvin explained.
The incorporation of more and more complexity is attributable to a combination of culture and genes.
"Behavior invents, and then little genetic changes come along that improve it," Calvin said.
He wonders if we might be headed into a second big bang of the mind. With "better-informed education" based on empirical methods, Calvin postulated that we might see a creative flourishing in the coming century, comparable to the advances made in medicine of the past century.