“As a general rule,” Stringer says, “the more meat on your bones, the more brain you need to control massive muscle blocks.”  An elephant's brain can weigh four times as much as a human’s.   However, EQ is not enough, as recent studies show that the brain shrank faster than the body in near-modern times.   

Chris Stringer Natural History Museum London

Some scientists point to warming in the earth’s climate that also began 20,000 years ago.  Since bulky bodies are better at conserving heat, larger frames fared better in the earlier, colder climate.  As the planet warmed, selection would favor "slighter" people, and the brain got smaller.

While this may be a factor, comparable warming periods occurred many times over the previous 2 million years, yet body and brain size still increased, so it's not the only factor. 

One popular theory is that with the advent of agriculture, we initially had poorer nutrition, as the first farmers were not very successful so there was a deficiency in protein and vitamins—critical for growth of body and brain.   However, the agricultural revolution did not arrive in Australia or southern Africa until much later, but brain size declined in those places, too.  Another important factor, but not the only one.  

David Geary, a cognitive scientist @ the University of Missouri, has a different approach; cranial size changed as our species adapted to an increasingly complex, but supportive, social environment between 1.9 million and 10,000 years ago.

Working with Drew Bailey, they used population density as a proxy for social complexity, reasoning that when more people are concentrated in a geographic region, trade springs up between groups, there is greater division of labor, and interactions among individuals become richer and more varied.

David Geary Univ. of Missouri

Population density did track closely with brain size; when population numbers were low, as they were for most of our evolution, the cranium kept getting bigger.   As population density climbed, cranial size declined with a sharp 3 to 4 percent drop in EQ starting about 15,000 years ago.  This trend occurred everywhere...Europe, China, Africa, Malaysia, etc.

As complex societies developed, the brain became smaller because people did not have to be as "smart" to stay alive.   As Geary explains, individuals who would not have survived by their wits alone could make it with the help of others—supported by the emerging social safety nets.  Likely a big factor in the story.  

Cro-magnon "smarter" than today's humans

The Cro-Magnons were likely "smarter" in terms of raw innate abilities.  Geary believes they were as “bright as today’s brightest” and might even have surpassed us.  However, he adds “our ancestors were not our intellectual or creative equals because they lacked the (social network) cultural support...our very brightest people can focus their efforts in the sciences, the arts, and other fields... Our ancient ancestors took all their efforts just to get through life.”

Other researchers believe selection against aggression is another important factor, i.e., we evolutionarily domesticated ourselves.  The leading proponent of this view is Richard Wrangham, a primatologist at Harvard.

As Wrangham points out, some 30 animals have been domesticated, and in the process all lost brain volume—typically 10 to 15 percent, and the builds become more slender.  Natural selection reduces aggressiveness by favoring those who have "a more juvenile brain, which tends to be less aggressive than that of an adult". 

Perhaps our increasingly hierarchical socialized structures evolutionarily sorted for "domestication" through laws, social pressures, different tasks, hierarchical promotions, different mates, etc.  As Wrangham says, "The story written in our bones is that we look more and more peaceful over the last 50,000 years.”

Our domestication has also transformed our cognitive style.  Wrangham's former graduate student Brian Hare, now at Duke University, compares domestic animals with their wild relatives.  He found that “...wild types and domesticates think differently.”

Richard Wrangham Harvard

In comparing the cognitive abilities of wolves and dogs, Hare found that wolves, with larger brains, have more flashes of insight, and solve problems on their own; dogs, with smaller brains, get humans to help them.  “Wolves persevere when dogs readily give up.”  However, dogs leave wolves in the dust when it comes to tracking the gaze and gestures of their masters.  As Hare puts it, “They are very good at using humans as tools to solve problems for them.” 

Hare is now studying other primates, notably bonobos. “Bonobos look and behave like juvenile chimps,” he continues. “They are gracile. They never show lethal aggression (scary video) and do not kill each other.  They also have brains that are 20 percent smaller than those of chimps.”

Hare thinks bonobos became domesticated by occupying an ecological niche that favored selection for less aggressive tendencies.  That niche offered more abundant sources of nutrition, so fighting over meals became less important to survival.  From that lineage came these highly cooperative primates known for their peaceful ways.  (BTW, when the Congo River formed 1.5 to 2 MM years ago, it split the common ancestor in two; one south of the river w/abundant resources > bonobos; the other north of the river w/fewer resources > chimps.)

bonobos

chimp

Both Wrangham and Hare see parallels between bonobo development and our own.  Our self-domestication may hold the key to our species’ extraordinary motivation to cooperate and communicate —arguably the twin pillars supporting the whole of our civilization.

However, perhaps human brain size is rising again.  Anthropologist Richard Jantz of the University of Tennessee measured the craniums of Americans of European and African descent from colonial times up to the late 20th century and found that brain volume was again moving upward.   

As this happened so rapidly, the explanation is “mostly nutrition.”  Jantz thinks the trend has “an evolutionary component because the forces of natural selection changed so radically in the last 200 years.”  With the unprecedented abundance of food in recent times, selective forces have relaxed, reducing the evolutionary cost of a large brain.

A recent study carried out by Chinese researchers looked at 500 endocasts from the past 7,000 years. They also confirmed that our brains are getting smaller.   However, they found that while the whole brain has been getting smaller, the frontal lobe, the region of the brain responsible for speaking, comprehending others' speech, reading and writing is actually increasing in size as we do more of that now compared to our ancient past. 

Interestingly, across the world the average IQ has increased over the last 100 years, a phenomenon known as the Flynn Effect.  Most of that jump probably resulted from better prenatal care, better nutrition and reduced exposure to brain-stunting chemicals such as lead.

“Natural selection is different from artificial selection in that it acts on every trait at once,” Stringer says. “It’s perfectly plausible our modern brain is smarter in some ways, dumber in others, and more docile overall.”



BTW, as mentioned in the last blogpost, Rich Doyle and i will be "dialoging" in a new webshow on "Awakening Beyond Thought; Who would you be without your stories?", Sunday, Oct 5, @ 8:00 pm EDT/NYC, 5:00 pm PDT/SFO, 1:00 am GMT/London; details in the link.