There are many ways to try to explain why human brains today are so big compared to those of early humans, but the major cause may be social competition, new research suggests.
But with several competing ideas, the issue remains a matter of debate.
Compared to almost all other animals, human brains are larger as a percentage of body weight. And since the emergence of the first species in our Homo genus (Homo habilis) about 2 million years ago, the human brain has doubled in size. And when compared to earlier ancestors, such as australopithecines that lived 4 million to 2 million years ago, our brains are three times as large. For years, scientists have wondered what could account for this increase.
The three major hypotheses have focused on climate change, the demands of ecology, and social competition. A new statistical analysis of data on 175 fossil skulls supports the latter hypothesis.
Behind the hypotheses
The climate idea proposes that dealing with unpredictable weather and major climate shifts may have increased the ability of our ancestors to think ahead and prepare for these environmental changes, which in turn led to a larger, more cognitively adept brain.
The ecology hypothesis states that, as our ancestors migrated away from the equator, they encountered environmental changes, such as less food and other resources. "So you have to be a little bit more clever to figure it out," said David Geary, a professor from the University of Missouri. Also, less parasite exposure could have played a role in the makings of a bigger brain. When your body combats parasites, it cranks up its immune system, which uses up calories that could have gone to boost brain development. Since there are fewer parasites farther away from the equator, migrating north or south could have meant that our predecessors had more opportunity to grow a larger brain because their bodies were not fighting off as many pathogens.
Finally, other researchers think that social competition for scarce resources influenced brain size. As populations grow, more people are contesting for the same number of resources, the thinking goes. Those with a higher social status, who are "a little bit smarter than other folks" will have more access to food and other goods, and their offspring will have a higher chance of survival, Geary said. Those who are not as socially adept will die off, pushing up the average social "fitness" of the group. "It's that type of process, that competition within a species, for status, for control of resources, that cycles over and over again through multiple generations, that is a process that could easily explain a very, very rapid increase in brain size," Geary said.
Weighing the options
To examine which hypothesis is more likely, Geary and graduate student Drew Bailey analyzed data from 175 skull fossils — from humans and our ancestors — that date back to sometime between 10,000 ago and 2 million years ago.
The team looked at multiple factors, including how old the fossils were, where they were found, what the temperature was and how much the temperature varied at the time the Homo species lived, and the level of parasites in the area. They also looked at the population density of the region in order to measure social competition, "assuming that the more fossils you find in a particular area at a particular time, the more likely the population was larger," Geary said.
They then used a statistical analysis to test all of the variables at once to see how well they predicted brain size. "By far the best predictor was population density," Geary said. "And in fact, it seemed that there was very little change in brain size across our sample of fossil skulls until we hit a certain population size. Once that population density was hit, there was a very quick increase in brain size," he said.
Looking at all the variables together allowed the researchers to "separate out which variables are really important and which variables may be correlated for other reasons," added Geary. While the climate variables were still significant, their importance was much lower than that of population density, he said. The results were published in the March 2009 issue of the journal Human Nature.
The social competition hypothesis "sounds good," said Ralph Holloway, an anthropologist at Columbia University, who studies human brain evolution. But, he adds: "How would you ever go about really testing that with hard data?"
He points out that the sparse cranium data "doesn’t tell you anything about the differences in populations for Homo erectus, or the differences in populations of Neanderthals." For example, the number of Homo erectus crania that have been found in Africa, Asia, Indonesia and parts of Europe is fewer than 25, and represent the population over hundreds of thousands of years, he said.
"You can't even know the variation within a group let alone be certain of differences between groups," Holloway said. Larger skulls would be considered successful, but "how would you be able to show that these were in competition?"
However, Holloway is supportive of the research. "I think these are great ideas that really should be pursued a little bit more," he said.
Holloway has another hypothesis for how our brains got so big. He thinks that perhaps increased gestation time in the womb or increased dependency time of children on adults could have a played role. The longer gestation or dependency time "would have required more social cooperation and cognitive sophistication on the part of the parents," he said. Males and females would have needed to differentiate their social roles in a complementary way to help nurture the child. The higher level of cognition needed to perform these tasks could have led to an increase in brain size.
Still other hypotheses look at diet as a factor. Some researchers think that diets high in fish and shellfish could have provided our ancestors with the proper nutrients they needed to grow a big brain.
And another idea is that a decreased rate of cell death may have allowed more brain neurons to be synthesized, leading to bigger noggins.
Ultimately, no theory can be absolutely proven, and the scant fossil record makes it hard to test hypotheses. "If you calculate a generation as, let's say, 20 years, and you know that any group has to have a minimal breeding size, then the number of fossils that we have that demonstrates hominid evolution is something like 0.000001 percent," Holloway said. "So frankly, I mean, all hypotheses look good."