How Did We Get So Smart?

Princeton and Bell Labs scientists have devised a simple but powerful method for analyzing brain anatomy, providing the first reliable measure of how brains of humans and other mammals are related to one another across evolution.

In a paper in the May 10 issue of Nature, the researchers show how comparing the relative sizes of 11 brain parts reveals a unique brain structure for each species. They calculated the percentage of total brain volume contributed by each part and created the term “cerebrotype” to describe the resulting 11-number characterization, just as the word “genotype” describes the unique DNA sequence for each species.

The analysis shows that mammals fall into a spectrum of cerebrotypes, with humans at one end and insect-eaters, such as hedgehogs, at the other.

“Intuitively, we know there is something about our brains that is extreme,” said Sam Wang, an assistant professor of molecular biology and the senior author of the paper. “What we have here is a direct measure of one way in which our brains are extreme.”

The findings support the “social intelligence” theory of primate evolution, which holds that prehuman ancestors were at an advantage for survival if they excelled at complex social dynamics such as working in groups and predicting the behavior of others.

Wang and colleagues based their work on a 20-year-old database assembled by German researchers who catalogued information about the brains of 300 animals. Wang’s co-authors on the paper are Princeton undergraduate Damon Clark and Partha Mitra, a scientist at Lucent Technologies’ Bell Labs.

The researchers were initially interested in a comparative study of brains as a means of identifying general principles of brain organization. “We were looking for underlying engineering or design principles that might account for how evolution led to one brain architecture and not another,” said Mitra.

The research may help scientists understand the selective forces that drove the evolution of humans and other animals. Brain areas that showed the most growth over the course of evolution are likely to perform functions that conferred a selective advantage, said Wang.

The research confirmed, for example, previous studies showing that one brain area, the neocortex, grew rapidly over the course of evolution, expanding from 16 percent of the brain in insect-eaters to 80 percent in humans. The neocortex is responsible for social interactions, reasoning and other complex cognitive tasks, suggesting that the outcome of social interactions has been a powerful evolutionary force. Interestingly, even when Wang and colleagues eliminated the neocortex from their analysis, humans still had a unique brain structure, appearing on the extreme end of the chart.

In general, the researchers found that animals with the most similar cerebrotypes were also the most closely related by evolution. Within groups of related species, total brain size varied by as much as 100-fold, but the relative sizes of their brain parts — their cerebrotypes — remained relatively constant. Shifts in cerebrotype occurred with the emergence of new groups, such as the evolution of older monkeys into the great apes into hominids.

Another implication of the research is that the genetic mechanisms that control the development of the brain’s structure may be much simpler than previously thought. Wang speculates that many differences between the brains of humans and those of the simplest mammals may result from evolutionary pressures on just a few genes.

The reason is that, as evolution progressed, the relative sizes of the 11 brain areas shifted in only limited ways. If there were hundreds of genes independently controlling the sizes of the brain areas, then there would be a great diversity of cerebrotypes among mammals — much more than what the researchers found. Instead, they were able to reduce the many variations between the 11 brain areas to a relatively simple, two-dimensional diagram.

Cerebrotype measurements also could lead to a better understanding of how the brain works by making it easier to correlate the cognitive abilities of various animals with their brain structures. For example, the researchers found that one brain region, the cerebellum, had the same approximate relative size in most mammals. However, whales, dolphins, and certain bats appeared to have larger cerebellums. They later learned that these species navigate by a kind of SONAR — bouncing sound waves off their surroundings. The measurements suggest that the cerebellum plays a unique role in the complex calculations involved in that function.

Previous brain-comparison studies had focused on either the whole brain or did not compare brain regions directly with each other. Some, for example, measured the relationship of overall brain size to body size and compared that measure among species of different body weights. That analysis yielded inconclusive results, suggesting that human brains were most closely related to those of spider monkeys, which are not considered to be close evolutionary relatives. However, the researchers’ new results follow widely accepted evolutionary charts: mammals that are closely related by evolution also proved to have similar cerebrotypes.

“It provides a little insight into who we are and how we got here,” said Wang.

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