Heads up: problem solving pushed bright primates toward bigger brains. (This Week).

Progressively larger brains evolved in primates of all stripes, not just humans. We can thank a common capacity for solving a broad range of problems, from coordinating social alliances to inventing tools, according to a new study.

This conclusion challenges a popular theory that big, smart brains arose primarily because they afforded advantages when it came to negotiating complex social situations during human evolution.

"The ability to learn from others, invent new behaviors, and use tools may have [also] played pivotal roles in primate-brain evolution," say Simon M. Reader of McGill University in Montreal and Kevin N. Laland of the University of Cambridge in England. In an upcoming report in the Proceedings of the National Academy of Sciences, the two zoologists chronicle links between an array of intelligent behaviors and enhanced brain size in primates.

Reader and Laland examined approximately 1,000 scientific studies of behavior in 116 of the world's 203 known primate species. They identified 553 instances of animals discovering new solutions to survival-related problems, 445 observations of individuals learning skills and acquiring information from others, and 607 episodes of tool use.

The researchers then consulted previously obtained data on brain size relative to body size in different primates. In particular, they focused on the volume of the structures that make up what scientists call the executive brain, a frontal region thought to be crucial for complex thinking.

Species that have the proportionately largest executive brains are the ones that most often innovate, learn from others, and use tools, Reader and Laland contend. These three facets of intelligence vary together as primate brains enlarge, they say. There's no evidence in any species of an evolutionary trade-off between these traits, such as an increase in innovation accompanying a decline in social learning.

A related report by neuroscientist Barbara L. Finlay of Cornell University and her colleagues concluded that different brain regions in mammals enlarged all together during mammalian evolution, not in piecemeal fashion related to specific functions. Whole-brain evolution was driven by changes in the timing of early brain development in individuals, says Finlay. In all species, late-generated structures--including the executive brain--have grown the largest, Finlay's team asserted in the April 2001 Behavioral and Brain Sciences. Reader and Laland provide "important new evidence" that wide-ranging thinking skills shared by many primate species encouraged the evolution of large brains, comment psychologist Robert M. Seyfarth and biologist Dorothy L. Cheney, both of the University of Pennsylvania in Philadelphia, in a comment published with the new report.

They suggest that intellectual accomplishments unique to people, such as language use, may have played a smaller role in the evolution of our sizable brains than has often been thought.