Previous research with tamarin monkeys (see image) suggested that the creatures seem to comprehend simple rules analogous to those that determine what types of word pairs belong together. In the new work, W. Tecumseh Fitch of the University of St. Andrews in Scotland and Marc D. Hauser of Harvard University designed a study to test whether the animals could follow more complicated sentence structures, in which words that depend on one another are separated. A classic example of this so-called phrase structure grammar is the "ifthen" sentence construction. Tecumseh and Hauser played recordings of human speech that mirrored the two sentence types and found that the monkeys reacted when the voices broke the simple grammar rules but could not discern when the more complicated rules were broken.
College students tested with the same grammar examples correctly identified all of the mistakes, even if they couldnt articulate the exact rules that were broken. As a result, the scientists suggest that acquiring the ability to process hierarchical word patterns may have represented a critical juncture in the evolution of the human language faculty.