Chicago: The arch of an eyebrow or the curve of a lip tells chimps a lot about each other, a finding that may give scientists new understanding about the evolution of human communication, researchers reported on Friday.

Human faces can be easy to read, but sometimes people must look in different places on the face to get an accurate picture.

"What we know from humans is that even a single movement added to an expression can change the entire meaning," said Lisa Parr, director of the Yerkes National Primate Research Center at Emory University in Atlanta.

"It can significantly affect the outcome of interactions,"she said. Until now, little research has been done on understanding how chimpanzees communicate through facial expressions, said Parr, speaking at an international conference of chimpanzee cognition at Chicago's Lincoln Park Zoo.

"There could be a whole realm of chimp communication we don't have the capability of understanding," she said. Chimpanzees are humans' closest relatives, with just a 1.23 percent difference between the genetic codes of people and chimps.

Scientists believe studying the behavior of chimpanzeeslends insights into human evolution.

FEAR OR SURPRISE?

In humans, some expressions are hard to distinguish because they use similar facial muscles. Fear and surprise are two examples of this. Parr found the same to be true of chimps. To understand the subtleties of chimpanzee facial expressions, Parr created a mapping program called ChimpFACS that isolates specific muscle movements in the face associated with various facial expressions.

Using ChimpFACS, Parr made three-dimensional, anatomically correct cartoon images of different chimpanzee facial expressions -- ranging from bared teeth to pout.

Chimps were then taught a simple computer matching game using a joystick to manipulate a cursor. They were first shown an image of a chimp expression, such as a scream. Then, they were shown two new images and asked to match up the original expression.

Once chimps mastered the easier task, they were shown a facial expression and then asked to match it with two individual movements that go into making that expression. For example, a smile could be a whole range of movements from retraction of the lips, opening of the lips, parting of the jaw and exposing of the teeth.

The goal was to identify which facial movements were primarily associated with an expression and which were more ambiguous.

Parr found that a scream was the easiest expression for chimps to identify. They also found it was easiest to distinguish between a scream and a whimper.

"As in humans, it's extremely difficult to identify when one expression stops and another starts," she said. "Chimps, like, us have to be able to interpret minimal changes and movements that can bleed into other interactions.

"Using these tools, Parr hopes to observe how chimps use facial expressions within a social context."We now have the ability to detect these movements and describe them and figure out how they affect social function,"she said.