New York, Sep 11 (IANS): Researchers have found new clues as to why humans look vastly different from chimpanzees - our nearest relative in the primate tree - despite nearly identical genetic backgrounds of the two species.

The key lies in how genes involved in facial development and human facial diversity are regulated -- how much, when and where the genes are expressed-- rather than dissimilarities among the genes themselves, according to a study.

In particular, the researchers found that chimps and humans express different levels of proteins known to control facial development, including some involved in jaw and nose length and skin pigmentation.

"We are trying to understand the regulatory changes in our DNA that occurred during recent evolution and make us different from the great apes," said Joanna Wysocka, associate professor at Stanford University School of Medicine in California, US.

To conduct the study, however, the researchers had to obtain a specialised type of cell present only in very early primate development.

The cells, called cranial neural crest cells, originate in humans within about five to six weeks after conception.

Their analysis revealed that two genes, PAX3 and PAX7, known to affect snout length and shape in laboratory mice, as well as skin pigmentation, were expressed at higher levels in chimpanzees than in humans.

In contrast, another gene known to be involved in determining the shape of the beaks of finches and the jaw of a fish called a cichlid was expressed at higher levels in humans than in chimpanzees.

In mice, overexpression of this gene, BMP4, in cranial neural crest cells causes a marked change in face shape, including a rounding of the skull and eyes that are more near the front of the face.

The study was published in the journal Cell.