Washington, Dec 20: According to a recent study by researchers at the Indiana University Bloomington six percent of human and chimp genes are unique to the two species.

Scientists however, say that this doesn’t mean that the 1.5 percent nucleotide-by-nucleotide difference between humans and chimps is wrong.

"Both estimates are correct in their own way. It depends on what you're asking. There isn't a single, standard estimate of variation that incorporates all the ways humans, chimps and other animals can be genetically different from each other," says IUB computational biologist Matthew Hahn, who led the research.

According to the study in the inaugural issue of Public Library of Science, by studying "gene families" -- sets of genes in every organism's genome that are similar (or identical) because they share a common origin -- it is possible to glean new information about the evolution of humanness.

For their study, Hahn and his research partners examined 110,000 genes in 9,990 gene families that are shared by humans, common chimpanzees, mice, rats and dogs.

They found that 5,622, or 56 percent, of the gene families they studied from these five species had grown or shrunk in the number of genes per gene family, suggesting changes in gene number have been so common as to constitute an evolutionary "revolving door".

The researchers also paid special attention to gene number changes between humans and chimps.

Using a statistical method they devised, they inferred that humans had gained 689 genes (through the duplication of existing genes) and lost 86 genes since diverging from their most recent common ancestor with chimps.

“Including the 729 genes chimps appear to have lost since their divergence, the total gene differences between humans and chimps was estimated to be about six percent,” Hahn said.

"Our results support mounting evidence that the simple duplication and loss of genes has played a bigger role in our evolution than changes within single genes," Hahn added.

He said that after surveying gene families common to both humans and chimps, the team further observed in the human genome a significant increase in the duplication of genes that influence brain functions.

IUB postdoctoral researcher Jeffery Demuth is the lead author of the paper.