April 16 (Bloomberg) -- Humans might use tools better, but chimps are more highly evolved, an analysis of chimpanzee and human genetic sequences found.

Chimpanzees have 233 genes that underwent natural selection -- the process in evolution where positive survival traits propagate across a species -- since splitting with the common ancestor shared with humans. Humans, in contrast, only changed 154 genes, many of which might have relationships to disease, researchers said.

Evolutionary theory predicts chimps should have more positive selection than humans, because chimps outnumbered people until a few thousand years ago, according to head researcher Jianzhi Zhang, an associate professor of ecology and evolutionary biology at the University of Michigan in Ann Arbor. No previous studies had been done comparing genetic variations between the species, he said.

``Very little has been done in the past because we humans are much more interested in humans,'' Zhang said in an April 13 telephone interview. ``In the last five years or so there has been lots of speculation about what genes make us human.''

When a beneficial mutation appears in a single individual, it tends to spread across the population in a process known as positive selection, Zhang said. Researchers have been interested in the positive selection underlying human origins since the genome was mapped, he said.

``The really interesting thing is that the varying genes, they're not necessarily the most obvious genes,'' said Matthew Hahn, an assistant professor of biology at Bloomington's Indiana University in a telephone interview today. ``It wasn't just brain-related genes.''

Disease-Related Genes

Almost 10 percent of the genetic variations in humans were found in genes that underlie known disorders, providing evidence to a hypothesis that adaptations that were advantageous thousands of years ago now sicken people.

``For instance, before, when we were out on the savannah, it made sense to conserve fat and salt, because food wasn't guaranteed,'' said Hahn, who wasn't affiliated with the study. ``And now those genes that were helpful may lead to diabetes.''

The researchers did a genome-wide comparison of the two species. They have provided a list of the genes that varied to individual labs to study which traits the genes influenced, Zhang said.

``We actually know very little about what makes us unique,'' Zhang said.

To contact the reporter on this story: Elizabeth Lopatto in New York at elopatto@bloomberg.net

Last Updated: April 16, 2007 17:02 EDT