The researchers performed large-scale sequence comparisons among human, chimpanzee, gorilla, orangutan, baboon, and rhesus monkey genomes to identify subtle differences in genomic change rates.

Among all these nonhuman apes, chimpanzee is genetically closest to human, scientists said in a paper published in the Jan.23 issue of The Proceedings of National Academy of Sciences.

The rate at which changes in the genomes of organisms accumulate can be represented as a molecular clock, measuring the speed with which an organism evolves.

"Our results confirm that there is very little difference in the alignable regions of the human and chimpanzee genomes," said Navin Elango of the Georgia Institute of Technology, lead author of the paper.

In addition, both humans and chimpanzees appeared to have slower molecular clocks than gorillas and orangutans.

"When two other hominoid species were included in the analysis,we found that both the human and the chimpanzee lineages exhibit slower molecular clocks compared to either gorilla or orangutan," the researchers wrote in the paper.

The researchers also found that the molecular clock of human evolution ticks more slowly than that of other species.

Time between generations is an important determinant of how quickly a species' molecular clock is moving because fewer generations mean fewer chances for germline mutations.

And humans, which take a relatively long time to reach sexual maturity, have the longest generation time of any hominoid, according to the researchers.

"Humans have a longer generation time than any other extant hominoid because of differences in several life history traits. Humans take almost twice as long to reach sexual maturity as chimpanzees and gorillas, have a longer lifespan, and have a longer gestation period as compared to any nonhuman hominoid," they said in the paper.

"These traits are believed to have played important roles in human evolution. In particular, life span and the length of gestation are highly correlated with the size of the brain, which is approximately three times larger in humans than in other hominoids."

The researchers also observed a slight variation in the speed of human versus chimpanzee clocks. This result may suggest a very recent evolution of human-specific generation time, potentially over the last 1 million years, the researchers said.

"The most parsimonious explanation for the slower molecular clocks in both humans and chimpanzees as compared to gorilla and orangutan, is a slowdown in the ancestral lineage leading to the common ancestor of humans and chimpanzees," they concluded. Enditem