Humans, chimps: Biggest difference is in brain
By Rosie Mestel
LOS ANGELES TIMES
Friday, April 12, 2002
Scientists have long puzzled over how humans and chimpanzees can share 98.7 percent of the same genes, yet have vastly different mental capacities.
Now an international team of researchers says it knows why. The genes in human brains, they report in today's issue of the journal Science, operate in different ways than the ones in chimps do, potentially giving humans a huge lead in mental prowess over their closest primate relative.
Figuring out precisely which genes differ, and how, could eventually provide clues toward treating human diseases from AIDS to malaria or shed light on the reasons why we age.
The brain appears to be the only organ where gene function varies so dramatically between humans and chimps, the researchers said. Genes elsewhere in the body, such as in the liver and blood, perform almost exactly the same in both species.
Examining more than 6,000 genes active in the human brain, researchers discovered that 175 of them produced proteins at rates up to five times greater or five times lower than the same genes in chimps, said the report's senior author, Svante Paabo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
Proteins serve not only as the building blocks of cells but also as the chemical messengers that allow brain cells to communicate. The absence of even one protein, for example, can lead to depression or severe mental retardation.
"The implication is that that might be the sort of crucial thing that has given us a special cognitive ability," Paabo said.
"This study suggests that changes in the brain were one of the main ways that humans evolved away from chimps," said Dr. Ajit Varki, a professor of medicine at the University of California at San Diego and co-author of the study.
Scientists suspect that many chimp-human differences will be caused by slightly different control of genes that both species share. Certain genes may be cranked up higher in one species or turned on earlier, later or in a different part of the body.
Each of these kinds of changes might have profound consequences -- perhaps causing embryonic brain cells to divide more times as an individual grows, thus creating a larger brain; or maybe creating subtle alterations in bone growth that help make walking upright easier.
To advance understanding of the chimpanzee's genetic code, Varki and many of his colleagues are urging the U.S. government to make deciphering the chimp genome the nation's next big genetic project.
Some scientists study the genetics of chimpanzees because comparing us with our nearest living relatives will deepen the understanding of who we are and where we came from.
Other biologists hope that the chimp genome will provide hard molecular data about how the human lineage evolved.
And scientists with a medical bent point to other key biological differences between chimps and humans: chimps don't get AIDS. They don't get malaria. They seem to have lower rates of cancers common in humans, such as those of the colon, stomach, breast and prostate.
Understanding the genetic reasons for such differences might potentially lead to therapies, Varki says.
Other scientists are exploring chimp-human differences in aging.
We live longer than chimps: in the wild they live 40 to 45 years, and even in zoos they live only to about 55 to 60. Humans, under the best of conditions, can live decades longer than that. Evolutionary biologists speculate that we evolved this longer life span because our culture is so complex and our young require long periods of learning to reach full productivity.
Scientists might be able to discover the genetic reasons why human aging is slower than chimp aging, said Caleb Finch, director of the University of Southern California's Alzheimer's Disease Research Center.
Finch thinks that a gene called ApoE might be involved in this aging difference. Both chimps and humans have this gene. But the gene exists in more than one form. All chimps examined so far have a version of the gene that -- when it is present in humans -- is known to increase the risk of Alzheimer's, a disease associated with aging. Most humans have a form of the gene that confers a lower risk.
Finch suspects that as humans evolved, the low-risk version of the gene became more prevalent among humans because those carrying the lower-risk gene would be more likely to live a longer life.
"It seems plausible that we may be able to discover genes that modify human aging and that could be new targets of intervention in disease" from the chimp gene research, he said.
This article contains material from other wire services. |
