What makes us human?
While some might turn to theology to ponder the possibilities, biologists have turned to our closest "cousin," the chimpanzee, and looked for genetic differences. New analysis finds that what sets us apart may lie more in how active our genes are, rather than in actual differences in our genetic makeups. And the biggest contrasts are found in our brains.
"It's not just one or two big differences — it's multiple ones," said Elaine Muchmore, a genetic researcher with VA San Diego Healthcare System and a professor of medicine at the University of California at San Diego. "I think this underscores how complex our differences are."
The finding could help in developing treatments for AIDS, Alzheimer's and malaria, since chimpanzees are less vulnerable to symptoms of these diseases than people.
An Active Brain
Since the 1970s, scientists have known that chimpanzees and people share about 98.7 percent of their genes. But while the genes may be identical, the amount of work they do is not always the same — particularly in the brain.
Genes provide the blueprint for all activity and development in the body, but they require messengers in the form of RNA, a molecule that carries information, and proteins to activate their coding.
The researchers used devices known as gene chips to compare the level of activity of these genetic messengers and proteins in blood and liver samples of chimpanzees and humans and then in brain samples from both. (Samples were collected from chimpanzees and humans who had died of natural causes.)
They found that the level of genetic activity was not that different in the liver and blood samples. But in the brain, it was about five times higher.
"This supports the fact that the human brain has undergone some radical evolution," said Ajit Varki, a professor of medicine at the University of California, San Diego.
Svante Paabo of the Max Planck Institute in Leipzig, Germany, was the senior author of the study, which is appearing in this week's issue of the journal Science. Varki and Muchmore, as well as Wolfgang Enard and Philipp Khaltovich of the Max Planck Institute and researchers from the Netherlands, also contributed.
Tracing Our Differences
Scientists believe that humans and chimpanzees shared a common ancestor 5 million to 7 million years ago. Since then people evolved separately and developed a brain about twice the size of chimps' and the ability to speak in a language.
When first scouting for the genetic roots that account for differences between people and chimps, scientists focused on the number and kinds of genes in both.
James Sikela, a genome scientist at the University of Colorado, has been pushing to launch a chimpanzee genome project that, like the human version, would set to decode all of the genes in the animal.
"It's possible there are a few extra genes in humans or chimps," Sikela said. "But that hasn't been worked out yet because the chimp genome isn't sequenced yet."
But researchers are also learning that other factors — like the level of gene expression or how the genes are arranged inside the chromosome — might also influence the biology of chimps and humans.
"It may be that a small number of differences in a few important genes could have a major effect on the expression and function of a large number of genes" said Sikela. "I think this study shows the value of looking at gene expression to gain insights into some of the genes and pathways that underlie the differences between humans and chimps."
Borrowing Chimp Resistance
Understanding the differences is not only interesting for biologists; Varki points out it could also pave the way to new medical treatments for people. He notes that chimpanzees can become infected with HIV, the AIDS virus, and never get sick. The animals also show no symptoms when infected with the most virulent form of malaria and they appear to be more resistant to the brain-degenerating Alzheimer's disease.
"By understanding the differences between humans and chimpanzees we can learn more about the genetics behind diseases that harm humans, but not chimpanzees," said Varki. "And that can lead to cures."