News Editor: Renato M.E. Sabbatini, PhD
Creative Use of Genes Makes Humans Unique
What makes humans different from chimps, apart from a little extra hair? It might be a more creative use of the genes that affect how the brain works, a study published on Thursday suggests.
Humans and chimps share 98.7 percent of their DNA, but are clearly very different. Scientists have long tried to determine how just over 1 percent of our genes can make such a difference.
It may not be an issue of quantity, but of quality, an international team of genetics experts reports in this week's issue of the journal Science. And the differences seem to lie mostly in the brain.
"When you look at blood, you don't see a lot of differences. Chimps look very, very much like us," Ajit Varki of the University of California San Diego, who helped direct the research, said in a telephone interview.
"But when you look at brain you see a lot of differences."
The findings could go a long way to soothing humans flustered at learning that we have fewer genes than even some plants. Public and private teams who sequenced the human genome (news - web sites) estimate that we have only about 30,000 to 40,000 genes as compared to, for instance, 50,000 for rice.
Genomics experts say it is not how many genes you have that counts, but what you do with them -- in this case the protein products of genes.
The work by Varki and colleagues, including noted genetics expert Svante Paabo of the Max-Planck-Institute for Evolutionary Anthropology in Leipzig, Germany, supports this.
They found five times as many changes in gene expression -- actual activity by the genes -- in the human brain than would be predicted by evolution.
"If two species have been apart for 5 million years, you expect a certain amount of differences," Varki said. But there are many more differences in the human brain than expected.
"Whereas if you look at liver and blood, you don't see that," Varki added.
The researchers look at messenger RNA, which is what translates the genetic recipe found in DNA into a final product -- a protein. Every cell contains a full complement of DNA, but what makes one cell become a liver cell and another function as a brain cell depends on what genes are expressed.
They took many different samples of tissue from human cadavers, chimps and orangutans that had died of natural causes, monkeys and mice, and compared them.
GRAY MATTER SHOWS GENETIC DIFFERENCES
In the case of the brain, they took gray matter from the left prefrontal lobe, one of the areas involved in thought as opposed to controlling movement or bodily function.
Using a gene chip, they checked to see which genes were actually being expressed by looking for messenger RNA.
The gene expression patterns of the chimpanzees and the macaque monkeys were more similar to one another than they were to humans. But the genes expressed in blood and liver were very similar in humans and chimps.
In addition, humans differed more from one another than chimps differ from one another, they found. As genomics experts predicted, it seems that humans mix and match their proteins, so that while there are only 30,000 genes, there are an estimated 250,000 different human proteins.
"With an understanding of the differences between humans and chimpanzees, we may be able to learn more about the genetics underlying diseases that seem to harm humans but not chimpanzees," Varki said. For instance, chimps do not die from AIDS (news - web sites) or malaria and do not get Alzheimer's or cancer in the way that humans do.
In 1998 Varki and colleagues documented the first genetic difference to be found between humans and chimps -- a cell-surface carbohydrate molecule called sialic acid that chimps have but that humans do not.
"The thing about this topic -- I never met a human being who was not interested in it," Varki said
By Maggie Fox
Reuters News, April 11, 2002