Chimpanzees and humans share a common ancestor, and even today 99 percent of the two species' DNA is identical. But since the paths of man and chimp diverged 5 million years ago, that one percent of genetic difference appears to have changed humans in an unexpected way: It could have made people more prone to cancer.
A comparative genetic study led by Cornell University researchers suggest that some mutations in human sperm cells might allow them to avoid early death and reproduce, creating an advantage that ensures more sperm cells carry this trait. But this same positive selection could also have made it easier for human cancer cells to survive.
"If we are right about this, it may help explain the high prevalence of cancer," says Rasmus Nielsen, lead author of the paper, and a former assistant professor of the Department of Biological Statistics and Computational Biology at Cornell who is now a professor at the University of Copenhagen, Denmark. The study, published in a recent issue of PLoS Biology (Vol. 3, Issue 6), a peer-reviewed, open-access journal published by the Public Library of Science (PLoS), focuses on identifying biological processes where positive selection -- adaptations that lead to new directions -- produced evolutionary changes that can be identified in the genomes of both humans and chimps.
To make these comparisons, the researchers used chimpanzee DNA sequence data generated by Celera Genomics of Rockville, Md. The chimpanzee and human versions of each gene were aligned, and on average they differ at only slightly more than 1 percent of the positions in the DNA.
The researchers' searched out the relatively few genes (13,731 sequences) that have diverged the most since sharing a common ancestor, most likely a primate that looked like a cross between a gorilla, chimp and human. While the scientists more or less expected to see that immune defense systems in each species have rapidly evolved separately to keep pace with attacking, mercurial bacteria and viruses, they were surprised to find that genes associated with the brain were practically the same.
One of the more interesting observations occurred in some genes that govern cell death in sperm cells and tumor cells alike. Both types of cells use a mechanism called apoptosis -- a pathway that includes genes that program a cell's demise and death. During the production of sperm cells, for example, apoptosis kills many of the cells before they reach maturity. But mutations in these genes could inhibit apoptosis in some sperm cells, allowing more sperm to reach maturity, reproduce again and ensure that future cells will carry the gene that defuses early self-destruction.