The evolutionary path that separated humans from chimps 5 million years ago may have made human sperm survive better but paradoxically may have made humans prone to cancer.

A comparison of chimpanzee genes to human genes shows a concentration of genes unique to people in areas associated with sperm production and cancer, and suggests the changes that make humans unique also make us uniquely prone to cancer.

"If we are right about this, it may help explain the high prevalence of cancer," said Rasmus Nielsen of the University of Copenhagen in Denmark, who led the study while at Cornell University in New York.

Nielsen and colleagues were studying the chimpanzee genome, the collection of all DNA, for clues about what make chimps and humans different. They used genetic sequences published by Maryland-based Celera Corp.

For the report, published in the journal Public Library of Science Biology, Nielsen's team at Cornell studied the 13,731 genetic sequences that are the most different between humans and chimps.

They knew that genes having to do with smell, making sperm and fighting bacteria and viruses were likely to be different.

"While we expected to find genes involved in olfaction, spermatogenesis, and immune defense among the 50 annotated genes ... we were surprised to find a very large proportion of cancer-related genes, especially genes involved in tumor suppression, apoptosis, and cell cycle control sequences," they wrote.

"It is surprising to find such a large proportion of genes that may be related to tumor development and control."

In cancer, cells lose their ability to self-destruct when they become faulty, a process called apoptosis. Cell cycling -- the process by which cells activate, divide, and grow into two separate cells -- is also disrupted in cancer.

"Eliminating cancer cells by apoptosis is one of the main processes used by the organism to fight cancer," Nielsen said.

"The connection that we saw that these genes involved in proliferation may be involved in spermatogenesis," Cornell's Andrew Clark, who worked on the study, said in a telephone interview.

Apoptosis also kills many developing sperm cells before they mature. But evolution could have interfered with this process, allowing more sperm to reach maturity, thus carrying the mutation into the next generation.

Clark said chimpanzees get cancer, too, but no one has been able to study enough of them in captivity to see if they do so at the same rate and in the same ways as humans do.

Cancer in people usually occurs in late adulthood, after they have reproduced, and thus has not been removed by natural selection -- the process that leads to evolution.