By LiveScience Staff Writer
updated 6/29/2006 6:21:27 PM ET 2006-06-29T22:21:27

Early humans living in Africa's open savannahs probably made easy pickings for large predatory cats, but a new study suggests that at least one of the meals didn't sit well.

A large cat dining on the entrails of one our early ancestors thousands of years ago contracted an ulcer-causing bacteria that spread to lions, cheetahs and tigers and which persists to this day, a new study concludes.

The strange finding will be detailed in an upcoming issue of the journal PLoS Genetics.

Too similar
Helicobacter pylori are bacteria that cause chronic stomach pains and ulcers in humans. Other animals, including non-human primates, are infected by other Helicobacter species that are only distantly related to H. pylori. The one exception is H. acinonychis, a microbe that infects large felines such as lions, tigers and cheetahs.

The microbes responsible for ulcers in humans and big cats are so similar that scientists speculated they were once one species. According to this theory, the microbe diverged after either a human ate a cat infected with the ulcer-causing bacteria or a cat ate an infected human.

To determine who or what ate whom, the researchers compared the genomes of the two bacteria species. They found that many of the inactive genes in H. acinonychis, the species that infects large cats, are more fragmented, or broken, than their still-functioning counterparts in H. pylori.

This strongly suggests that the direction of the host jump was from humans to cats, and not the other way around, the researchers say.

"It is very unlikely that such gene fragmentations could be restored to yield intact genes, which is what would have been necessary" if the bacteria jumped from cats to humans, said study team member Mark Achtman of the Max-Planck Institute in Germany.

The researchers think the host jump happened only once because many of the gene fragmentations are uniform among different strains of H. acinonychis.

Fighting to get sick
Based on similarities in the genomes of the two bacteria species, the researchers estimate that the host jump from humans to large cats took place about 200,000 years ago.

"The feline was probably a tiger, cheetah or lion but we don't know which," Achtman told LiveScience.

After the first cat was infected, the bacteria probably spread to other feline species during deadly fights for territory, explained another researcher on the project, Stephan Schuster of Pennsylvania State University.

"Behavior like this has been observed numerous times among the big cats in Africa," Schuster said.

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