Many of our primate ancestors probably ended up in the bellies of big cats. How else to explain bite marks on the bones of ancient hominins, the apparent gnawing of leopards or other African felines?

Big cats still pose a threat to primates. In one study of chimpanzees in Ivory Coast, for example, scientists estimated that each chimp ran a 30 percent risk of being attacked by a leopard every year.

A new study suggests that the big cats may be getting some tiny help on the hunt. A parasite infecting the brains of some primates, including perhaps our forebears, may make them less wary.

What does the parasite get out of it? A ride into its feline host.

The parasite is Toxoplasma gondii, a remarkably successful single-celled organism. An estimated 11 percent of Americans have dormant Toxoplasma cysts in their brains; in some countries, the rate is as high as 90 percent. Infection with the parasite poses a serious threat to fetuses and to people with compromised immune systems. But the vast majority of those infected appear to show no serious symptoms. Their healthy immune systems keep the parasite in check.