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July 2, 2007

Did human evolution make us more sickly?

How’s this for an interesting theory – the dietary switch that paved the way for the evolution of our impressive intelligence also made us more prone to illness.

The suggestion was made by Benjamin Hart of the University of California, Davis, who gave a talk on the four pillars of medicine, and whether they are practised by animals. The four cornerstones of medical practice – medication, nursing, immunization and quarantine – all have equivalents in the animal world, although only humans have adopted them all, perhaps through evolutionary necessity.

In the animal world, chimps medicate themselves by eating leaves to remove intestinal parasites. Elephants are notable nurses – if one of them is unwell, the whole group will slow down to allow them to keep up. Many carnivores drag meat along the ground before feeding it to young, which might partly function to inoculate them with bacteria and build up the immune system. And many animals practise a form of quarantine, by remaining in close-knit tribes to avoid coming into contact with too many individuals who might pass on diseases.

But humans are the undoubted masters of medicine, Hart points out. And he suggests that the switch from a plant diet to a meaty one, which happened several hundred thousand years ago with our ancestors Homo erectus, may have paved the way not only for the rapid expansion of our brains, but also removed many of the valuable, antioxidant-rich plants from our diet, which led to us becoming much more sickly.

A big brain would have come in handy in figuring out how to stave off all this new disease, Hart says. But it’s worth noting that animals seem to get sick far less often than we do. Yet another reason why meat-rich diets might give doctors food for thought.

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