Nobody in the Ice Age lived on berries, and nobody chased rabbits for a living, and if our ancestors had followed the evidence-based diet none of us would be here to be told about it. It's northern Europe, 30,000 years ago, October. Grass to the horizon, a wind off the ice sheet that hasn't stopped since the summer, a line of willow along the river. The healthy-eating poster says five portions of fruit and vegetables a day. The berries were three weeks in August and the birds had most of them. The roots are the size of a thumb, bitter, and an hour of digging gets a handful that needs cooking before it stops trying to poison anyone. The grass is grass, and a human has one stomach for it. The poster says wholegrains. There's a wild grass seed the size of a flea, four months off, and a day of threshing it with a flat stone makes a cupful. The poster says choose lean protein. So the hare. Two hours of running for a kilo of meat with no fat on it, and a fortnight of hares leaves the body sick, weak and hungrier than it started, because protein with nothing to burn it with makes the body eat itself. The poster, read faithfully, has killed the tribe by December, and the dietitian who wrote it would have been eaten first, on account of being the only one with any fat on him. Across the river there's a herd of bison, and behind them horses, and twice a year the reindeer come through in their thousands, and now and then a mammoth that is, on its own, a village's winter. Big, slow in the cold, walking the same path to the same water every evening, carrying a layer of fat on their backs that renders into something that keeps in a skin for months. The marrow keeps in the bone. The brain is fat. The hump is fat. One kill feeds forty people for weeks and the fire does the rest. The people who lived there looked at the bitter roots, the hare, and a thousand kilos of dinner strolling to the river at the same time every evening, and they made the choice that put a brain in your skull. Nobody who ate the roots left descendants to write guidelines. The diet that would have wiped us out by Christmas is now on a poster in every surgery in the country, with a government logo on it, and the people handing it out are descended from the ones who ignored it.