After 10m years, we're still not used to meat

A single enzyme in our liver betrays our plant-eating evolutionary roots, but our bodies have not quite adapted yet to the relatively recent introduction of meat in our diet.

For the past 20 years, Professor Chris Danpure from University College London has studied the liver enzyme AGT in mammals. In carnivores like lions and tigers, AGT is found in the mitochondrion, but herbivores like sheep and cows have it in their cells' peroxisome.

Omnivorous humans, meanwhile, have their AGT in the same cellular area as sheep - a genetic throwback to the great apes' common ancestors.

This was, however, not a reason to go completely vegan, said Professor Danpure, who is in New Zealand for the Queenstown Molecular Biology Meeting.

"All I'll say is that our ancestral condition ten million years ago is vegetarian and we may be suffering the consequences now of meat eating. But people that don't eat meat have other problems, especially vegans.

"I think the most important thing is a varied diet. If you look at other aspects of our body, we're clearly supposed to be omnivorous - it's only this particular enzyme that looks as if it's in herbivorous distribution. But it may catch up to our omnivorous lifestyles in years to come."

While working in the department of rare metabolic diseases in 1985, he was presented with the chance to study the liver of a patient with Primary Hyperoxaluria Type 1 (PH1), a rare liver disease characterised by kidney stones.

Professor Danpure said many PH1 patients had significant amounts of AGT but the enzyme was located in the mitochondrion, and not the peroxisome.

"It was all there, but it was in the wrong place.

"These patients were like lions and tigers - they had a carnivorous distribution, which is clearly very bad unless you eat only meat."

The finding intrigued the biochemist, and he started looking into other mammals and their evolution.

In mammalian evolution, AGT was found to have moved around frequently between peroxisomes and mitochondria, "which is really, really surprising, because almost all other enzymes stay in the same place.

"We calculated the distribution must have changed in at least 20 occasions during the evolution of mammals. The only thing we have been able to relate this to is diet."

The common ancestor for great apes like chimpanzees, gorillas - and humans - was most probably herbivorous, he said. Sometime during the evolutionary track, a mutation occurred.

"So all the mitochondrial protein was lost, and it all became peroxisome, which was very compatible with herbivorous diets.

"And it was probably herbivorous dietary selection which caused this mutation to be maintained. And that's the mutation that's still found in all modern great apes."

Human diets, however, have become more omnivorous. Professor Danpure said rises in meat consumption have been paralleled by an increase in kidney stone disease.

"It would seem that the distribution of AGT is not optimal for our current diet."

But in some individuals, a common mutation, or polymorphism, allows for small amounts of AGT to be sent back to the mitochondria.

"It makes the individuals that carry this polymorphism look a bit more omnivorous, if you like, as far as AGT is concerned."

In Western societies, this polymorphism has a frequency of 15 to 20 per cent. But among the Sami people, who live in the Arctic circle and have an almost completely meat-based diet, the frequency is 30 per cent, while among Indian Hindu populations in Mumbai, who subsist on a completely vegetarian diet, the frequency was 3 per cent.

"We know it's increasing because we know that this is not found in orangutans, gorillas, or chimpanzees. And we know it occurred maybe 15,000 years ago in human ancestors. It may only have started to increase when it became a selective advantage and one of the advantages is meat eating."