为何人类爱吃肉

　　It is no less remarkable¹ for people today, even those of us whose tastes run more to sirloin. Because if you look at where our species came from, none of our primate² cousins could ever survive such a meat-intensive diet. Like so much else that makes us unique, we owe our ability to eat all of that meat to changes in our DNA³.

　　Monkeys and apes have molars and stomachs adapted to pulping⁴ plant matter, and in the wild they eat mostly vegan diets. A few primates⁵, like chimpanzees, do eat a few ounces of termites⁶ or other animals each day. But for most monkeys and apes, a high-fat, high-cholesterol⁷ diet trashes their insides. Captive primates with regular access to meat and dairy products often end up wheezing⁸ around inside their cages, their cholesterol pushing 300 and their arteries⁹ paved with lard.

　　Our protohuman ancestors certainly ate meat; they left too many stone cleavers¹⁰ lying next to piles of bones for it all to be coincidence. And it isn't hard to figure out why they indulged. To most people, meat tastes really good. It provides precious protein, and it's packed with fatty calories, an important consideration when food sources were precarious¹¹. But for eons early humans probably suffered no less than other primates for their love of flesh─Paleolithic Elvises wandering the savanna¹².

　　Twice, however, since human beings diverged¹³ from chimpanzees a few million years ago, the human gene¹⁴ called apoE has mutated, giving us distinct versions. Overall it is the strongest candidate around for a human 'meat-eating gene' (though it isn't the only candidate).

　　The first mutation¹⁵─well before humans learned to control fire some 500,000 years ago─seemed to have boosted the performance of killer¹⁶ blood cells that attack microbes, like the deadly microbes lingering in mouthfuls of raw flesh. This mutation also protected against chronic¹⁷ inflammation, the collateral¹⁸ tissue damage that occurs when microbial infections never quite clear up.

　　Unfortunately, this version of apoE may have mortgaged our long-term health for short-term gain: We could eat more meat, but it left our arteries looking like the insides of Crisco cans. Luckily, a second mutation appeared around 226,000 years ago, which helped us break fats down and whisk cholesterol from our blood. What's more, it kept cells fitter and made bones denser¹⁹ and tougher to break in middle age, further insurance against early death.

　　ApoE probably also boosted our brains. To function properly, brain cells need to sheathe²⁰ their axons in myelin, which acts like rubber insulation²¹ on wires and helps brain signals travel much faster. Cholesterol is a major component²² of myelin, and while the cholesterol in our bellies²³ doesn't end up in our brains (the brain manufactures its own cholesterol), the version of apoE that helps whisk cholesterol from our blood also helps to distribute brain cholesterol where it's needed and therefore helps prevent myelin deterioration²⁴. The ability to eat more meat was perhaps just a side benefit of boosting our brain power.

　　Before we congratulate ourselves on our spiffy apoEs, however, consider this: Bones with hack²⁵ marks and other archaeological evidence indicate that we started dining on meat at least 2.5 million years ago, eons before the more recent fat- and cholesterol-fighting apoE emerged. So for millions of years we were either too dim to link eating meat and early demise²⁶, too pathetic to get enough calories without meat or too brutishly indulgent to stop ingesting food we knew would kill us. Even less flattering is what the germicidal properties of the earlier apoE mutation imply: that protohumans scavenged carcasses and ate putrid²⁷ leftovers²⁸.

　　Still, eating meat did help our ancestors survive, and live long enough to pass down their traditions to future generations. Now we celebrate most every holiday by eating (or avoiding) meat.

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