Nature already tried that, and look how it turned out.

These were the first farmers, staying in one place to plant and harvest. Some groups began to domesticate animals and then to milk them. Switching from hunting to a sedentary farming lifestyle generated more calories with fewer carbon emissions, because milk is much more efficient than meat. They weren't worried about atmospheric greenhouse gases, of course, but their new lifestyle allowed for a more efficient use of Earth's natural resources.

What do these lifestyle changes have to do with genetic engineering? They wouldn't have been possible without modifications to human biology. Prehistoric children naturally lost the ability to digest lactose, or milk sugar, as they got older. So in order to create a culture of dairying, which allows for a reduction in meat intake, our ancestors had to undergo some kind of genetic mutation—one that allowed them to consume lactose throughout their lives. We've now identified five versions of this shift in the human genome. That means there were five milky X-Men born less than 10,000 years ago, and who today have hundreds of millions of descendants around the world.

Most people today descend from early farmers, and are thus likely to carry a bigger genetic change. Saliva drips with an enzyme called amylase, which allows you to start converting starchy foods into sugar before they even hit your stomach. Lots of modern humans carry duplications of amylase in their genome, taking some of us from the original two copies of the gene to as many as 14. More copies make more amylase, breaking down starches faster. Having so many copies of the amylase gene is another bioengineered trait that has flourished over the last 10,000 years, allowing for a faster and more efficient caloric return from grains.

When it comes to cutting meat, natural selection has acted more like an entrepreneur than a eugenicist. Instead of giving us an aversion to meat, it lures us away from meat by offering a milkshake sweetened with corn syrup.

What about Liao, Sandberg, andRoache's other suggestions? Should we make children smaller, so they'll use less metabolic energy?

Early Holocene people did shrink. By developing a smaller mass, smaller stature, and smaller brains than their ancestors, our ancient relatives lived this specific form of bioengineering. Shrinking brains may have been favored by selection, but much of the change in height and weight came from malnutrition relative to hunter-gatherers. Their fabulous diet, which seems like it should have been more efficient and healthier, actually led to a raft of problems: bad teeth, stunted growth, and new viruses and parasites. At least initially the new, low-impact lifestyle brought with it a lower standard of living.

But humans' real bioengineering coup came later: We became bioengineers ourselves. Centuries and then millennia of tinkering led to the development of hybrid corn, fatted cattle, and tomatoes that survive shipping. The Green Revolution made us bigger, but whatever the impacts of pesticdes and nitrogren fertilizers, it also allowed us to raise more calories in less space.

So if we’ve undergone all these extreme changes, why are we still suffering from a damaged environment? It's true that we've learned and evolved ways to cut our dependence on meat, increase our reliance on sustainable and renewable foodstuffs, and pack ourselves into densely-populated urban areas. Humans today can draw hundreds of times more calories from each acre of land than they did as ancient hunter-gatherers. But these changes haven’t reduced our impact on the Earth; they've merely enabled us to grow the population a thousand times over. The bioengineering that came about 10,000 years ago led to more people, not less consumption.

Here's another problem: Natural selection is often a good engineer, but it’s slow. Ten thousand years later, milk-drinking still isn't universal. We resist many old diseases, but can't outrun some new ones. The cruelty of evolution is a lottery that exacts a high death rate for every useful mutation. Nature’s bioengineering doesn’t bother with human ethics. But then, maybe our future bioengineers won’t, either.

Population growth was the subject of Jonathan Swift’s original, trolling call for legalized cannibalism. It seems like bioengineering won’t be of much use unless we dare to adopt a truly Swiftian approach. Don’t breed meat-hating children; breed children with a taste for human flesh.