The mouth-watering smokiness of a rack of pork ribs. The juicy gluttony of a medium-rare bacon cheeseburger. The simple pleasure of a salami sandwich on rye. One thing is clear—humans love meat. But why do we eat so much more meat than our primate cousins and why are we wired to drool at the sound and smell of steaks sizzling on the grill?

Scientists still have plenty of unanswered questions about the origins and evolution of human meat-eating, but there are some strong theories as to when, how and why we started to incorporate larger amounts of meat in our omnivorous diet.

Blame an ancient climate shift.

Between 2.6 and 2.5 million years ago, the Earth got significantly hotter and drier. Before that climate shift, our distant human ancestors—collectively known as hominins—were subsisting mostly on fruits, leaves, seeds, flowers, bark and tubers. As the temperature rose, the lush forests shrank and great grasslands thrived. As green plants became scarcer, evolutionary pressure forced early humans to find new sources of energy.

The grassland savannas that spread across Africa supported growing numbers of grazing herbivores. Archaeologists have found large herbivore bones dating from 2.5 million years ago with telltale cut marks from crude stone tools. Our ancient hominin ancestors weren’t capable hunters yet, but likely scavenged the meat from fallen carcasses.

“More grasses means more grazing animals, and more dead grazing animals means more meat,” says Marta Zaraska, author of Meathooked: The History and Science of Our 2.5-Million-Years Obsession With Meat.

Once humans shifted to even occasional meat eating, it didn’t take long to make it a major part of our diet. Zaraska says there’s ample archaeological evidence that by 2 million years ago the first Homo species were actively eating meat on a regular basis.

Tools became our ‘second teeth.’

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A saber-toothed tiger hunting its prey.

It’s not a coincidence that the earliest evidence of widespread human meat-eating coincides in the archaeological record with Homo habilis, the “handyman” of early humans. At sites in Kenya dating back to 2 million years ago, archaeologists have discovered thousands of flaked stone “knives” and fist-sized hammerstones near large piles of animal-bone fragments with corresponding butcher marks.

While our ancient human relatives had stronger jaws and larger teeth than modern man, their mouths and guts were designed for grinding up and digesting plant matter, not raw meat. Even crude stone tools could function as a second set of teeth, stripping hunks of flesh from a zebra carcass or bashing open bones and skulls to get at the nutrient-rich marrow or brains inside. By pre-processing meat with tools originally designed to dig tubers and crack open nuts, our ancestors made animal flesh easier to chew and digest.

Thank you, saber-toothed tigers.

Primitive stone hand tools are fine for carving up carcasses or smashing open large bones, but they are lousy for hunting live prey. This is why zooarchaeologists believe our meat-eating human ancestors living more than a million years ago were scavengers, not hunters.

One theory for why so many butchered animal bones entered the archaeological record around 1.8 million years ago is that while early humans were lousy hunters, they were living among some of the most efficient killers to ever roam the earth: saber-toothed cats.

Briana Pobiner, who studies the origins of human meat-eating, wrote that “Between one- and two million years ago the large carnivore communities of the African savanna consisted not only of lions, hyenas, leopards, cheetahs and wild dogs, as we see today, but also at least three species of saber-toothed cats, including one that was significantly larger than the largest male African lions. These cats may have hunted larger prey, leaving even more leftovers for early humans to scavenge.”

It’s unclear if humans “actively” scavenged by waiting for the big cats to kill their prey and then scaring them off by throwing stones or making loud noises, or if they “passively” scavenged what was left when the saber-toothed hunters abandoned their kill. Active scavenging would preserve more fresh meat but carries some serious risks.

Meat was the original ‘brain food.’

Henry Guttmann Collection/Hulton Archive/Getty Images
A reconstruction of a pre-historic cave man, at the Chicago Field Museum, eating meat.

The modern human brain is far larger than that of other primates and three times the size of the one possessed by our distant ancestor Australopithecus, the predecessor of Homo. But those big brains come at a cost in that they require tons of energy to operate. Zaraska says our brains consume 20 percent of our body’s total energy. Compare that to cats and dogs, whose brains require only three to four percent of total energy.

Meat, Zaraska says, played a critical role in boosting energy intake to feed the evolution of those big, hungry brains. “Some scientists argue that meat is what made us human,” she says.

When ancient hominins subsisted exclusively on fruits, plants and seeds, they expended a lot more energy on digestion. Millions of years ago, the human gut was longer and slower, requiring more effort to derive limited calories from forage foods. With all of that energy being spent on digestion, the human brain remained relatively small, similar to other primates today.

Compared to foraged fruits and plants, Zaraska says, meat is a “high-quality” food — energy dense with lots of calories and protein. When humans began adding meat to their diet, there was less of a need for a long digestive tract equipped for processing lots of plant matter. Slowly, over hundreds of thousands of years, the human gut shrunk. This freed up energy to be spent on the brain, which grew explosively in size.

When humans began cooking meat, it became even easier to digest quickly and efficiently and capture those calories to feed our growing brains. The earliest clear evidence of humans cooking food dates back roughly 800,000 years ago, although it could have begun sooner.

Humans continue to eat meat because we like it, not because we need it.

Meat was clearly pivotal in the evolution of the human brain, but that doesn’t mean that meat is still an irreplaceable part of the modern human diet. Zaraska says any calorie-dense food would have had the same effect on our ancient evolving brains—“it could have been peanut butter”—but that meat happened to be available.

We crave meat today, in part, because our brains evolved on the African savanna and are still wired to seek out energy-dense sources of protein. It’s similar to our penchant for sugar, a rare calorie-rich commodity to our foraging ancestors whose brains rewarded them for finding ripe fruit.

But we also crave meat because of its cultural significance. Different cultures are more or less meat-centric, although there’s a clear correlation between wealth and meat consumption. Industrialized Western nations average more than 220 pounds of meat per person per year, while the poorest African nations average less than 22 pounds per person.

An overly meaty diet has been linked to heart disease, diabetes and certain cancers—things our distant ancestors never had to worry about, because they didn’t live long enough to fall victim to chronic disease. “The goals of life for our ancestors was very different than ours,” says Zaraska. “Their goal was to survive to the next day.”

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