Our earliest ancestors subsisted on plants, seeds, and nuts. What spurred them to change their diets so dramatically?
The tale of how humans became such avid carnivores begins 65 million years ago. The dinosaurs have just gone extinct, together with over half of Earth’s species. In rainforests that carpet vast areas of the planet, among soaring trees ribboned with vines, our next ancestor has just evolved. It’s the first primate ever known: Purgatorius. It doesn’t look much like you or me, or even like a chimp. It resembles a cross between a mouse and a squirrel. And if it were still alive today, it would likely pass for a cute pet.
Purgatorius was an accomplished tree climber—and a vegan. It gave up the insect-based diet of its ancestors in favor of newly abundant fruits and flowers, carving for itself a comfortable niche high in the branches. For tens of millions of years, the descendants of Purgatorius were committed to their plant-based diets. From small monkeys to gorilla-size apes, they survived mostly on tropical fruits, spicing their meals with occasional worms (often by accident). Around 15 million years ago, they diversified a bit, adding hard seeds and nuts to their diets, but stayed true to their vegan roots.
Then, around 6 million years ago, Sahelanthropus tchadensis entered the African primate scene. With the advent of Sahelanthropus, our lineage likely split from that of our closest cousins, the chimps and bonobos. In the language of paleoanthropology, the word hominin stands for modern humans and all the extinct species closely related to us—and Sahelanthropus was the first. A short, flat-faced, small-brained creature, it most likely walked upright on two legs. It had smaller canine teeth than its ancestors and thicker tooth enamel, which suggests that its diet required more chewing and grinding than Purgatorius-like meals of fruits and flowers.
Nevertheless, meat-eating still hadn’t caught on among our ancestors.Sahelanthropus probably ate tough, fibrous plants supplemented with seeds and nuts. Later on, the several species of Australopithecus that lived between 4 and 3 million years ago in woodlands, riverine forests, and on seasonal floodplains of Africa weren’t hooked on meat, either. Their dental microwear—the pattern of microscopic pits and scratches left on the surface of their teeth by the foods they ate—suggests a diet similar to that of modern chimps: some leaves and shoots, lots of fruits, flowers, a few insects here and there, and even tree bark.
Did australopiths ever eat meat? It’s possible. Just as modern chimps occasionally hunt colobus monkeys, our ancestors may have occasionally dined on the raw meat of small monkeys, too. Yet the guts of early hominins wouldn’t have allowed them to have a meat-heavy diet, like the one Americans eat today. Their guts were characteristic of fruit-and-leaf eaters, with a big caecum, a bacteria-brimming pouch at the beginning of the large intestine. If an australopith gorged himself on meat—say, ate a few zebra steaks tartare in one sitting—he likely would have suffered twisting of the colon, with piercing stomach pains, nausea, and bloating, possibly resulting in death. And yet in spite of these dangers, by 2.5 million years ago, our ancestors had become meat eaters.
It seems that our bodies had to adjust gradually, first getting hooked on seeds and nuts, which are rich in fats but poor in fiber. If our ancestors ate a lot of them, such a diet would have encouraged the growth of the small intestine (where the digestion of lipids takes place) and the shrinking of the caecum (where fibers are digested). This would have made our guts better for processing meat. A seed-and-nut diet could have prepared our ancestors for a carnivorous lifestyle in another way, too: It could have given them the tools for carving carcasses. Some researchers suggest that the simple stone tools used for pounding seeds and nuts could have easily been reassigned to cracking animal bones and cutting off chunks of flesh. And so, by 2.5 million years ago, our ancestors were ready for meat: They had the tools to get it and the bodies to digest it.
But being capable is one thing; having the will and skill to go out and get meat is quite another. So what inspired our ancestors to look at antelopes and hippos as potential dinners? The answer, or at least a part of it, may lie in a change of climate approximately 2.5 million years ago. As the rains became less abundant, so did the fruits, leaves, and flowers that our ancestors relied on. Much of the rain forest turned into sparsely wooded grasslands, with few high-quality plants to eat but with more and more grazing animals. During the long, dry spell from January through April, our ancestors would have had problems getting enough food, and to find their usual fare, they would have had to expend more time and calories. Early hominins were at an evolutionary crossroads. Some, like the australopiths, chose to eat large quantities of lower-quality plants; others, like early Homo, went for meat. The australopiths ended up extinct, but early Homo survived to evolve into modern humans.
Interestingly, while these proto-humans chose to profit from the new wealth of savannah herbivores and their flesh, the ancestors of chimps and gorillas never did. One of the reasons might have been their inability to walk on two legs. Searching for meat is costly, requiring more long-distance walking—and, in turn, more energy—than eating grass or fruit. Moving on two legs is more energy efficient than chimp- or gorilla-style knuckle walking, and longer legs better dissipate temperature, which prevents overheating and boosts endurance. It seems that if Sahelanthropus or its ancestors didn’t stand up straight (or at least straight-ish) 6 million years ago, early Homo wouldn’t have been so well equipped to search for meat a few million years down the road, and might not have developed a taste for animal flesh—and there might not now be steaks or burgers on the dinner tables of today.
Still unanswered, however, is the question of what actually happened to spark the very first foray into carnivory. Maybe a few of our ancestors were walking among acacia trees and saw a saber-toothed cat feed on a gazelle. Maybe they stumbled upon a dead zebra, with its guts spilling out and meat exposed, and thought, hey, why not give it a try?
Even dedicated herbivores such as deer or cows will sometimes try meat if they chance upon it. There are records of cows devouring live chicks and munching dead rabbits, of deer eating birds, and of the duiker, a tiny African antelope, hunting frogs. (If you want to see a few of these carnivorous herbivores caught on camera, check out YouTube.) So it comes hardly as a surprise that our ancestors, who might have already been supplementing their diets with the meat of an occasional small monkey, saw the new abundance of savanna grazers as a way to get a few additional calories. The hominins were already omnivorous and opportunistic. If something was edible and it was there, they ate it. By 2.6 million years ago, there was a lot of meat around. Just as Purgatorius took advantage of the climate change and a new wealth of fruits, their descendants, early Homo, successfully adapted their diets to the changes in their environment. But this time, it meant going after meat.