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.