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.
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.
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.
Source: The Atlantic