Humans share a common herbivorous, or plant-eating, ancestor with living apes and early hominins were mostly vegetarians who ate fruit, nuts, tubers (roots), and also ate insects like termites. Like other herbivorous mammals, including monkeys and apes, humans cannot synthesize vitamin C—an unnecessary skill with a diet comprised of vitamin C-rich vegetable matter. But the drastic shift in hominin brain and body size around 2 Mya is linked to a shift in diet toward carnivory. Scavenging and hunting during the hot daytime hours on the East African savannah is a significant component of the evolution of the suite of characters that differentiated early Homo from australopiths and Paranthropus and eventually led to humans.
Although meat-eating holds an important place in our evolutionary history, it did not emerge uniquely in our lineage. Mostly fruit-eating chimpanzees at Gombe, Tanzania, are known for their habit of killing and eating red colobus monkeys to the point of significantly reducing monkey populations in the region. They have even been known to kill small bovids and wild pigs. Chimpanzees also use tools to hunt for meat. Jill Pruetz and Paco Bertolani observed male, female, and even juvenile chimpanzees in Senegal fashioning spears from branches (similar to the way females fashion termite fishing wands) and using them to kill bush babies (nocturnal strepsirhines that sleep during the day). They jabbed the spears into hollow trees where bush babies were nesting in order to immobilize them before breaking into the tree to catch and eat them.
It is hard to imagine that our hominin ancestors lacked such hunting sophistication but unfortunately spears like those made by chimpanzees do not preserve in the fossil record for us to find out. In order to track the evolution of human meat-eating by scavenging and hunting, we must follow the record of stone tools and devoured animal bones. Stone tools are found as early as 2.6 Mya and, soon after that, around 2.5 Mya, the earliest cut-marked bovid fossils come from Bouri, Ethiopia. Although these dates are normally cited as the earliest considerable incorporation of meat into the hominin diet, neither site has both cut-marked bones and stone tools.
The beds at Olduvai Gorge (dating to as early as about 2.5 Mya) provide direct association between tools and butchered bones. Pat Shipman and Rick Potts studied the cut marks and the carnivore tooth marks on the Olduvai faunal assemblage and suggested that hominins were scavenging meat. Cut marks on bones indicate butchery and with the aid of a microscope they can be distinguished from carnivore tooth marks. A stone flake, no matter how fine-grained the material is, will never have a perfect straight edge, so that when it is dragged across a bone during use, its jagged edge leaves many fine parallel lines. On the contrary, carnivore teeth leave a singular, rounded groove on bones.
At Olduvai, cut marks were found superimposed on tooth marks and vice versa implying that either hominins were the primary hunters/scavengers and carnivores were secondarily scavenging from kill sites, or that hominins were scavenging from carnivore kills. The second scenario, with hominins as scavengers, is usually the case at the earliest sites. Marrow inside the bones is especially nutritious and scars from bashing open bones with stone tools are also distinguishable from the marks from a carnivore's bone crushing teeth.
In the absence of carnivore tooth marks for hunting versus scavenging evidence, there are other means of establishing meat-eating. Stone tool cut marks located at areas of skeletal articulation are signs of butchery (e.g., at the joints where ligaments and tendons are connecting the bones). Depending upon whether the archaeological site is a kill site (where the animal is butchered and carried "home") or a living site (or "living floor"), the absence or presence of the bones associated with the meaty and delicious parts of animals also indicate whether or not hominins had first dibs on the carcass or whether they scavenged after a carnivore. Even if there are no cut marks it is sometimes possible to identify a fossilized carnivore kill because each carnivore species has a particular way of devouring an animal. Some eat the feet and ribs and leave the rest of the bones, but some eat everything, even the vertebrae, and leave only the skull and teeth. Bob Brain was one of the first to document the different eating preferences of carnivores because he actually fed them baboon carcasses and then analyzed the parts that were left behind.
Selection must have been very high on hominins who obtained meat since scavenging from a carnivore kill would have been extremely dangerous. Hominins could have developed sly methods of creeping toward a carcass, but also could have coordinated groups to chase away the carnivores from their food. After acquiring the skills to chase away a carnivore from a kill, it hardly seems like a big behavioral leap for hominins to hunt and kill their own prey. Because chimpanzees do not obtain meat through scavenging we would not necessarily assume hominins did that either if it were not for the abundant evidence from sites like Olduvai Gorge and the overlapping of cut marks and tooth marks.
The evolution of meat-eating and then predatory behavior is probably the root of many modern sports and athletic endeavors. Endurance running and overarm throwing no doubt helped early hominins hone their hunting skills. Genes like ACE, although it does not exist in all populations, help explain how humans can work harder with less fuel and increase their endurance. Throwing would have been of great importance since it allows action-at-a-distance. And William Calvin has even suggested that because of the complex neural control involved in accurate overarm throwing, it is linked evolutionarily to brain size increase and language.
Meat intake could was definitely beneficial and possibly necessary for growing a large brain. Leslie Aiello postulated that to balance out the energetic cost of growing and maintaining a big brain, hominins evolved shorter guts (i.e., selection favored hominins with shorter intestinal segments). Humans have relatively smaller guts for their body size compared to apes, which is a shared feature with carnivores. But intestinal material is nearly as energetically costly as the brain and perhaps the body compromised for the development of more "expensive tissue" in the brain and less in the gut, thus reinforcing a shift in diet.
Meat-eating and related strategies separated Paranthropus from the Homo lineage. While Paranthropus was eating plant materials and termites, early Homo had shifted their diet to include meat and bone marrow and this is perhaps indicative of their sympatric speciation. Around the time that we see an abundance of archaeological data for meat-eating, there is a transition in hominin body size and brain size. A
growing brain needs more energy for maintenance and the incorporation of meat into the diet may have selected for bigger brains and increased intelligence (needed for hunting skills) while feeding them. Several predictions can be made about a species that becomes a predator. Based on knowledge from our current fossil record, H. erectus fits many of these predictions. Namely H. erectus shows an increase in body size, an increase in geographic range, a major technological shift (from Oldowan to Acheulean tools), and an increase in sociality. The last point is debatable because evidence is indirect, but some would point to the evolution of group hunting as support.
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