Meat-Eating: Evolution of Our Brain and Gut
Source: www.americanscientist.org Read the full article by clicking on the link. There are some wonderful pictures there depicting the evolution of the size of our brain and gut compared to primates.
Evidence of meat-eating among our distant human ancestors is hard to find and even harder to interpret, but researchers are beginning to piece together a coherent picture.
by Briana Pobiner
Over the course of six million years of human evolution, brain size increased 300 percent. Our huge, complex brains can store and process decades worth of information in split seconds, solve multifactorial problems, and create abstract ideas and images. This would have been a big advantage to early humans as they were spreading out across Africa and into Asia just under two million years ago, encountering unfamiliar habitats, novel carnivore competitors, and different prey animals. Yet our large brains come at a cost, making childbirth more difficult and painful for human mothers than for our nearest evolutionary kin. Modern human brains take up only about 2 percent of our body weight as adults, but use about 20 percent of our energy. Such a disproportionate use of resources calls for investigation. For years, my colleagues and I have explored the idea that meat-eating may have played a role in this unusual aspect of human biology.
In 1995, Leslie Aiello and Peter Wheeler developed the expensive tissue hypothesis to explain how our huge brains evolved without bringing about a tremendous increase in our rate of metabolism. Aiello, then of University College London, and Wheeler, then of Liverpool John Moores University, proposed that the energetic requirements of a large brain may have been offset by a reduction in the size of the liver and gastrointestinal tract; these organs, like the brain, have metabolically expensive tissues. Because gut size is correlated with diet, and small guts necessitate a diet focused on high-quality food that is easy to digest, Aiello and Wheeler reasoned that the nutritionally dense muscle mass of other animals was the key food that allowed the evolution of our large brains. Without the abundance of calories afforded by meat-eating, they maintain, the human brain simply could not have evolved to its current form.
Although the modern “paleodiet” movement often claims that our ancestors ate large amounts of meat, we still don’t know the proportion of meat in the diet of any early human species, nor how frequently meat was eaten. Modern hunter-gatherers have incredibly varied diets, some of which include fairly high amounts of meat, but many of which don’t. Still, we do know that meat-eating was one of the most pivotal changes in our ancestors’ diets and that it led to many of the physical, behavioral, and ecological changes that make us uniquely human.
Our Omnivorous Ancestors
The diet of our earliest ancestors, who lived about six million years ago in Africa, was probably much like that of chimpanzees, our closest living primate cousins, who generally inhabit forest and wet savanna environments in equatorial Africa. Chimpanzees mainly eat fruit and other plant parts such as leaves, flowers, and bark, along with nuts and insects. Meat from the occasional animal forms only about 3 percent of the average chimpanzee’s diet. In 2009, Claudio Tennie, of the Max Planck Institute for Evolutionary Anthropology, and his colleagues developed a hypothesis that offered a nutritional perspective on the group hunting they had observed in the chimpanzees in Gombe National Park, in Tanzania. According to this hypothesis, the micronutrients gained from meat are so important that even small scraps of meat are worth the very high energy expenditure that cooperative hunting entails. Important components of meat include not only vitamins A and K, calcium, sodium, and potassium, but also iron, zinc, vitamin B6, and vitamin B12; the latter, although necessary for a balanced primate diet, is present only in small quantities in plants. In addition, macronutrients such as fat and protein, hard to come by in the environments where chimpanzees live, may be important dietary components of meat-eating.
The fossil record offers evidence that meat-eating by humans differs from chimpanzees’ meat-eating in four crucial ways. First, even the earliest evidence of meat-eating indicates that early humans were consuming not only small animals but also animals many times larger than their own body size, such as elephants, rhinos, buffalo, and giraffes, whereas chimpanzees only hunt animals much smaller than themselves. Second, early humans generally used tools when they procured and processed meat. (Of course, meat-eating by human ancestors could have taken place before early humans developed the ability to procure meat by means of tools —but so far no one has determined whether the fossil record would show any evidence of it or what the evidence would look like.) Third, as we will see later, it’s likely that much of the first meat eaten by early humans came not from hunting but from scavenging; by contrast, observations of chimpanzees scavenging are extremely rare. Fourth, like humans today, our early ancestors didn’t always eat food as soon as they encountered it. Sometimes they brought it back to a central place or home base, presumably to share with members of their social group, including unrelated adults. This behavior, the delaying of food consumption, is not observed in chimpanzees, and it holds important implications for how these early humans interacted with one another socially.
Hunters or Scavengers?
The investigation of early human meat-eating in Africa began in 1925, with the earliest discovery of human fossils there. Raymond Dart, a professor of anatomy at the University of the Witwatersrand in Johannesburg, South Africa, named a new early human species Australopithecus africanus (meaning “southern ape from Africa”) after a small fossil skull from the site of Taung. The skull has since been identified as that of a three-year-old child who died about 2.8 million years ago. In other fossils at the same site Dart saw evidence of meat-eating, such as baboon skulls bearing signs of fracture and removal of the brain case prior to fossilization, with v-shaped marks on the broken edges and small puncture marks in the skull vault. He concluded the Taung child had belonged to a predatory, cave-dwelling species he described as “an animal-hunting, flesh-eating, shell-cracking and bone-breaking ape” and “a practised and skilful wielder of lethal weapons of the chase.” The concept of the killer ape was born.
Also put forth as evidence for early human hunting are patterns in the types of animal bones found in the fossil record at early human sites in East and South Africa; such patterns are called skeletal part profiles. The bulk of the data came from the well-known site of Olduvai Gorge in Tanzania, excavated by the famous duo Mary and Louis Leakey, mainly in the 1950s. Three decades later Henry Bunn, an anthropologist at the University of Wisconsin-Madison, studied the animal fossils from 1.8-million-year-old sediments at the site known as FLK Zinj (after a fossil found there that had at one time been designated Zinjanthropus). He inferred from the abundance of meat- and marrow-rich limb bones that early humans had had first choice among the parts of these animal carcasses by virtue of having hunted them.
This interpretation was supported by simple stone tools the Leakeys had found in the same deposits. The Oldowan technology (named for these kinds of tools, found at Olduvai Gorge) includes sharp stone knives, or flakes; cores from which those flakes were struck; and fist-sized rounded hammerstones used to strike the flakes from the cores. These seemingly basic tools allowed early humans to gain access to a much broader array of foods. The sharp flakes could be used to slice meat from bones or to whittle sticks to dig for underground roots or water; the cores and hammerstones could be used to process plants and bash open bones to get access to the fat-rich marrow and brains inside.
A problem came up as early as 1957, however, when Sherwood Washburn, of the University of California, Berkeley, reported on carnivore kills he observed in Wankie Game Reserve in (then) Rhodesia. He noticed that the parts of skeletons most often remaining more or less intact after carnivores had eaten their fill were skulls and lower jaws, the least edible parts of the animals. According to Washburn, the meat-bearing bones, often broken up, that Dart had found in australopithecine deposits must have been brought there by some other animal and not the australopithecines—perhaps hyenas, which he and others had observed accumulating such bones around their dens.
In 1981, C. K. “Bob” Brain published a seminal book titled The Hunters or the Hunted?, which came to the same conclusions. After studying a large collection of goat bones in the central Namib desert that had been discarded by modern people and then chewed by dogs, Brain hypothesized that the skeletal part profiles could best be attributed to the durability of those particular bones themselves, rather than to selection by early human hunters. Brain applied a similar explanation to fossil assemblages from early human sites in the Transvaal region of South Africa. In his view, a number of nonhuman agencies that could cause accumulations in modern caves had done so in these fossil assemblages: fluvial activity, carnivores such as hyenas and leopards, porcupines, owls, and natural deaths. It was clear that more evidence was needed to evaluate whether early human activities were in fact responsible for the accumulation of animal fossils at prehistoric sites.
In that same year, 1981, incontrovertible evidence of early human butchery came to light, in the form of linear striations on fossils which were identified as cut marks made by the stone tools found in abundance at the FLK Zinj site. Bunn and, in a separate study, Rick Potts from the Smithsonian Institution and Pat Shipman from Pennsylvania State University, used a scanning electron microscope to demonstrate that these marks were different from the shallow, chaotically oriented scratches seen on some fossils. This sedimentary abrasion is thought to be the result of sand grains rubbing against the bones as they tumbled around in rivers or were trampled on by animals. The cut marks, by contrast, were shorter, deeper, and often located on the parts of bones where muscles attach. They seemed to show conclusively that early humans were proficient hunters of the extinct antelopes, zebras, and similar animals found alongside the early human fossils and stone tools in the 1.8-million-year-old deposits.
Although more than one kind of early human had been found at Olduvai, for several decades the thousands of Oldowan stone tools and hundreds of cut-marked bones were attributed exclusively to the fossils of our genus, Homo. With a new report of a jaw from the site of Ledi Geraru in the Afar region of Ethiopia, the fossil evidence of our genus now extends back 2.8 million years. Until recently, Ethiopia has also yielded the earliest evidence of stone tools and cut marks on animal fossils (from 2.5 to 2.6 million years ago) at the sites of Bouri and Gona, and the earliest Oldowan stone tools from Gona, dated to 2.5 million years ago. Altogether, a tidy package of archaeological evidence of the earliest butchery and stone tools—in other words, carnivory—seems to have emerged by at least 2.5 million years ago with the origins of our genus.