Behaviorial Patterns of the Earliest HominidsEdit
One of the most important and intriguing questions in human evolution is about the diet of our earliest ancestors.
The presence of primitive stone tools in the fossil record tells us that 2.5 million years ago, early hominids (A. garhi) were using stone implements to cut the flesh off the bones of large animals that they had either hunted or whose carcasses they had scavenged.
Earlier than 2.5 million years ago, however, we know very little about the foods that the early hominids ate, and the role that meat played in their diet. This is due to lack of direct evidence.
Nevertheless, paleoanthropologists and archaeologists have tried to answer these questions indirectly using a number of techniques.
- Primatology (studies on chimpanzee behavior)
- Anatomical Features (tooth morphology and wear-patterns)
- Isotopic Studies
What does chimpanzee hunting behavior suggest about early hominid behavior?Edit
Earliest ancestors and chimpanzees share a common ancestor (around 5-7 million years ago). Therefore, understanding chimpanzee hunting behavior and ecology may tell us a great deal about the behavior and ecology of those earliest hominids.
In the early 1960s, when Jane Goodall began her research on chimpanzees in Gombe National Park (Tanzania), it was thought that chimpanzees were herbivores. In fact, when Goodall first reported meat hunting by chimpanzees, many people were extremely sceptical.
Today, hunting by chimpanzees at Gombe and other locations in Africa has been well documented. We now know that each year chimpanzees may kill and eat more than 150 small and medium-sized animals, such as monkeys (red colobus monkey, their favorite prey), but also wild pigs and small antelopes.
Did early hominids hunt and eat small and medium-sized animals? It is quite possible that they did. We know that colobus-like monkeys inhabited the woodlands and riverside gallery forest in which early hominids lived 3-5 Myrs ago. There were also small animals and the young of larger animals to catch opportunistically on the ground. Many researchers now believe that the carcasses of dead animals were an important source of meat for early hominids once they had stone tools to use (after 2.5 million years ago) for removing the flesh from the carcass. Wild chimpanzees show little interest in dead animals as a food source, so scavenging may have evolved as an important mode of getting food when hominids began to make and use tools for getting at meat. Before this time, it seems likely that earlier hominids were hunting small mammals as chimpanzees do today and that the role that hunting played in the early hominids' social lives was probably as complex and political as it is in the social lives of chimpanzees.
When we ask when meat became an important part of the human diet, we therefore must look well before the evolutionary split between apes and humans in our own family tree.
What do tooth wear patterns suggest about early hominid behavior?Edit
Bones and teeth in the living person are very plastic and respond to mechanical stimuli over the course of an individual's lifetime. We know, for example, that food consistency (hard vs. soft) has a strong impact on the masticatory (chewing) system (muscles and teeth). Bones and teeth in the living person are therefore tissues that are remarkably sensitive to the environment. As such, human remains from archaeological sites offer us a retrospective biological picture of the past that is rarely available from other lines of evidence. Also, new technological advances developed in the past ten years or so now make it possible to reconstruct and interpret in amazing detail the physical activities and adaptations of hominids in diverse environmental settings.
Some types of foods are more difficult to process than others, and primates tend to specialize in different kinds of diets. Most living primates show three basic dietary adaptations:
- insectivores (insect eaters);
- frugivores (fruit eaters);
- folivores (leaf eaters).
Many primates, such as humans, show a combination of these patterns and are called omnivores, which in a few primates includes eating meat.
The ingestion both of leaves and of insects requires that the leaves and the insect skeletons be broken up and chopped into small pieces. The molars of folivores and insectivores are characterized by the development of shearing crests on the molars that function to cut food into small pieces. Insectivores' molars are further characterized by high, pointed cusps that are capable of puncturing the outside skeleton of insects. Frugivores, on the other hand, have molar teeth with low, rounded cusps; their molars have few crests and are characterized by broad, flat basins for crushing the food.
In the 1950s, John Robinson developed what came to be known as the dietary hypothesis. According to this theory there were fundamentally two kinds of hominids in the Plio-Pleistocene. One was the "robust" australopithecine (called Paranthropus) that was specialized for herbivory, and the other was the "gracile" australopithecine that was an omnivore/carnivore. By this theory the former became extinct while the latter evolved into Homo.
Like most generalizations about human evolution, Robinson's dietary hypothesis was controversial, but it stood as a useful model for decades.
Detailed analyses of the tooth surface under microscope appeared to confirm that the diet of A. robustus consisted primarily of plants, particularly small and hard objects like seeds, nuts and tubers. The relative sizes and shapes of the teeth of both A. afarensis and A. africanus indicated as well a mostly mixed vegetable diet of fruits and leaves. By contrast, early Homo was more omnivorous.
But as new fossil hominid species were discovered in East Africa and new analyses were done on the old fossils, the usefulness of the model diminished.
For instance, there is a new understanding that the two South African species (A. africanus and A. robustus) are very similar when compared to other early hominid species. They share a suite of traits that are absent in earlier species of Australopithecus, including expanded cheek teeth and faces reemodeled to withstand forces generated from heavy chewing.
What do isotopic studies suggest about early hominid behavior?Edit
Omnivory can be suggested by studies of the stable carbon isotopes and strontium(Sr)-calcium(Ca) ratios in early hominid teeth and bones.
For instance, a recent study of carbon isotope (13C) in the tooth enamel of a sample of A. africanus indicated that members of this species ate either tropical grasses or the flesh of animals that ate tropical grasses or both. But because the dentition analyzed by these researchers lacked the tooth wear patterns indicative of grass-eating, the carbon may have come from grass-eating animals. This is therefore a possible evidence that the australopithecines either hunted small animals or scavenged the carcasses of larger ones.
There is new evidence also that A. robustus might not be a herbivore. Isotopic studies reveal chemical signals associated with animals whose diet is omnivorous and not specialized herbivory. The results from 13C analysis indicate that A. robustus either ate grass and grass seeds or ate animals that ate grasses. Since the Sr/Ca ratios suggest that A. robustus did not eat grasses, these data indicate that A. robustus was at least partially carnivorous.
Much of the evidence for the earliest hominids (Sahelanthropus tchadensis, Orrorin tugenensis, Ardipithecus ramidus) is not yet available.
Australopithecus anamensis shows the first indications of thicker molar enamel in a hominid. This suggests that A. anamensis might have been the first hominid to be able to effectively withstand the functional demands of hard and perhaps abrasive objects in its diet, whether or not such items were frequently eaten or were only an important occasional food source.
Australopithecus afarensis was similar to A. anamensis in relative tooth sizes and probable enamel thickness, yet it did show a large increase in mandibular robusticity. Hard and perhaps abrasive foods may have become then even more important components of the diet of A. afarensis.
Australopithecus africanus shows yet another increase in postcanine tooth size, which in itself would suggest an increase in the sizes and abrasiveness of foods. However, its molar microwear does not show the degree of pitting one might expect from a classic hard-object feeder. Thus, even A. africanus has evidently not begun to specialize in hard objects, but rather has emphasized dietary breadth (omnivore), as evidenced by isotopic studies.
Subsequent "robust" australopithecines do show hard-object microwear and craniodental specializations, suggesting a substantial departude in feeding adaptive strategies early in the Pleistocene. Yet, recent chemical and anatomical studies on A. robustus suggest that this species may have consumed some animal protein. In fact, they might have specialized on tough plant material during the dry season but had a more diverse diet during the rest of the year.