Early Hominin Social Dynamics

Three main archaeological interpretations of Plio-/Pleistocene hominid(in) behavior have been proposed. While the models emphasize the behavioral repertoire of early Homo — for they focus on stone tool technologies and their distribution over the landscape — they are probably relevant to the behavioral features of the australopithecines too. The first was proposed by the Harvard archaeologist Glynn Isaac, who became famous for his archaeological excavations and interpretations of the localities centered around Koobi Fora and Olorgesailie (see Isaac, 1977). Isaac (1976, 1978, 1986) formulated the Central Place Foraging model, which argues that early hominins required a central location to which they could retreat in order to eat in safety and at leisure. The main emphasis is on a safe refuge, with secondary importance given to food sharing and development of a sexual division of labor. In this model, males focused on large animal hunting and the scavenging of animal carcasses, while females and juveniles focused on opportunistic hunting of small animals and on the gathering of edible plant material.

The second model, proposed by the North American archaeologist Louis Binford, is the Scavenging hypothesis. Binford (1981) argues that early hominins were not involved in hunting at all but, rather, roamed the African savanna, scavenging the kills from other carnivores, thus obtaining only low-food-utility items, and then, like other scavengers, moved off to a more protected location. His hypothesis is based on a statistical interpretation of patterns observed at known carnivore scavenging sites, which were then compared to early hominin archaeological sites. Binford argued, based on his number crunching, that the early hominin sites were more similar to the carnivore scavenging sites than to known later hominin hunting sites. He believed that the focus on low-food-utility items (those not consumed by large carnivores) enabled the early hominins to carve out a niche for themselves as marginal scavengers.

Finally Richard Potts (1988, 1996), of the Smithsonian Institution, has proposed a third model called the Stone Caching hypothesis, which argues that early hominins were not using a central place foraging system but, rather, were bringing hunted and/or scavenged animal remains back to a number of stone caches situated optimally throughout the landscape. These stone caches, he suggests, were an aggregation of transported stone, including modified and unmodified pieces, which were repeatedly visited to obtain or manufacture tools and to use them in processing food. The model is based on the presence of rock types (some of which have been made into tools) that are not naturally found at Olduvai, but farther afield. Hominids, he argues, must have brought these rocks from the surrounding regions; whenever they were able to kill, or scavenge from other carnivores, the carcass or parts of it were carried to the nearest stone cache in order to speed up the butchering process and reduce the chance of confrontation between them and other carnivores.

These three models are of course based on the archaeological evidence. In the early 1990s, the authors decided to test these three models to determine which, if any, was the most likely (Cameron, 1993a; Cameron & Groves, 1993), but we used a completely different data set. We examined the literature of ape (Pongo, Gorilla, and both species of Pan) and human behavior, and defined a number of behavioral features common to all species; these features were also likely to have been present in the earliest hominins, by parsimony. We further examined the behaviors of humans and the two species of chimpanzee, and looked for common features shared by all three; again, the earliest hominins would also likely share these more derived features (Figure 3.4, Table 3.1).

Our study strongly indicated that the social system of the immediate common ancestor of chimpanzees and humans was one where both hunting and an incipient division of labor were present, with little evidence for scavenging as a major component of early hominin behavior. This is not to argue that some scavenging did not occur, just that it is not likely to have reflected the primary strategy for early hominin activities. Stone caching is also unlikely because it is a unique condition of just one group of common chimpanzees (from the Tai Forest, Ivory Coast). There is evidence, however, that some form of central place foraging strategy, based on

Pongo pygmaeus Gorilla gorilla Pan troglodytes Pan paniscus Homo sapiens

Pongo pygmaeus Gorilla gorilla Pan troglodytes Pan paniscus Homo sapiens

15 Ma

Figure 3.4 ► Cladogram of the extant hominids based on the molecular evidence. See text and Table 3.1 for further details. From Cameron (1993a).

TABLE 3.1 ► Behavioral Features Characterizing Each Node (Character Complex) in Figure 3.4

Character Complex 1

1. Variety in diet

2. Ranging pattern depended on seasonality of available foods Character Complex 2

1. Social Character Complex 3

1. Community not necessarily controlled by dominant male

2. Omnivore

3. Food-sharing present

4. Engaged in opportunistic and/or hunting activities to obtain meat Character Complex 4

1. Sexual relationships that are polygynous and promiscuous

2. A medium degree of emphasis placed on a sexual division of labor

3. Known to move food away from source location

4. Males retained, females migrate

Character Complex 5

1. Females that have exclusive core areas, while males have larger overlapping areas

2. Social systems based on lone individuals, not group structure

3. Lone male territorial patrol

4. Polygamous and opportunistic mating strategy

5. No sexual division of labor

6. Females and males very rarely form into groups, and when they do it is for a very short period of time

7. Nests rarely reused Character Complex 6

1. Polygamous mating strategy

2. Weak sexual division of labor

Character Complex 7

1. A group of males may patrol alone or may be accompanied by estrus females

2. Weak female-female bonding but strong male-male bonding

3. Dependent on small food patches

4. Adult males frequently share meat among themselves and with estrus females

5. Meat moderately important to the diet

6. Food occasionally taken back to a nest

7. Nests infrequently shared

8. Food may be taken back to optimally placed material caches for processing Character Complex 8

1. Increased period of estrus

2. Weak male-male bonding but strong female-female bonding

3. Meat of only minor importance to the diet

4. No pirating of meat

5. No scavenging of meat

6. Nests commonly shared

Character Complex 9

1. Group of males usually patrol alone

2. Extreme periods of estrus

Early Hominin Social Dynamics TABLE 3.1 ► Continued

3. Strong bonding among all classes

4. Mating strategy extremely variable

5. Extreme sexual division of labor

6. Dependent on concentrated food patches

7. Female attitudes to new group female members extremely variable

8. Capture of prey based on complex organization

9. Meat important to the diet

10. Food usually removed to another location

11. Food usually taken back to a nest

12. Nests almost always shared

13. Same nest may be reused for a prolonged period of time

14. Food usually processed at camp, not at optimally dispersed material caches. Synapomorphies of Pan troglodytes and Homo

1. Both based on unstable ranging pattern

2. Both marked by intercommunity relations that are based on physical agression

3. Males the predominant hunters, and they tend to hunt in male groups

4. Both use stone and plant tools in food processing

5. Both known to pirate meat

6. Scavenging infrequent, although it does occur Synapomorphies of Pan paniscus and Homo

1. Meat usually shared with little distinction between the sexes.

From Cameron (1993a).

shared derived behavioral conditions of the chimpanzee and human lineages, applied. For example, common chimpanzees have been observed to take meat back to a night nest, although it is not reported that they share the meat there. This is close to central place foraging and differs only in degree, not kind. In both common and pygmy chimpanzee groups, food sharing is usual, so this is a common derived feature of the human and chimpanzee lineages, and it is clear that a sexual division of labor is incip-iently present in all African apes (gorillas, chimpanzees, and humans). In addition to these features, the earliest hominin social systems were probably characterized by the sharing of meat, with little distinction between the sexes. Hunting of small mammals as well as scavenging of small and large mammals took place; infrequent pirating of meat is also likely to have occurred. The hunting parties were cooperative and consisted predominantly of male members. These Pliocene hominid groups were probably based on unstable ranging patterns, which were dependent on seasonality of available foods; intercommunity relationships would have been based on physical aggression. Some stone and plant tools were also probably used in food processing. Sexual relationships were probably based on promiscuous behavior and occasional consortships.

Recently, Wrangham (2001) has proposed a behavioral model similar to the one we propose, but it has the addition of a few derived features to help explain the original emergence of the proto-human from the proto-chimpanzee. He suggests that the earliest proto-humans, like extant apes, originally occupied a forest habitat (with many of the behavioral features we have just discussed), but at least one of these populations had acquired the ability to search for and obtain underground storage organs (USOs), for example, tubers, corms, rhizomes, and other roots. With the depletion of the forests around the time of the Mio-/Pliocene transition, most proto-human populations must have died out. But some that had already adapted to these foods would be able to survive in the increasingly drier habitats. These foods would be an important food source, because they are not readily available to other animals (they are hard to dig up and often tough or rich in toxins). Numerous "digging tools" have been found in association with fossil hominin specimens. Indeed, the increased thickness of molar enamel and increased molar size in later fossil hominins may be associated with such an adaptation. As such, these foods can be seen as a fallback position to preferred foods that may have become increasingly difficult to obtain. As suggested by Wrangham (2001:128) "This was a critical adaptation enabling the woodland apes to survive when natural selection was at its most intense."

We suggest, then, that the evolution of the Pliocene hominins emphasized a number of behavioral features present in the last common ancestor of the chimpanzee and human lineage. The unique derived features of living human groups cannot alone be used to explain the behavior of the earliest hominins, just as the behavior of living chimpanzee groups cannot alone be used as an analogy for early hominin behavior, though we do accept that Wrangham's USO hypothesis is an intriguing model for the initial split between the proto-chimpanzee and proto-humans around 5 million years ago. In examining the more primitive and shared derived features of the apes, we can say that of the three archaeological interpretations proposed so far for early hominin behavior, Isaac's Central Place Foraging model is the most likely.

Interlude 2 The Importance of Being an Ape

We are animals, of course. (A creationist might dispute that. What are we, then — plants? fungi?) We are vertebrates — animals with backbones. We are mammals — vertebrates with body hair, and "we" secrete milk for the newborn ("Man is an animal that suckles his young," in prenonsexist parlance). We are primates — mammals with grasping hands and feet, Meissner's corpuscles to enhance the sense of touch, and stereoscopic vision, and the males have dangling penises. We are haplorrhines — primates with a dry nose, a macula in the retina of the eye, and born of a vascular placenta of the type called haemochorial. We are catarrhines — haplorrhines with an auditory tube conducting sound from the external ear to the middle ear, and only two premolar teeth (in each half of each jaw). And we are apes — catarrhines that sit upright and sometimes stand upright and have a shortened lumbar spine, lowering the center of gravity for this purpose; that have shoulder joints with an all-round hemisphere of rotation; and that have loose, flexible wrists and no tail; that have simple molar patterns and shortened canine teeth; and that have an appendix in the gut. The other apes are gibbons (called lesser apes), orangutans, gorillas, and chimpanzees (called great apes).

Until the 1960s, textbooks placed humans in one family, Hominidae, and "the apes" in another, Pongidae. Or the lesser apes might be classified in a third family, Hylobatidae. The evolutionary diagrams had the human stem separating from the ape stem way back, perhaps in the Oligocene. Work in the 1960s on a mid-Miocene ape, Ramapithecus, apparently confirming it as a human ancestor, seemed to corroborate this.

Then came the work of Morris Goodman. In a classic paper in a 1963 book (Classification and Human Evolution, edited by Sherwood L. Washburn), Goodman illustrated the results of his comparisons of the serum proteins of humans and great apes. According to the "traditional" model, the serum proteins of humans should have been the most distinct; instead, those of the orangutan were the most distinct. Next most distinct were those of the gorilla. Those of humans and chimpanzees were very alike indeed. Goodman proposed that the gorilla and chimpanzee should be taken out of the Pongidae and placed in the Hominidae.

Goodman was way ahead of his time. What price serum proteins, when we had all that anatomy telling us that man was so very different from the great apes: the habitually bipedal locomotion, the hairless body, the noble brow, the huge brain, the ability to make tools and television sets.

Another biologist, meanwhile, was white-anting the establishment in a different way. Jane Goodall had begun her successful long-term field study of chimpanzees in Gombe National Park (as it is called today), in Tanzania. In her paper in a 1965 book (Primate Behavior, edited by Irven DeVore), she revealed that chimpanzees make tools — not television sets, but simple tools of twigs and grass stems, to get termites and ants out of their nests to eat.

The collapse of the traditional model probably started about 1970, as other work in both genetics and psychology showed how close humans and great apes really are, and the genetic work in addition confirmed Goodman's conclusion that chimpanzees are our closest relatives. In 1980 the final stroke that brought down the old edifice was the demonstration, in a classic paper by Peter Andrews and Jack Cronin, that our mid-Miocene supposed ancestor, Ramapithecus, had been outrageously misrepresented and that the available evidence showed it as barely, or not at all, different from Sivapithecus, which is, in its turn, a member of the orangutan lineage.

All primates are intelligent, as mammals go, and monkeys are intelligent as primates go. But with the apes, it is not just a matter of intelligence. The great apes have a theory of mind: They know how their own and others' minds work, so they can anticipate others' reactions, empathize with them, and anticipate their own reactions. They can imitate others in a way that monkeys cannot; and, being self-aware, they can learn to recognize themselves in mirrors. A monkey will continue for hours on end threatening that other monkey it sees in the mirror, or looking behind the mirror for it; most chimpanzees will take a matter of days, at the most, to realize who they are looking at, and most orangutans, too, though only about 25% of gorillas (so far).

Having an insight into ones own and others' mental processes has other implications. Great apes can learn from each other and take turns at doing some complex actions. One chimpanzee works the lever, and the other operates the food tray; then they change places — the one who was operating the food tray wants a turn at the lever. Kanzi, the bonobo (so-called "pygmy chimpanzee") who lives in the Language Research Center in Atlanta, Georgia, strikes a flake off a stone core, tests it for sharpness, and uses it to cut a rope and thereby release the door of a food box. Orangutans are more patient and methodical than chimpanzees. One used to escape from his cage in the London Zoo in the early 20th century by fashioning a key to the cage door out of wood, hiding the half-finished key under the straw whenever a keeper came into view.

"Ape language" has always been controversial — needlessly so, in fact. When, in the early 1970s, the first chimpanzees were taught "sign language" (using hand signs as symbols for objects and actions), there was ill-tempered polemic over whether they "had language" or not. This sort of argument is ultimately sterile; it just depends where you want to put the barrier between language and not-language. A much more productive argument would have been to ask what language-like features this chimpanzee hand-signing has and whether the symboling abilities are homologous with human linguistic abilities or whether they arise from some other aspect of the complexity of chimpanzee cognition. Because people asked the wrong questions, ape-language trainers became incautious and claimed more and more for the apes (mostly chimpanzees) in their care. So when, in 1981, Herb Terrace and his colleagues showed that the signs the chimpanzees were making were not sentence-like and that they were not taking turns in their conversations with their trainers but that the trainers were often inadvertently cuing them, it was too easy for human chauvinists to breathe a sigh of relief and say, "I knew it — apes ain't got language."

It is due largely to the work of Sue Savage-Rumbaugh and her indefatigable colleagues in the Language Research Center that ape-language studies have been revived and brought back on track, first using common chimpanzees (Pan troglodytes) and then turning to bonobos (Pan paniscus). The work at the Language Research Center with Kanzi, Panbanisha, and other bonobos has finally allowed nonhuman great apes to approach us human great apes in the linguistic sphere as they have done in other cognitive aspects. The way this has been done is really so simple. Consider this amazing fact: Children learn language not by making the sounds and then associating them with objects and actions, but by understanding first. So did Kanzi: He watched over his mother's shoulder while she was being taught, unavailingly. Just like a child, he understood first — only then did he utter.

The Great Ape Project, first proposed in 1994 by Peter Singer and Paola Cavalieri in the book of that name, promotes the idea that nonhuman great apes deserve a version of human rights. With such impressive psychological support, it is no wonder that the progress of this proposal has been accelerating. It would be a foolhardy biomedical researcher nowadays who admitted to performing disabling or distressing research on chimpanzees. On October 7, 1999, the New Zealand Parliament passed an amendment to its Animal Welfare Act that "nonhuman hominoids . . . must not be used in research, testing or education, unless the government official responsible for animal welfare is satisfied that such use is for their benefit, either as individuals or as a species." Some other countries have legislation that is similar in tenor, if weaker; thus the British government in November 1997 stated that it would "not issue any licenses to use great apes in scientific procedures."

Why great apes have these cognitive skills is obscure. The best that anyone can suggest is that it is something to do with their large size. But whether it is somehow selected for because they are large or is a simple epiphenomenon of it is still argued.

Other animals? Whales and dolphins seem to have complex cognition but are harder to test than apes, for obvious reasons. Elephants too.

If great apes, maybe gibbons? Maria Ujhelyi has found mirror recognition in the large Siamang gibbon (Symphalangus syndactylus) and in the smaller white-cheeked gibbon (Nomascus leucogenys), but she has failed to elicit mirror recognition in the still smaller white-handed gibbon (Hylobates lar). Does this support the body-size hypothesis? It is too early to judge. But it seems to make sense to us that, rather than a cognitive divide between great apes and everybody else, we have a sort of gradation, though the nature of this gradation remains to be explored.

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