Every time some human attribute is said to be unique, whether tool-making or language or warfare, biologists soon find some plausible precursor in animals that makes the ability less distinctive.
Still, humans are vastly different from other animals, however hard the difference may be to define. A cascade of events, some the work of natural selection, some just plain accidents, propelled the human lineage far from the destiny of being just another ape, down an unexpected evolutionary path to become perhaps the strangest blossom on the ample tree of life.
And what was the prime mover, the dislodged stone that set this eventful cascade in motion? It was, perhaps, the invention of weapons — an event that let human ancestors escape the brutal tyranny of the alpha male that dominated ape societies.
Biologists have little hesitation in linking humans’ success to their sociality. The ability to cooperate, to make individuals subordinate their strong sense of self-interest to the needs of the group, lies at the root of human achievement.
“Humans are not special because of their big brains,” says Kim Hill, a social anthropologist at Arizona State University. “That’s not the reason we can build rocket ships — no individual can. We have rockets because 10,000 individuals cooperate in producing the information.”
The two principal traits that underlie the human evolutionary success, in Dr. Hill’s view, are the unusual ability of nonrelatives to cooperate — in almost all other species, only closely related individuals will help each other — and social learning, the ability to copy and learn from what others are doing. A large social network can generate knowledge and adopt innovations far more easily than a cluster of small, hostile groups constantly at war with each other, the default state of chimpanzee society.
If a shift in social behavior was the critical development in human evolution, then the answer to how humans became unique lies in exploring how human societies first split away from those of apes.
Paleoanthropologists often assume that chimp societies are a reasonably good stand-in for the ancestral ape society that gave rise to the chimp and human lineages. Living hunter-gatherers may reflect those of long ago, since humans always lived this way until the first settled societies of 15,000 years ago.
The two species’ social structure could scarcely be more different. Chimp society consists of a male hierarchy, dominated by the alpha male and his allies, and a female hierarchy beneath it. The alpha male scores most of the paternities, cutting his allies in on others. The females try to mate with every male around, so each may think he’s the father and spare her child. How did a chimplike society ever give rise to the egalitarian, largely monogamous structure of hunter-gatherer groups?
A new and comprehensive answer to this question has been developed by Bernard Chapais of the University of Montreal. Dr. Chapais is a primatologist who has spent 25 years studying monkey and ape societies. Recently he devoted four years to reading the literature of social anthropology with the goal of defining the transition between nonprimate and human societies. His book, “Primeval Kinship,” was published in 2008.
Dr. Chapais sees the transition as a series of accidents, each of which let natural selection exploit new opportunities. Early humans began to walk on two legs because it was a more efficient way of getting around than knuckle-walking, the chimps’ method. But that happened to leave the hands free. Now they could gesture, or make tools.
It was a tool, in the form of a weapon, that made human society possible, in Dr. Chapais’s view. Among chimps, alpha males are physically dominant and can overpower any rival. But weapons are great equalizers. As soon as all males were armed, the cost of monopolizing a large number of females became a lot higher. In the incipient hominid society, females became allocated to males more equally. General polygyny became the rule, then general monogamy.
This trend led to the emergence of a critical change in sexual behavior: the replacement of the apes’ orgiastic promiscuity with the pair bond between male and female. With only one mate, for the most part, a male had an incentive to guard her from other males to protect his paternity.
The pair bond was the pivotal event that opened the way to hominid evolution, in Dr. Chapais’s view. On the physiological level, having two parents around allowed the infants to be dependent for longer, a requirement for continued brain growth after birth. Through this archway, natural selection was able to drive up the volume of the human brain until it eventually reached three times that of a chimpanzee.
On the social level, the presence of both parents revealed the genealogical structure of the family, which is at least half hidden in chimp societies. A chimp knows who its mother and siblings are, because it grows up with them, but not its father or father’s relatives. So the neighboring bands to which female chimps disperse at puberty, avoiding incest, are perceived as full of strange males and treated with unremitting hostility.
In the incipient hominid line, males could recognize their sisters and daughters in neighboring bands. They could also figure out that the daughter’s or sister’s mate shared a common genetic interest in the welfare of the woman’s children. The neighboring males were no longer foes to be killed in sight — they were the in-laws.
The presence of female relatives in neighboring bands became for the first time a bridge between them. It also created a new and more complex social structure. The bands who exchanged women with each other learned to cooperate, forming a group or tribe that would protect its territory from other tribes. Though cooperation became the norm within a tribe, tribes would wage warfare just as relentlessly as chimpanzee bands.
“There is no single pressure that made us human,” Dr. Chapais said in an interview. He sees human evolution as having progressed through a series of accidents. “The fact that you can recognize patrilineal kin was not selected for, but as soon as you had that you could move forward and establish peaceful relations with other groups,” he said.
The new social structure would have induced the development of different social behaviors. “I personally am hung up on cooperation as being what really differentiates humans from nonhuman apes,” said Michael Tomasello, a developmental psychologist at the Max Planck Institute for Evolutionary Anthropology in Germany. A system of cooperative bands “provides the kind of social infrastructure that can really get things going,” he said.
In a series of experiments comparing human and chimpanzee infants, Dr. Tomasello has shown that very young children have an urge to help others. One of these skills is what he calls shared intentionality, the ability to form a plan with others for accomplishing a joint endeavor. Children, but not chimps, will point at things to convey information, they will intuit others’ intentions from the direction of their gaze, and they will help others achieve a goal.
Early humans venturing out into the savannah from the apes’ ancestral forest refuge would have been surrounded by predators and in fierce competition for food. Cooperation may have been forced on them as a condition of existence. “Humans were put under some kind of collective pressure to collaborate in their gathering of food — they became obligate collaborators — in a way that their closest primate relatives were not,” Dr. Tomasello writes in a recent book, “Why We Cooperate.”
Humans wear the mark of their shared intentionality, he notes, in a small but significant feature — the whites of their eyes, which are three times larger than those of any other primate, presumably to help others follow the direction of gaze. Indeed, chimps infer the direction of gaze by looking at another’s head, but infants do so by watching the eyes.
So if ever a visiting Martian biologist should ask you what made your species the master of its planet, point first to your mother and all her relatives, then to the whites of your eyes, and only lastly to your prominent forehead.
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