Trade Made Us Human

The journal Science is publishing a fascinating new study (subscription required) in its March 11 edition in which researchers suggest that the conventional anthropological explanation for the development of humanity's unique penchant for cooperation may be wrong. First, what's so unique about humans? As the researchers note: 

Evolutionary behavioral scientists have attempted to understand how cooperative temperament might spread in small-scale human societies and how it might account for "prosocial" and "other-regarding" behaviors (9), exceptional social cognition and shared intentionality (10), orientation toward teaching and learning (11), a taste for equitable distribution (12), and a widespread willingness to punish norm violators, even when not directly affected by the noncooperative behavior (13). These traits appear to be derived in Homo because they are rare or absent in other apes (14).

Earlier anthropologists and evolutionary psychologists have argued humanity's robust prosociality evolved because "being nice made evolutionary sense when we lived in small bands surrounded by relatives, because helping them helped our genes survive." This is known as the inclusive fitness hypothesis. As the new study observes: 

Traditionally, anthropologists have suggested that hunter-gatherer co-residence is almost entirely based on kinship…and if foraging bands are mainly collections of close kin, inclusive fitness gains might be the primary motivator of ancestral human cooperation. 

However, it turns out that contemporary hunter-gatherer bands are not chiefly composed of close kin. The researchers analyzed data on band composition for 32 hunter-gatherer societies. They found

… bands are mainly composed of individuals either distantly related by kinship and/or marriage or unrelated altogether. In our sample of 32 societies, primary kin generally make up less than 10% of a residential band.

If this finding holds for our Stone Age ancestors, the development human cooperation among strangers must have another basis than inclusive fitness based on kinship. The researchers go on to suggest … 

…we cannot necessarily assume that cognitive features such as inequality aversion and enhanced prosocial emotions evolved in ancestral environments composed mainly of close kin. Given the constant flow of individuals between groups, genetic group selection at the level of the band also seems improbable. Instead, cultural group selection (27) may lead to the spread of cooperative institutions within ethnic groups, which might then create a context favoring the genetic evolution of prosocial cognitive mech- anisms through individual-level selection.

Second, mathematical models suggest that large interaction networks may be required for culture to accumulate (21, 23). In small populations, cultural innovations can be lost because of infrequent interaction between potential models and imitators and/or stochastic events that eliminate models with particular cultural knowledge. For example, one of the authors (K.R.H.) observed that the Northern Ache, isolated from their ancestral core territory in the 19th century, were unable to make fire by the time they were contacted in the 1970s. However, older informants stated that their parents and grandparents had told them that their ancestors could make fire, and had partially described to them the technique, even though none had ever observed it directly. In contrast, the Southern Ache groups did maintain fire-making knowledge until their first contact in the late 20th century. Likewise, Tasmanians failed to maintain previously known methods for fishing when their island was cut off from mainland Australia in the early Holocene (23), and fishing technology in Polynesia shows reduced complexity on islands with smaller populations (28). In contrast, Wiessner [(29) and references therein] conducted studies on style and social information in material culture among Kalahari Bushman bands from four language families connected by intermarriage, exchange partnerships, and visiting. She found that tapered bone points were rapidly replaced everywhere by iron tips when fence wire became available in the 20th century, and that new point styles emerged and became relatively homogeneous within language groups over a period of 40 years.

When people reside together, they have frequent opportunities to observe innovations, evaluate their success, and imitate traits judged most successful or most common. Our analyses suggest that the increased network size that follows a unique shift in ancestral human residential struc- ture may have led to greater exposure to novel ideas worth copying, and may explain why hu- mans, but not other animals, evolved costly social learning mechanisms (such as high-fidelity over-imitation or conformity-biased transmission) that may have resulted in cumulative cultural evolu-tion (21). This unique expansion of network size in our hominin ancestors can be detected archaeologically by the emergence of long-distance flows of tools and raw materials that appear at least as early as the middle Pleistocene (30).

To summarize: trade and the division of labor are hallmarks of human cooperation. These findings bolster the arguments made in my friend Matt Ridley's superb new book, The Rational Optimist: How Prosperity Evolves. Ridley argues that groups that took advantage of the division of labor and traded peaceably with strangers outcompeted less cooperative groups. And more cooperation leads to more invention and more prosperity over time.