Ancient human social networks exhibited several of the same properties as those of modern networks, a new Nature study suggests. The study, co-authored by professor of sociology Nicholas A. Christakis, is “one of the first, and maybe only, studies to map social cooperation in humans,” according to  Coren L. Apicella, a post-doctoral fellow in Christakis’s lab who spearheaded the research.

Over the past few years, the team of researchers mapped the social networks of the Hadza, an ethnic group of hunter-gatherers in Tanzania.

The researchers used a photographic census, which Apicella called “a pencil and paper version of Facebook,” to determine social ties among the Hadza.

“The Hadza provide a window into our ancient past, as we probe the evolutionary and biological origins of this crazy thing we do: making social networks,” Christakis wrote in an email.

Of particular note among the shared properties between the Hadza and modern populations is homophily, the tendency for friends to be similar in a number of traits. The researchers found that cooperators tend to be friends with other cooperators, and defectors acted in a similar manner.

“You do see cooperation in other animals for sure, but humans are really unique in the sense that we form long-lasting non-reproductive bonds with other individuals,” said Apicella.

Apicella explained that there are two main hypothesized explanations for homophily.

The first is that “birds of a feather flock together,” meaning that individuals seek others who are like them.

The second explanation is that individuals influence their friends’ behavior in a cascading effect, leading to communities with similar traits.

According to Apicella, the Hadza social structure is set up in a way consistent with the “birds of a feather” hypothesis: as hunter-gatherers, the Hadza have few possessions and are not tied down to a particular location.

“We see that camp membership is very fluid,” said Apicella.

Homophily is one of the many social traits that Christakis’s lab seeks to explain.

“My lab is interested in the deep roots of human social organization: how and why do we assemble ourselves into a sort of human super-organism?” Christakis asked.

In the field, the researchers asked Hadza individuals whom they would like to live with in their next camp and to whom they’d give a stick of honey, using the “pencil and paper version of Facebook” to identify social ties.

“[The Hadza] loved looking at these photos, just as we like looking at photos of our friends on Facebook,” said Apicella.

“Even when they weren’t conducting fieldwork, they would come and ask Coren, ‘can we look at the pictures?’” she added. “It’s really touching upon something that’s very human.”

—Staff writer Sabrina A. Mohamed can be reached at smohamed@college.harvard.edu.