Social networks in modern societies are highly structured, usually involving frequent contact with a small number of unrelated 'friends'(1). However, contact network structures in traditional small-scale societies, especially hunter-gatherers, are poorly characterized. We developed a portable wireless sensing technology (motes) to study within-camp proximity networks among Agta and BaYaka hunter-gatherers in fine detail. We show that hunter-gatherer social networks exhibit signs of increased efficiency(2) for potential information exchange. Increased network efficiency is achieved through investment in a few strong links among non-kin 'friends' connecting unrelated families. We show that interactions with non-kin appear in childhood, creating opportunities for collaboration and cultural exchange beyond family at early ages. We also show that strong friendships are more important than family ties in predicting levels of shared knowledge among individuals. We hypothesize that efficient transmission of cumulative culture(3-6) may have shaped human social networks and contributed to our tendency to extend networks beyond kin and form strong non-kin ties.