Once thought to be universal, this "embodied cognition of time" is in fact strictly cultural. Over the past decade, encounters with various remote tribal societies have revealed a rich diversity of the ways in which humans relate to time (see "Attitudes across the latitudes"). The latest, coming from the Yupno people of Papua New Guinea, is perhaps the most remarkable. Time for the Yupno flows uphill and is not even linear.
Rafael Núñez of the University of California, San Diego, led his team into the Finisterre mountain range of north-east Papua New Guinea to study the Yupno living in the village of Gua. There are no roads in this remote region. The Yupno have no electricity or even domestic animals to work the land. They live with very little contact with the western world.
Núñez and his colleagues noticed that the tribespeople made spontaneous gestures when speaking about the past, present and future. They filmed and analysed the gestures and found that for the Yupno the past is always downhill, in the direction of the mouth of the local river. The future, meanwhile, is towards the river's source, which lies uphill from Gua.
This was true regardless of the direction they were facing. For instance, if they were facing downhill when talking about the future, a person would gesture backwards up the slope. But when they turned around to face uphill, they pointed forwards (Cognition, DOI: 10.1016/j.cognition.2012. 03.007).
Núñez thinks the explanation is historical. The Yupno's ancestors arrived by sea and climbed up the 2500-metre-high mountain valley, so lowlands may represent the past, and time flows uphill.
But the most unusual aspect of the Yupno timeline is its shape. The village of Gua, the river's source and its mouth do not lie in a straight line, so the timeline is kinked. "This is the first time ever that a culture has been documented to have everyday notions of time anchored in topographic properties," says Núñez.
Within the dark confines of their homes, geographical landmarks disappear and the timeline appears to straighten out somewhat. The Yupno always point towards the doorway when talking about the past, and away from the door to indicate the future, regardless of their home's orientation. That could be because entrances are always raised, says Núñez. You have to climb down - towards the past - to leave the house, so each home has its own timeline.
"This study is an important landmark," says Pierre Dasen, an anthropologist at the University of Geneva in Switzerland who was not involved in the work. "It demonstrates both universality of cognitive processes and a fascinating cultural difference."
Lera Boroditsky of Stanford University in California agrees. "Each one of these discoveries isn't just telling us something about other people, it's telling us something about us," she says. "A lot of English speakers think that it's natural to think of time as a straight line. But that's an illusion. It doesn't have to be that way."
Ananthaswamy, Anil. 2012. "Time flows uphill for remote Papua New Guinea tribe ". New Scientist. Posted: May 31, 2012. Available online: http://www.newscientist.com/article/mg21428675.400-time-flows-uphill-for-remote-papua-new-guinea-tribe.html