Talking about smelling, we know that scientists have long considered that humans are "blind".

Indeed, compared to most mammals, human olfactory sense is ridiculous.

The first mammals evolved from nocturnal beings; that's why the olfactory sense is quite developed amongst mammals. But the arboreal life of primates put forward the visual sense and the smelling sense was left behind. But a recent study proved that the human nose is still able to sniff and pick up a scent trail, a sensorial ability thought as impossible for people.

It seems that the human brain compares information it gets from each nostril, to get clues about where a smell is coming from, just like it may occur in other mammals. People can detect the source of a noise by comparing the difference between the signals received in each ear. But scientists thought that mammals can't follow the same strategy for smells, because their nostrils are too close together to get distinct signals. "We debunked that," said Noam Sobel of the University of California, Berkeley.

The new study "opened the doors for full consideration" of the two nostril hypothesis. Five experiments were made. Using tiny particles employed in theatrical fog the team proved that each human nostril really does sample a distinct region in space.

One outdoor experiment checked if people could follow just using their noses a 30-foot-long trail of chocolate scent, which traced a dogleg course through the grass. The trail was created with scented twine. But the 32 participants were blindfolded and equipped with thick gloves, earmuffs, kneepads and elbow pads to make sure they couldn't see or feel it. Before they began, they were shown a video of proper scent-tracking form, which requires putting the nose on the ground. Two-thirds of the participants succeeded in following the scent. But when they tried it again with their noses plugged, nobody could do it.

Another experiment found that people got better with practice. Yet another experiment, with 14 participants, found that the volunteers did better if they used two nostrils than if one nostril was taped shut. They succeeded 66 % of the time with two nostrils, versus 36 % with one nostril.

But did that really mean their brains were benefiting from two independent signals? Maybe both signals are the same, but the olfactory system just works better if it gets input from both nostrils. Or maybe the real explanation is that people take in fewer odors with one nostril than with two, giving a weaker signal.

To check out, researchers retested four of the participants who had gone through the practice sessions, but this time they wore devices over their nostrils that controlled the airflow into their noses. One version of the device was basically an extension of a normal nose, with two holes to sniff through, each supplying air to one nostril. The other version had only one hole, taking in the same amount of air as the first version, but simulated the effect of having only one big nostril. The one nostril device rendered the subjects less successful and slower. "That supports the idea that people benefit from having two," researchers said.