The origin of eyes has been traced back to 500 million years ago, but our ability to smell? It's even more ancient than that. Before the first life on Earth - single-celled organisms - developed clusters of light-sensitive proteins to help them locate prey and hiding places, these creatures could detect scents. And the fact that life was led by its sense of smell hundreds of millions of years ago hasn't changed one bit today.

As Joe Hanson points out in the latest episode of It's Okay To Be Smart, we navigate our worlds primarily using our eyes and ears, but on every step of that journey, we are led by our noses. Both literally, and figuratively.

Think about this - our eyes and ears can be very easily fooled, say by optical illusions, and birds making laser gun sounds, but it's very difficult to fool our noses. And our sense of taste has got nothing on the vast array of scents we can pick up. According to Joe, of the human genome's 20,000 or so genes, almost 1,000 of those are responsible for olfactory receptors, which pick up and help process different smells. That's 5 percent of our genes dedicated entirely to smelling things!

Unfortunately, through the process of evolution, we lost function in 600 of those genes as we became progressively dependant on our eyes, but of the 400 we're left with, each of them is specific to certain chemical odours. "During each of the 20,000 or so breaths that you take every day, air molecules float back and land on mucus-covered tissue with the area of a couple of postage stamps," says Hanson in the episode above. "They're packed with about 40 million special scent-detecting nerves."

These olfactory receptor neurons are the only nerves in your body that are directly exposed to the environment, and each one is tipped with just one of the 400 receptor types. Believe it or not, these types are so specific, there's one for cinnamon, and one for the the smell of a Christmas tree. 

And because each of these nerves can be triggered by a multitude of different scents, we're not limited to 400 different types of chemical odours. When we smell a rose, for example, we're being exposed to more than 200 chemical odours, and we can detect some of these at the minuscule, minuscule amount of two molecules in a billion.

So smell is obviously important to us as humans, but why is it linked so vividly to our memories? Why does something as simple as the scent of rain hold so much significance for so many of us? Watch the latest episode of It's Okay To Be Smart above to find out what science has to say about that.

Source: It's Okay To Be Smart