Today’s topic sits at the center of the Venn diagram comprised of “fascinating science,” “fun vocabulary,” and “fantastic aromas.” This trio might rest near the top of the rankings for possible The Mountains Are Calling Venn diagrams. 

Some scents occupy unique positions in our experiences and memories. We all have things we enjoy smelling, but most of them fall into larger buckets. The fragrance of baking bread is wonderful. Perhaps there exist connoisseurs whose noses are so delicate they can easily distinguish different breads, but, to me, baking bread is largely baking bread. The rising doughs themselves fall into the larger category of food scents. Certain odors, however, have few corollaries.

One of my favorites is petrichor

This marvelous word combines two Ancient Greek words. Pétra for “rock” or pétros for “stone” prepends ikhṓr, which was a supernatural fluid that formed the blood of the gods.

If you’re on a walk and your nose detects oncoming rain, or the first few drops of precipitation hit the ground and your olfactory sense lights up with joy, you have experienced petrichor. This “stone blood” is the smell of rain.

Rain hitting soil - photo by US Department of Agriculture

Water pounding into soil produces an earthy scent that nearly every human finds pleasant. The reason for the smell and the reason we like it are both interesting and, perhaps, not what one might expect.

The loam-like odor does not actually come from the soil itself. Instead, our noses whiff a molecule called geosmin. This little conglomeration, with a chemical formula of C12H22O, comes from bacteria in the ground, named Streptomyces. Unlike some bacteria, including the similarly named Streptococcus, this one is a beneficial organism. They are a sign of healthy soils and we have used them to produce commercial antibiotics.

When a raindrop hits a porous surface, air from the holes forms bubbles, which float to the surface and release into the broader environment. These aerosols carry the geosmin to our noses.

Geosmin - graphic by Xplus1

The amount of geosmin released by rain is actually extraordinarily tiny. Most times, the human sense of smell pales considerably next to other large organisms. Yet, for some reason, our noses are acutely sensitive to geosmin. We can detect the molecule at just 4 parts per trillion! 

Why would we evolve the superpower of smelling petrichor?

To today’s humans, the trait is simply a pleasing part of being alive. The rain smells nice. Some scientists believe the ability to discern small amounts of geosmin might stem from a point in our history where subsistence was not so easy or compartmentalized. Early humans lived in largely arid climates and spent most of their time finding nourishment. Water is, obviously, necessary for survival. The capacity to detect the faintest flicker of water mixing with soil, wafting in on the wind, might signal a river or spring. A few researchers believe that camels, who can notice water over 50 miles away in deserts, might employ geosmin, as well.

This potential evolutionary ability also explains how the phenomenon of petrichor sometimes occurs before the rain reaches us.

If you can smell rain coming – which we now know is our noses picking up the geosmin – it means you are downwind from the shower. Drops have contacted soil somewhere away from you and the wind picked up the little gift from bacteria, sending you petrichor. It’s a rare instance where we can predict the future.

Interestingly, what we find so pleasing to smell, we tend to abhor as a taste. Too much geosmin makes food and drinks taste like soil. Soil is unpalatable to the tongue.

The sensation of petrichor is unparalleled. Its power, perhaps, comes from a time-traveling attribute. It connects us to the ancients, to whom water and soil were life-bearing elements. When we smell the rain, we recall their gifts of genetics and aromas, as if our own bare feet walked across the dry soils of age-old Africa searching for stone blood.

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