Sunday, September 24, 2023

Evolution and The Secrets of First Rain

Humans can detect the smell of fresh new rains—Petrichor—a million times better than most other smells. The reason lies in evolution.

This article has no preventive health tips. However, it discusses an intriguing question.

The Unique Smell

We have heard that sharks and dogs have far superior smelling abilities compared to us. For example, sharks can detect blood even in concentrations of one part in a million (1 ppm). At their best, the sharks can catch some other smells as weak as one part in 10 billion.

But there is one natural smell that we humans can make out at concentrations of one part in 100 billion. For perspective, if a typical raindrop (50 microlitres) falls in a standard Olympic size swimming pool (2.5 million litre water), it would be 2 parts in 100 billion.

Why are humans bestowed with a superhuman ability to detect this special odour—the smell of first rains?

As I am typing this, the first rain of the year has started outside my window. Monsoons have arrived in my home town—Mumbai, India—after a very hot summer and nearly a week later than normal.

As a part celebration of the occasion, I chose to write this article about the vital role the first rains have played in our survival and evolution.


All of us are familiar with the earthy, all-pervasive smell that wafts through the air after the first rains—Petrichor. In Greek language and mythology, Petra means stone and Ichor is the Golden blood that flows through the veins of divine beings.

Perfumers have elaborate techniques to extract this earthy fragrance from soil to add to their blends.

When raindrops hit the soil, this chemical is released into the air as a fine aerosol, just like your deodorant spray. When light rains fall on clay or sandy soils, the aerosol formation is at its best. In heavy downpours, the speed of rain droplets prevents bubble formation that precedes aerosol release. So petrichor is best experienced when light rains arrive a long time after previous drizzles.


Petrichor is the odour of a terpene compound in soil called Geosmin. In Ancient Greek, Geo is earth and Osme is smell.

This chemical is unstable and breaks down in a few hours. So how is it formed in the soil again and again?

The scientists found that a particular type (genus) of bacteria makes geosmin. These bacteria normally produce many chemicals with antibiotic, antifungal and anti-cancer properties. But geosmin does not have any such special attribute. Why does it exist?

Geosmin and Springtails

The researchers discovered that those bacteria produce geosmin to attract some soil-dwelling animals—springtails—to eat the bacteria. Bizarre! Why would a living being secrete a chemical that asks it to be eaten up?

The biologists also found that this smell has been on our earth for 400 million years along with these bacteria. So there had to be some evolutionary advantage to this strange ritual.

The scientists learned that the bacteria produce geosmin to signal those hungry springtails to come and eat them and in return, those animals spread the bacteria’s spores all around through their excreta and skin.

Geosmin and Fruit Flies

The geosmin smell gives a different message to fruit flies, who typically eat yeasts that grow on rotting fruit.

The flies need to be careful—if they eat fruits that are too decayed, they would die by consuming ‘bad’ bacteria and fungi. Their eggs laid in such fruits would produce larvae that would not survive.

So how does a fruit fly know when a fruit is excessively decayed? It does so by detecting geosmin that is released by bacteria in significantly rotten fruit.

Geosmin and Humans

So what does the geosmin smell signal to humans? Scientists hypothesize—and we will never know for sure—that geosmin indicated to ancient humans that an area of fresh rainwater was nearby. Having a hygienic source of water had an evolutionary advantage and humans who could pick up the geosmin scent could locate fresh rainwater areas better.

But why at one part per hundred billion? The ability to detect such minute concentrations indicates the supreme importance of fresh water in human survival.

We are the descendants of those nomads. As we settled into an agrarian society, the rains still played an important role in our lives and the ability to smell geosmin persevered. Today, we no longer need to detect geosmin, or do we?

We are in the era of space travel, nuclear power, and genetic engineering, but we are still fully reliant on rain for our water needs.

Over the coming centuries, we may conquer this dependence and lose the ancient trait. But until then, every time we get the first rains and the waft of geosmin pervades all senses, we must celebrate this smell of life.

As the light drizzle continues outside, I remember the ballad from the musical ‘The Sound of Music’. Tweaking its words slightly: “Petrichor, petrichor, bless my homeland forever.”

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First Published on: 13th Jun 2023
Image credit: Image by Freepik
Last Updated on: 21st July 2023


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