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Secondary Metabolites

How Did Plant Evolution Give Us Aromatherapy?

Some Context For Evolution and Aromatherapy

Millions of years ago, the first humans evolved in the forest and in the savanna . They inhaled the air that was in their environment, which was a lot purer than today’s air is. By ‘pure’, we mean ripe with the naturally occurring aromatic compounds that essentially all plants give off as part of their metabolism. Concrete, unfortunately, does not produce these compounds. And to add insult to injury, pollution from industrialization in urban areas further dirties the air. Aromatherapy and essential oils allow you to recapture a taste of the air that your ancestors breathed.

Primary and Secondary Metabolites

As you probably learned many times in school, plants grow by way of photosynthesis. Sunlight hits the leaves, and through a sophisticated chemical process, the plant cells combine carbon dioxide and water to produce new plant material. Thankfully for us, one of the byproducts of this process is the oxygen we need to survive.

However, the structural plant materials (the primary metabolites) are not the only products of photosynthesis. Plants also produce secondary metabolites that do not necessarily become part of the structure of the plant, but rather are released into the atmosphere. Why would this occur? Why would a plant expend energy-producing substances that aren’t part of its growth?

As evolution would dictate, there must be some benefit to this. These secondary metabolites must help the plant survive and, more importantly, reproduce. Let’s put ourselves into the position of a plant in the forest or the plains and consider its main concern when it comes to survival.

Poisonous to Predators

One of the primary "concerns" of a plant is that it not get eaten. There is an incredible number of herbivorous animals that like to nibble essentially every plant. These predators are not picky, and yet different plants get eaten at substantially different rates. Why is this?

One rather obvious explanation is that the plant could produce a chemical that makes it taste bad to predators, or even causes physical illness in the predators.

Interestingly, many of the secondary metabolite poisons are very strong alkaloids that we use in medicine. Some can be used directly as they come from the plant, while others are modified by pharmacologists. Consider cocaine as produced by the coca plant, morphine as produced by the opium poppy, and atropine as produced by deadly nightshade . These are all examples of alkaloids that plants produce naturally as a poison to ward off predators. This happens pervasively throughout the plant kingdom, but it is not the only method that plants use to protect themselves.


It has been shown that essential oil production within a plant helps plants to adapt to factors in the environment such as drought, high temperatures, radiation, and pollution.


Aromatic compounds can also help plants attract pollinators, like bees and butterflies. These insects are critical to the plants' reproduction.


There is another way that plants have evolved to protect themselves more indirectly. Co-evolution is the process by which two or more species evolve dependently on one another.

Essentially all herbivores have predators - animals that eat the herbivore. For example, insects (the most threatening herbivore from most plants' perspectives) are prey for frogs and birds.

Imagine this: a particular type of insect eats a plant. Predatory relationships (habits) between particular insects and plants develop due to proximity and evolutionary learning on the part of the insects. In order to protect itself from these insects, the plant emits a fragrance. This fragrance is appealing to birds, but the birds don't eat the plants - they eat the insects. As a result, the plants which emit fragrances that are pleasant to birds have fewer insects eating them!

This type of evolutionary learning occurs over hundreds, if not thousands of years according to the survival of the fittest, which is essentially lots and lots of trial and error.

Beneficial Aromatics

The fragrant substances responsible for this type of co-evolution are “aromatic”, and they comprise the foundation of aromatherapy.

The secondary metabolites above fall into a variety of categories, including alkaloids (which tend to be strong and poisonous), antibiotics, phenols, terpenes, and terpenoids.

At MONQ, we focus on the terpenes , which are one of the largest classes of organic compounds (compounds produced by living organisms). Plants readily produce and release terpenes into the air, but animals do not readily produce these compounds. There are tens of thousands of different terpenes. Some of them have names that might appear familiar, such as limonene, geraniol, eucalyptol, pinene, citronellol, and others. These terpenes are what give the plants which often share their names the fragrances they emit. Interestingly enough, Vitamin A is actually a terpene!

To read more about terpenes and the role they play in aromatherapy and in your life, check out our other blog posts:

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