Walk through a pine forest on a summer day, and you will notice the scent long before you see the towering trees. The sharp, resinous aroma that fills the air is more than a pleasant fragrance—it is part of an ancient chemical defense system. Conifers such as pines, spruces, and firs release sticky resins that ooze from bark wounds and harden into protective barriers. These resins do more than seal injuries: they are saturated with natural compounds that repel insects and fight off invading fungi. Among the most important of these compounds are diterpenes, specialized molecules that act as chemical shields against some of the forest’s most dangerous enemies, including bark beetles.
The enzymes responsible for making these defensive compounds are called diterpene synthases. For decades, scientists have wondered how these enzymes arose and whether conifers inherited them from a single ancient ancestor or developed them independently over time. Now, new research led by scientists at the Max Planck Institute for Chemical Ecology in Germany and Iowa State University in the United States is unraveling the evolutionary story of these remarkable enzymes—and in doing so, offering a glimpse into how trees have defended themselves for hundreds of millions of years.
Tracing the Origins of Enzymes
Every tree in a coniferous forest carries its own internal chemical laboratory. Within their cells, diterpene synthases assemble molecules that become the building blocks of resin. What makes these enzymes fascinating is their sensitivity: even slight changes in their structure can produce entirely new defensive compounds. This flexibility may help explain how conifers evolved such a vast chemical repertoire to ward off threats.
The research team set out to reconstruct the evolutionary timeline of diterpene synthases. Did these enzymes emerge once, early in conifer history, or did different tree lineages independently stumble upon similar chemical defenses? To answer this, the scientists turned to genetic analysis. They compared the DNA of diterpene synthases from multiple conifer species, then used these sequences to computationally “resurrect” ancient versions of the enzymes.
In the laboratory, the team brought these ancestral enzymes back to life by inserting their genes into bacteria. Once the bacteria produced the enzymes, the scientists supplied the raw materials needed for diterpene synthesis and carefully analyzed the resulting products. This approach allowed them to see which defensive molecules ancient enzymes might have generated and how they changed over evolutionary time.
A Chemical Timeline Written in Resin
The results revealed a surprising mix of old and new. Some diterpenes found in modern resin first appeared over 300 million years ago, during a period when conifers themselves were still taking shape in Earth’s ecosystems. These ancient compounds have survived the passage of geological time, still present in the resin that protects trees today.
Other diterpenes, however, appear to be much younger. Genetic and chemical evidence shows that several important compounds evolved independently in multiple conifer species, long after the first trees appeared. This means that pine, spruce, and fir trees, though separated by millions of years of evolution, often developed identical diterpenes on their own—parallel solutions to the same evolutionary problem.
But why did some compounds arise quickly while others took so long to appear? The researchers found that a genetic principle called epistasis holds the key. In simple terms, epistasis means that one genetic change often depends on earlier ones. Certain traits cannot develop until a particular sequence of mutations has already unfolded. Over millions of years, conifers accumulated these hidden genetic prerequisites. Once they were in place, the path to new diterpenes opened dramatically, allowing similar defensive compounds to emerge multiple times in different tree species.
Bark Beetles and the Evolutionary Arms Race
The timing of these evolutionary developments is no coincidence. Fossil evidence shows that bark beetles were already present when some of the newer diterpenes appeared. This suggests that conifers may have fine-tuned their chemical defenses in direct response to beetle attacks.
Bark beetles are small, but their impact on forests is massive. By boring into the bark, they create entry points for themselves and for symbiotic fungi that weaken or kill the tree. Resin packed with diterpenes provides a critical line of defense: it can entrap beetles, repel them with its pungent chemistry, and slow the spread of fungi.
Today, the resin of conifers is a chemical mosaic—a blend of ancient molecules forged in deep evolutionary history and newer compounds that arose in response to more recent threats. This mixture may be one reason why conifers remain resilient in the face of pests. Their defensive chemistry is not a single weapon but a diverse arsenal, refined over millions of years of natural selection.
The Prehistory That Shapes the Present
One of the study’s central insights is that a tree’s ability to adapt to modern challenges depends heavily on its evolutionary past. A conifer facing bark beetles today does not start from scratch; it builds upon the molecular foundations laid by countless generations before it. Past genetic changes, whether ancient or recent, shape the possibilities for future adaptations.
As Jonathan Gershenzon, head of the Department of Biochemistry at the Max Planck Institute, explained, this evolutionary prehistory determines not only what new traits can arise but also how effectively trees can respond to threats. In other words, the past sets the stage for the present—and for the future survival of entire forests.
Looking Ahead: Trees in a Changing World
The work of tracing diterpene evolution is not just an exercise in understanding the past. It carries urgent relevance for the future of forests under pressure. Climate change, invasive species, and increasing bark beetle outbreaks pose serious risks to conifers worldwide. Knowing how trees have historically adapted their defenses can help scientists predict which species are most vulnerable and which might be resilient.
The researchers now plan to explore how diterpene synthases continue to influence conifer defenses today, especially against the dual threat of beetles and fungi. Early evidence suggests that no single compound is enough. Instead, trees rely on a carefully balanced mixture of substances—an evolutionary cocktail that provides flexible, layered protection.
The Forest’s Chemical Story
What began as a study of enzymes has become a story about survival, adaptation, and resilience. Conifers have endured for hundreds of millions of years not by standing still, but by evolving a sophisticated chemical language of defense. Their resins are both time capsules and weapons—containing whispers of ancient forests and shields against modern threats.
The research reminds us that every pine-scented breeze carries echoes of deep evolutionary history. When resin glistens on a tree trunk, it tells the story of ancestors who survived mass extinctions, shifting climates, and relentless pests. It is the story of enzymes changing shape, of molecules arising and reappearing, and of forests learning to fight back, again and again.
The next time you walk beneath towering conifers, breathing in their sharp, resinous scent, remember that what you smell is not only the essence of a tree—it is the fragrance of survival, crafted by evolution over hundreds of millions of years.
More information: O’Donnell, Andrew J., Favorable epistasis in ancestral diterpene synthases promoted convergent evolution of a resin acid precursor in conifers, Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2510962122. doi.org/10.1073/pnas.2510962122
