On the younger, black-rock islands of the Galápagos archipelago, wild-growing tomatoes are doing something peculiar. They're shedding millions of years of evolution, reverting to a more primitive genetic state that resurrects ancient chemical defenses.
These tomatoes, which descended from South American ancestors likely brought over by birds, have quietly started making a toxic molecular cocktail that hasn't been seen in millions of years, one that resembles compounds found in eggplant, not the modern tomato.
In a study published recently in Nature Communications, scientists at the University of California, Riverside, describe this unexpected development as a possible case of "reverse evolution," a term that tends to be controversial amongst evolutionary biologists.
That's because evolution isn't supposed to have a rewind button. It's generally viewed as a one-way march toward adaptation, not a circular path back to traits once lost. While organisms sometimes re-acquire features similar to those of their ancestors, doing so through the exact same genetic pathways is rare and difficult to prove.
However, reversal is what these tomato plants appear to be doing.
"It's not something we usually expect," said Adam Jozwiak, a molecular biochemist at UC Riverside and lead author of the study. "But here it is, happening in real time, on a volcanic island."
The key players in this chemical reversal are alkaloids. Tomatoes, potatoes, eggplants, and other nightshades all make these bitter molecules that act like built-in pesticides, deterring insect predators, fungi, and grazing animals.
While the Galápagos are famous as a place where animals have few predators, the same is not necessarily true for plants. Thus, the need to produce the alkaloids.
The researchers began this project because alkaloids in crops can be problematic. In high concentrations they are toxic to humans, hence the desire to understand their production and reduce them in the edible parts of fruits and tubers.
"Our group has been working hard to characterize the steps involved in alkaloid synthesis, so that we can try and control it," Jozwiak said.
What makes these Galápagos tomatoes interesting isn't just that they make alkaloids, but that they're making the wrong ones, or at least, ones that haven't been seen in tomatoes since their early evolutionary days.
The researchers analyzed more than 30 tomato samples collected from distinct geographic locations across the islands. They found that plants on eastern islands produced the same alkaloids found in modern cultivated tomatoes. But on western islands, the tomatoes were churning out a different version with the molecular fingerprint of eggplant relatives from millions of years ago.
That difference comes down to stereochemistry, or how atoms are arranged in three-dimensional space. Two molecules can contain exactly the same atoms but behave entirely differently depending on how those atoms are arranged.
To figure out how the tomatoes made the switch, the researchers examined the enzymes that assemble these alkaloid molecules. They discovered that changing just four amino acids in a single enzyme was enough to flip the molecule's structure from modern to ancestral.
They proved it by synthesizing the genes coding for these enzymes in the lab and inserting them into tobacco plants, which promptly began producing the old compounds.
The pattern wasn't random. It aligned with geography. Tomatoes on the eastern, older islands, which are more stable and biologically diverse, made modern alkaloids. Those on the younger, western islands where the landscape is more barren and the soil is less developed, had adopted the older chemistry.
The researchers suspect the environment on the newer islands may be driving the reversal. "It could be that the ancestral molecule provides better defense in the harsher western conditions," Jozwiak said.
To verify the direction of the change, the team did a kind of evolutionary modeling that uses modern DNA to infer the traits of long-extinct ancestors. The tomatoes on the younger islands matched what those early ancestors likely produced.
Still, calling this "reverse evolution" is bold. While the reappearance of old traits has been documented in snakes, fish, and even bacteria, it's rarely this clear, or this chemically precise.
"Some people don't believe in this," Jozwiak said. "But the genetic and chemical evidence points to a return to an ancestral state. The mechanism is there. It happened."
And this kind of change might not be limited to plants. If it can happen in tomatoes, it could theoretically happen in other species, too. "I think it could happen to humans," he said. "It wouldn't happen in a year or two, but over time, maybe, if environmental conditions change enough."
Jozwiak doesn't study humans, but the premise that evolution is more flexible than we think is serious. Traits long lost can re-emerge. Ancient genes can reawaken. And as this study suggests, life can sometimes find a way to move forward by reaching into the past.
"If you change just a few amino acids, you can get a completely different molecule," Jozwiak said. "That knowledge could help us engineer new medicines, design better pest resistance, or even make less toxic produce. But first, we have to understand how nature does it. This study is one step toward that."
