Hundreds of miles off the coast of Ecuador, in the very place that inspired Charles Darwin’s seminal theory of evolution, a wild-growing species appears to have hit rewind.
A small tomato found in the Galápagos, known scientifically as Solanum pennellii, first caught the attention of researchers in 2024 during a study of alkaloids, natural compounds produced by plants that can act as a built-in pesticide. As the scientists analyzed tomatoes found across the area, they noticed something peculiar: Solanum pennellii from the younger, western islands of the archipelago were producing compounds that hadn’t been seen in tomato plants for millions of years.
The researchers then compared the unusual plants with Solanum pennellii samples on the older islands. They found that the tomatoes on the eastern islands had a modern defense system, implying that the younger, western plants were not left behind on the species’ evolutionary journey, but instead displayed a possible case of “reverse evolution.”
“It’s not very common to see reverse evolution,” said Adam Jozwiak, a molecular biochemist at the University of California, Riverside, who was part of the team that made the discovery. The scientists reported their findings in June in the journal Nature Communications.
“We think that maybe environmental conditions put the pressure on these tomatoes to revert back to original or to ancestral state,” Jozwiak said, adding that the finding “shows that nature is very flexible, and it’s not the way we thought — that everything goes only forward.”
While the fruit of the western plants looked slightly different — with a purplish color and darker vines instead of their usual bright and warm hues — the biggest differences between them and tomatoes from the eastern islands was found on a molecular level.
Analyzing more than 30 tomato samples, the researchers observed that the western Solanum pennellii had a molecular fingerprint similar to that of an eggplant, another member of the nightshade family of plants that share a common ancestor. While modern tomatoes have evolved to no longer produce the eggplant alkaloids, those on the western Galápagos Islands had seemingly re-evolved, or de-evolved, to contain this ancestral gene.
By studying these molecules and investigating why the tomatoes have reverted to ancient genes, scientists could design better crops for eating, stronger pesticides or even medicine, Jozwiak said. It could also help researchers further understand evolution in various species, including humans — and whether it is more flexible than once thought.
Reviving ancient tomato defenses
Solanum pennellii has South American origins and likely made its way over to the Galápagos Islands through birds carrying the plant’s seeds 1 million or 2 million years ago, before the younger islands formed as a result of volcanic activity, according to Jozwiak. While experts aren’t sure exactly when the tomatoes arrived on the younger islands, the plant’s evolution had to have happened within the last half million years, since that’s around the time the youngest islands first emerged, Jozwiak said.
On the eastern islands, the environment is more stable and biologically diverse, whereas the younger islands have a barren landscape with less developed soil. The toxic molecular cocktail of the tomatoes with the ancient genes not only helps the plants ward off predators, but Jozwiak hypothesizes, it could also help the roots collect more nutrients or possibly protect them from disease.
When analyzing the tomatoes, the researchers found that it was a very simple change in the makeup of the amino acids that caused the plant to revert to its ancestral traits. Then, they genetically modified tobacco plants in the same way to observe the production of the ancestral compounds and confirm how it works.
But further study is needed to understand the benefit of this transformation — and why the reversion is occurring in the first place, Jozwiak said.
The case of Solanum pennellii sheds light on how plants evolve diverse chemistry under different conditions, according to Anurag Agrawal, an evolutionary ecologist and the James A. Perkins Professor of Environmental Studies at Cornell University in Ithaca, New York. But he added that he does not find the idea of reverse evolution in the tomatoes to be particularly surprising.
“Most evolutionary biologists would reject evolution as a forward process, it is more of a tinkering process that frequently takes detours and reversions,” Agrawal said in an email.
He pointed to examples such as eye loss in cave dwelling animals, flightlessness in birds that evolved from flying ancestors — such as penguins, ostriches and kiwi birds — and the loss of hind limbs in aquatic mammals such as whales, dolphins and porpoises, when their four-legged forebears returned to the sea.
Further research, including experiments to figure out the timing and conditions under which the tomato plants evolved to this ancestral state, would help confirm what caused the reversion.
Alkaloids in high concentrations aren’t safe for people to eat, which makes studying the compounds and how to control them valuable, Jozwiak said. But, as of now, the wild-growing tomatoes have no impact on human health, since they are not grown for human consumption.
Jozwiak said he hopes to return to the islands to look for answers to these questions, as well as other traits potentially influenced by the ancestral molecules, such as the plants’ interactions with insects and the rate at which they decompose.
Challenging a law of evolution
For certain species to develop island-specific traits is nothing unusual. Darwin observed this phenomenon while working in the Galápagos in 1835, noticing for example that finches had varying beak shapes suited to their food sources found on the differing islands.
Nevertheless, the term “reverse evolution” can be seen as controversial in the evolutionary biology world, as evolution is usually not thought to go backward, Jozwiak said.
Furthermore, “since evolution has no predetermined goal, it’s a bit problematic to talk about ‘forward’ and ‘reverse.’ Change is change,” said Eric Haag, a professor of biology at the University of Maryland in College Park, in an email. Haag was not involved with the study.
Haag referred to the rule in evolutionary biology known as Dollo’s Law, which states that once a trait has been lost in evolution, it will not be regained the exact same way. For example, dolphins once evolved to be land mammals, then returned to the sea millions of years ago, but their tails were positioned differently, and they still had to breathe air, Haag explained.
Because of this, the paper “represents somewhat of a challenge to Dollo’s Law,” he added. “It appears the specific amino acid changes… in the Galápagos species are some of the same ones found in much more distant ancestors. Converging down to that level is interesting, for sure.”
But it is complicated, Haag noted, because it also appears that the tomatoes have some differences from their ancestral counterparts — the tomatoes on the younger islands that regained the ancestral gene produced both the modern alkaloids and the ancient ones. More study is needed to understand what is really going on, and whether natural selection favored the ancestral mutations, he said.
While Jozwiak does not study humans, he said that by viewing evolution as more flexible, scientists could observe other species in which this may be happening — and could also explore the possibility of humans one day “reverse evolving,” or reinstating ancestral genes over time.
The concept is similar to the rare cases of humans who are born with rudimentary tails, a trait that was seen in primate ancestors over 25 million years ago, according to Brian Hall, research professor emeritus in evolutionary cell biology at Dalhousie University in Halifax, Canada. What remains now are tail bones that have the ability to produce more, he said.
However, the term reverse evolution is “non-sensical because it implies that we have reverted to an ancestral state, which obviously, is impossible,” Hall told CNN in an email. He equated it more to a “retention of evolutionary potential.” This is also seen in horses that have one toe, but occasionally can be born with three, like their ancestors, he noted.
“What is lost in modern horses is three toes. What is retained from ancestors is the potential to form three digits,” Hall added.
On the other hand, Beth Shapiro, a professor of ecology and evolutionary biology at University of California, Santa Cruz, said she finds the term to be a great way to engage more people in the concept of evolution.
“It’s simply a human-centric rather than purely scientific way of talking about it. Evolution isn’t directional; it’s random,” Shapiro said in an email. “As time moves on evolution continues — and sometimes that means gene variants that aren’t common anymore become common once again. But it’s just evolution.”
While the concept that evolution “can go in any direction” is not readily accepted by some scientists, Jozwiak said it is still an important topic worth further study.
“Evolution was always kind of forced by environmental conditions, by competition,” he added. “It would be nice to show that the traits that species had in the past were perfect for that situation in the past, and if these conditions change now, we can go back to that trait that we had, or other species had.”
Taylor Nicioli is a freelance journalist based in New York.
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