Evolution Running Backwards? That's What This Unlikely Organism Appears To Be Doing

We typically think of evolution as progressing in one direction, with a species getting “better” and “better” as it goes. But evolution is far more complex than that, as has recently been illustrated by an unusual organism: Galápagos tomatoes. These wild-growing fruits are shedding millions of years of evolution in order to reproduce long-lost chemical defences.

The tomatoes are descended from South American ancestors that were probably brought to the Galápagos by birds, and they have now started to produce a toxic molecular cocktail that hasn’t been seen in millions of years. The compounds in question are alkaloids, which are produced by nightshades like potatoes, eggplants, peppers, and tomatoes. They are bitter-tasting molecules that serve as a built-in pesticide to deter insects, fungi, and grazing animals from noshing on the plants or tubers

Generally speaking, animals on the Galápagos have few predators to worry about, but the same is not true for plants. As such, the production of these alkaloids can help protect vulnerable species. In low concentrations, these alkaloids are not harmful to humans, but higher concentrations can be toxic. This is why researchers from the University of California, Riverside (UC Riverside), started to examine the Galápagos tomatoes. They wanted to understand how the plants are producing these alkaloids.

In this instance, it isn’t that the tomatoes are producing the compounds, it's that they are making the “wrong” ones, as in they are creating chemicals that have not been seen in tomatoes since their early evolution.

“It’s not something we usually expect,” Adam Jozwiak, a molecular biochemist at UC Riverside explained in a statement. “But here it is, happening in real time, on a volcanic island.”

Contrary to what some people believe, evolution is not always a one-way march towards greater adaptation. Sometimes it can act like a circular path whereby an organism gains traits similar to ones it once lost. It’s rare, but it does happen. However, this doesn’t tend to involve them regaining lost traits through the exact same genetic pathways, which is difficult to prove. And yet the tomatoes appear to have achieved this rare reversal.

“Our group has been working hard to characterize the steps involved in alkaloid synthesis, so that we can try and control it,” Jozwiak added.

The team analyzed over 30 tomato samples taken from distinct geological locations across the Galápagos islands. They found that plants on eastern islands produced the same alkaloids as those in modern cultivated tomatoes, but ones taken from western islands produced compounds with a chemical fingerprint of their ancient eggplant relatives.

The difference between these the eastern and western island tomatoes comes down to stereochemistry, how molecules are arranged in three-dimensional spaces. Two molecules can contain the same atoms but will behave differently depending on how they are arranged. During their analysis, the team found that the tomatoes made the switch by changing four amino acids in a single enzyme. This simple development was enough to flip the molecule’s structure from modern to ancient.

The researchers proved this change by synthesizing genes coding for these enzymes in their lab and inserting them into tobacco plants. As expected, the plants promptly started producing the old compounds.

It seems this development was not a random event; it aligned with geography. The eastern islands are biologically diverse and more stable. Here the tomatoes produced modern alkaloids. In contrast, the western islands are younger, and the landscape is more barren and the soil less developed. This is where the ancient alkaloids are being produced.

“It could be that the ancestral molecule provides better defense in the harsher western conditions,” Jozwiak said.

Although the tomatoes have gained this unexpected trait, how do we know it is due to an evolutionary reversal? To verify the direction of this change, the team conducted a kind of evolutionary modeling using modern DNA to infer the traits of long-extinct ancestors. The western tomatoes matched what the early ancestors probably produced.

So is this really a reversal? The process of evolutionary reversal has been documented in snakes, fish, and some bacteria, but it’s a rare and often unclear process to examine.

“Some people don’t believe in this,” Jozwiak explained. “But the genetic and chemical evidence points to a return to an ancestral state. The mechanism is there. It happened.”

And if this change can happen in one species, it could happen in others too.

“I think it could happen to humans,” Jozwiak said. “It wouldn’t happen in a year or two, but over time, maybe, if environmental conditions change enough.”

This is obviously still extremely theoretical, but it does offer a surprising development in our understanding of evolution. It seems that life can sometimes find a way forward by circling back to aspects of its 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.”

The study is published in Nature Communications.