This week, a friend sent me our horoscope—we’re both Gemini—from Seven Days, a beloved Vermont weekly, because, improbably, it was about the sea slug I’d been telling her about just days before.“The sea slug Elysia chlorotica is a small, unassuming creature that performs a remarkable feat: It eats algae and steals its chloroplasts, then incorporates them into its own body,” the horoscope explained. Years ago I had incorporated this fact into my own view of the world, and it had changed my understanding of the rules of biology.This particular slug starts life a brownish color with a few red dots. Then it begins to eat from the hairlike strands of the green algae Vaucheria litorea: It uses specialized teeth to puncture the alga’s wall, and then it slurps out its cells like one might slurp bubble tea, each bright-green cellular boba moving up the algal straw. The next part remains partially unexplained by science. The slug digests the rest of the cell but keeps the chloroplasts—the plant organelles responsible for photosynthesis—and distributes these green orbs through its branched gut. Somehow, the slug is able to run the chloroplasts itself and, after sucking up enough of them, turns a brilliant green. It appears to get all the food it needs for the rest of its life by way of photosynthesis, transforming light, water, and air into sugar, like a leaf.The horoscope took this all as a metaphor: Something I’d “absorbed from another” is “integrating into your deeper systems,” it advised. “This isn’t theft, but creative borrowing.” And in that single line, the horoscope writer managed to explain symbiosis—not a metaphor at all, but an evolutionary mechanism that may be more prevalent across biology than once thought.Elysia chlorotica is a bewitching example of symbiosis. It is flat, heart-shaped, and pointed at the tail, and angles itself toward the sun. Its broad surface is grooved by a web of veins, like a leaf’s is. Ignore its goatish head, and you might assume this slug was a leaf, if a particularly gelatinous one. Sidney Pierce, a marine biologist retired from the University of South Florida, remembers his surprise when a grad student brought a specimen into his office in the Marine Biological Laboratory at Woods Hole, on Cape Cod, more than two decades ago. Photosynthesis requires specialized equipment and chemistry, which animals simply do not have—“yet here was an animal that’s figured out how to do it,” he told me. He spent the next 20-odd years trying to find the mechanism. “Unfortunately, I didn’t get all the way to the end,” he said.No one has, as my colleague Katherine J. Wu has written. The algae and the slug may have managed some kind of gene transfer, and over time, produced a new way of living, thanks not to slow, stepwise evolution—the random mutation within a body—but by the wholesale transfer of a piece of code. A biological skill leaked out of one creature into another.All of us are likely leakier than we might assume. After all, every cell with a nucleus, meaning all animal and plant cells, has a multigenetic heritage. Mitochondria—the organelles in our cells responsible for generating energy—are likely the product of an ancient symbiosis with a distant ancestor and a microbe, and have their own separate DNA. So we are walking around with the genetic material of some other ancient life form suffused in every cell. And the earliest ancestor of all plants was likely the product of a fusion between a microbe and a cyanobacterium; plants’ photosynthesizing organelles, too, have distinct DNA. Lynn Margulis, the biologist who made the modern case for this idea, was doubted for years until new genetic techniques proved her correct.Her conviction about the symbiotic origins of mitochondria and chloroplasts was a monumental contribution to cell biology. But Margulis took her theory further; in her view, symbiosis was the driving force of evolution, and many entities were likely composites. Evolution, then, could be traced not only through random mutation, but by combination. “Life did not take over the globe by combat, but by networking. Life forms multiplied and complexified by co-opting others, not just by killing one another,” she wrote, with her son, in 1986. This remains pure conjecture, and an exaggeration of the role of symbiosis beyond what mainstream evolutionary theory would support; random mutation is still considered the main driver of speciation.Yet more scientists now wonder if symbiosis may have played a larger role in the heritage of many species than we presently understand. Phillip Cleves, a geneticist at the Carnegie Institution for Science who studies the symbiotic relationship between corals and their algae symbionts, told me how, as an undergraduate, he was blown away by the fact that corals’ alliance with algae made possible ecosystems—coral reefs—that support a quarter of all known marine life. The algae cells live, whole, inside coral cells, and photosynthesize as normal, sustaining the coral in nutrient-poor tropical waters. “I realize now that that type of interaction between organisms is pervasive across the tree of life,” he said.It’s probable that the ancestors of all eukaryotes were more influenced by bacteria in their environments than modern evolutionary theory has accounted for. “All animals and plants likely require interactions with microbes, often in strong, persistent symbiotic associations,” Margaret McFall-Ngai, a leading researcher of the role of microbes in animal development, wrote in 2024. These interactions, she argued, are so fundamental to life that the animal immune system should perhaps be thought of as a sort of management system for our many microbial symbionts. Although biology has been slow to recognize symbiosis’s significance, she thinks this line of research should now take center stage, and could alter how all stripes of biologists think about their work.Cleves, too, sees himself as working to build a new field of science, by training people on how to ask genetic questions about symbiotic relationships in nature: When I called him, he was preparing to teach a four-week course at the Marine Biological Laboratory in Woods Hole on exactly that. Genomic research has only relatively recently been cheap enough to apply it routinely and broadly to all sorts of creatures, but now scientists can more easily ask: How do animals’ interactions with microbes shape the evolution of individual species? And how does that change dynamics in an ecosystem more broadly?Elysia chlorotica is also a lesson in how easily the boundaries between an organism and its environment can be traversed. “Every time an organism eats, a whole wad of DNA from whatever it’s eating passes through the animal. So DNA gets transferred all the time from species to species,” Pierce told me. Most times it doesn’t stick, but on the rare occasions when it does, it can reroute the fate of a species. “I think it happens more than it’s recognized, but a lot of times it’s hard to recognize because you don’t know what you’re looking for. But in these slugs, it’s pretty obvious,” he said. They’re bright green.Patrick J. KrugStill, attempts to understand what is happening inside Elysia chlorotica have mostly fallen short. Scientists such as Pierce presume that, over time, elements of the algal genome have been transferred to the slug, allowing it to run photosynthesis, yet they have struggled to find evidence. “It’s very hard to find a gene if you don’t know what you’re looking for,” Pierce said—plus, slug DNA is too muddled to parse a lot of the time. Slugs are full of mucus, which can ruin samples, and because the chloroplasts are embedded inside the slug cells, many samples of slug DNA end up picking up chloroplast DNA too. After years of trying, and at least one false start by a different lab, Pierce and his colleagues did manage to find a gene in the slug that was involved with chloroplast repair, hinting that a genetic transfer had occurred, and offering a clue as to how the animal manages to keep the plant organelles alive.But another research team showed that related species of photosynthesizing slugs can survive for months deprived of sunlight and actual food: They may simply be hardy. Why, then, if not to make nutrients, might the slugs be photosynthesizing? Perhaps for camouflage. Or perhaps they’re stashing chloroplasts, which themselves contain useful fats and proteins, as food reserves. (Pierce, for one, is skeptical of those explanations.)Whatever benefit Elysia chlorotica derives from the chloroplasts, there couldn’t be a leakier creature. It crosses the divide between plant and animal, one species and another, and individual and environment. I first read about the slug in a book titled Organism and Environment by Sonia Sultan, an evolutionary ecologist at Wesleyan University, in which she forwards the argument that we should be paying more attention to how the environment influences the way creatures develop, and how those changes are passed generationally, ultimately influencing the trajectory of species.While Elysia chlorotica is an extreme example of this, a version of it happens to us, and our bodies, all the time. Encounters with the bacteria around us reshape our microbiomes, which in turn affect many aspects of our health. Encounters with pollution can reroute the trajectory of our health and even, in some cases, the health of our offspring. Researchers think access to healthy foods—a factor of our environments—can modify how our genes are expressed, improving our lives in ways that scientists are just beginning to understand. We are constantly taking our environment in, and it is constantly transforming us.