Scientists have long linked the massive size of prehistoric animals to warmer climates, environmental shifts, or extreme evolutionary pressures. But a new study shows that one of the world’s largest snakes, the green anaconda, has maintained its giant size since it first appeared more than 12 million years ago. The findings challenge long-held assumptions about how large animals evolve as climate and ecological conditions change.
The study, titled An early origin of gigantism in anacondas (Serpentes: Eunectes) revealed by the fossil record, was published on December 1, 2025. Led by Andrés Alfonso-Rojas of the University of Cambridge, the research analyzed 183 vertebral fossils from at least 32 ancient anacondas discovered in the Urumaco Formation in Venezuela, an area that was one of South America’s largest wetland systems during the Miocene epoch.
Using ancestral state reconstruction techniques within a phylogenetic framework, the team was able to estimate the body size of the earliest anacondas based on fossil evidence. Their results were surprising: ancestral anacondas already measured around four to five meters in length when they first appeared, the same size range as modern anacondas today. This suggests that gigantism in anacondas was not a gradual evolutionary process or a later adaptation to environmental conditions—it has been a defining feature of the species from its earliest origins.
Not Change
According to Alfonso-Rojas, initial assumptions among researchers pointed in the opposite direction. Many scientists previously believed that snakes of the Miocene—an era marked by warmer global temperatures and expansive tropical wetlands—would have grown to much larger sizes, potentially rivaling Titanoboa, the 12–14-meter giant snake that lived around 58 million years ago. But the fossil evidence did not support this idea.
“We expected ancient anacondas to be significantly larger, but all the evidence shows they were already within the same size range as modern anacondas,” Alfonso-Rojas said. “This indicates that a large body size was the starting condition, not something that changed later.”
Jorge Carrillo-Briceño of the University of Zurich, another researcher involved in the study, added that the stability of body size in anacondas is highly unusual among large reptiles. He noted that many giant reptiles of the Miocene—including large crocodilians and massive freshwater turtles—shrunk in size or went extinct as environmental conditions shifted toward the end of the period.
“Anacondas appear to be the exception,” he said. “They maintained their large size even as dramatic changes occurred across South American ecosystems. This suggests that their semi-aquatic lifestyle and wetland habitats provided an evolutionary advantage that remained stable over time.”
Evolutionary Consistency
Environmental factors played a crucial role in this evolutionary consistency. According to the team, the presence of large river systems and vast tropical wetlands in South America offered a stable habitat for anacondas for millions of years. Although the region experienced climatic fluctuations and geological shifts, wetlands remained abundant enough to support the lifestyle of these giant snakes.
Alfonso-Rojas emphasized that ecological conditions—especially the availability of medium- to large-sized prey such as ancient capybaras, large fish, and other reptiles—helped sustain the species’ large body size. As long as food remained sufficiently available and major predators did not emerge, there was no evolutionary pressure for anacondas to grow smaller. “If a large body works perfectly well, evolution has no reason to change it,” he said.
The research also highlights how fossil reconstruction can reshape scientific understanding of snake evolution in South America. The morphology of the fossil vertebrae closely resembles that of modern anacondas, reinforcing the conclusion that the species has undergone minimal morphological change over a vast evolutionary timescale. The team describes this phenomenon as “evolutionary stasis,” a condition in which a species retains its core physical characteristics for millions of years.
These findings expand scientific understanding of why the anaconda lineage succeeded while other giant animals did not. As giant crocodilians and turtles declined or disappeared due to environmental pressures, anacondas persisted—thriving as apex predators in the Amazon and Orinoco wetlands up to the present day.
The research provides new insights into how long-term ecological stability in tropical freshwater habitats may have been the key to the anaconda’s evolutionary success. In his concluding remarks, Alfonso-Rojas stated, “Anacondas are an extraordinary example of how, in some cases, the best evolutionary strategy is simply to stick with a design that works.” (Sulung Prasetyo)
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