Imagine stumbling upon evidence that proves one of the world's most fearsome creatures has been a giant for millions of years longer than we thought—fascinating, right? But here's where it gets controversial: Did ancient anacondas really hit their peak size early on, or could there be hidden reasons they're not even bigger today? Dive in as we explore groundbreaking fossils from Venezuela that challenge our ideas about these colossal snakes.
Anacondas stand out as some of the biggest snakes alive today, typically stretching 4 to 5 meters in length, though records show they've occasionally grown up to 7 meters. In a fresh study, scientists examined ancient anaconda fossils unearthed in South America to uncover when these tropical giants first achieved their enormous proportions. Surprisingly, the research indicates these serpents reached their maximum size about 12.4 million years ago—and they've pretty much stayed that way ever since. For beginners wondering what that timeline means, 12.4 million years ago falls within the Miocene epoch, a period of Earth's history from roughly 23 to 5 million years back, when climates and landscapes were quite different from now.
The green anaconda (Eunectes murinus), captured in this image, exemplifies their impressive scale. Image credit: MKAMPIS / CC BY-SA 4.0.
'As researchers like Ph.D. student Andrés Alfonso-Rojas from the University of Cambridge and his team point out, living snakes vary widely in size, from tiny thread snakes just 10 centimeters long to massive pythons and anacondas topping out around 7 meters,' they explain, referencing details on anaconda species and their relatives. 'Among the anacondas in the Eunectes genus, the green anaconda reigns as the largest, averaging 4 to 5 meters with confirmed records of 6 to 7.2 meters.'
Our understanding of anaconda evolution has been pieced together from fossils found in places like Brazil, Colombia, Peru, and Venezuela, dating back to the Middle to Late Miocene. However, details about the earliest anacondas' sizes were scarce, especially since they shared environments with other behemoths like huge crocodiles, turtles, hoofed mammals, and armored creatures such as sloths and armadillos.
In this new investigation, the team meticulously analyzed 183 fossilized anaconda vertebrae—backbones—from at least 32 different snakes, all discovered in Venezuela's Falcón State. By combining these measurements with data from other South American fossil sites, they calculated that prehistoric anacondas measured roughly 4 to 5 meters long, matching the averages we see in today's specimens.
'And this is the part most people miss: While giants like enormous crocodiles and turtles vanished during the Miocene due to dropping global temperatures and shrinking habitats, anacondas endured as super-resilient survivors,' notes Alfonso-Rojas. 'Our measurements reveal they developed their large bodies soon after emerging in tropical South America around 12.4 million years ago, and their size has remained stable ever since.'
To ensure accuracy, the scientists verified their findings through a technique called ancestral state reconstruction. This method builds a 'family tree' of snakes to estimate body lengths of extinct ancestors by comparing them to living relatives, such as tree boas and rainbow boas. The results reinforced that anacondas averaged 4 to 5 meters from their Miocene beginnings.
Anacondas thrive in watery habitats like swamps, marshes, and major rivers such as the Amazon, where they hunt prey like capybaras and fish. During the Miocene, much of northern South America mirrored today's Amazon basin, and anacondas roamed more extensively than they do now. Yet, plenty of suitable environments persist today, allowing modern anacondas to maintain their impressive stature.
Previously, experts assumed anacondas might have been even larger in the past when Earth was warmer, since snakes' growth can be temperature-sensitive. But here's where it gets controversial: 'This outcome caught us off guard, as we anticipated finding evidence of 7- or 8-meter giants from the warmer Miocene era,' Alfonso-Rojas admits. 'Yet, no fossils support larger sizes during that time.' Could climate alone explain their evolution, or might factors like competition with other predators have capped their growth? It's a debate worth pondering—does this mean anacondas are perfectly adapted as they are, or could they grow bigger under future conditions?
These discoveries, detailed in the Journal of Vertebrate Paleontology (available at https://www.tandfonline.com/doi/full/10.1080/02724634.2025.2572967), were released this week. The study, titled 'An early origin of gigantism in anacondas (Serpentes: Eunectes) revealed by the fossil record,' was authored by Andrés F. Alfonso-Rojas and colleagues, with publication online on December 1, 2025 (doi: 10.1080/02724634.2025.2572967).
What do you think—should we be surprised that anacondas didn't evolve to be even larger, or do you suspect there's more to uncover about their ancient world? Share your thoughts in the comments: Agree that climate change doomed the other giants while anacondas thrived, or disagree and argue for other hidden influences? Let's discuss!
