Anacondas became giants long before humans walked the Earth

Snakes come in all sizes today, from tiny thread snakes no longer than a pencil to green anacondas that rival small cars in length. You already know the anaconda as one of nature’s heavyweights. Adults often stretch four to five meters, and a few verified giants have reached beyond six.

What has not been clear is when this living legend grew into a giant. New fossil work from Venezuela now offers an answer that feels both surprising and intimate. Anacondas reached their massive size more than 12 million years ago, and they have held onto it ever since.

A team led by the University of Cambridge studied ancient anaconda bones from northwestern Venezuela and compared them with snakes alive today. Their conclusion feels like a story of survival. While other Miocene giants vanished as Earth cooled and wetlands shrank, anacondas endured. They did not just survive. They stayed big.

The fossils came from a remote place called the Urumaco region in Venezuela’s Falcón Basin. This landscape is a gift to paleontologists, packed with stone layers that capture millions of years of change. Those layers tell of seas that came and went, rivers that spilled into swamps, and forests that crowded the shore. Sharks, turtles, crocodiles, birds, and mammals once moved through this world. Among them lived early anacondas.

Researchers measured 183 fossilized backbones from at least 32 snakes found in Falcón State. Many remains were no more than a single bone, yet that was enough. Snakes have hundreds of vertebrae, and the size of one can speak for the whole body. By comparing the width of each fossil bone with those of modern snakes, the team could estimate how long each ancient anaconda was.

The results were striking. Most of these snakes measured four to five meters, essentially matching their living relatives. Some were even larger. There was no sign of a lost era of giant eight meter anacondas roaming ancient swamps.

The work drew on fossil finds collected through years of fieldwork by partners at the University of Zurich and the Museo Paleontológico de Urumaco in Venezuela. The study appears in the Journal of Vertebrate Paleontology.

If you assumed warmer weather made bigger snakes, you were not alone. Scientists long thought ancient anacondas must have been larger, because reptiles depend on heat to run their bodies. The Miocene world, around 12 million years ago, ran warmer than today and held sprawling wetlands filled with prey. Bigger bodies should have flourished.

The lead author, Andrés Alfonso-Rojas, expected just that. Instead, the bones told a different story.

“This is a surprising result because we expected to find the ancient anacondas were seven or eight metres long. But we don’t have any evidence of a larger snake from the Miocene when global temperatures were warmer,” Alfonso-Rojas said.

Other animals in that same period grew to epic sizes. A crocodile called Purussaurus reached about 12 meters. A freshwater turtle, Stupendemys, carried a shell wider than a small car. Both went extinct as climates cooled and habitats changed. Anacondas endured.

“Other species like giant crocodiles and giant turtles have gone extinct since the Miocene, probably due to cooling global temperatures and shrinking habitats, but the giant anacondas have survived, they are super-resilient,” Alfonso-Rojas said.

To test their findings, the scientists turned to genetics. They built a family tree using DNA from dozens of living snake species. That let them estimate the size of long dead ancestors, a method known as ancestral state reconstruction.

The tree revealed that anacondas split from close relatives tens of millions of years ago. By the time the group fully emerged, their ancestors already averaged about four meters. That overlaps with the fossils from Venezuela. Two lines of evidence, bones and genes, said the same thing. Anacondas became giant early and never looked back.

“By measuring the fossils we found that anacondas evolved a large body size shortly after they appeared in tropical South America around 12.4 million years ago, and their size hasn’t changed since,” Alfonso-Rojas said.

During the Miocene, much of northern South America resembled a vast marshland, similar to today’s Amazon region but even wetter. Shallow lakes merged with sluggish rivers. Thick forest pressed against open water. Food was everywhere.

Anacondas thrived in this richness. They fed on fish, birds, reptiles, and mammals. That menu has changed little. Capybaras, white birds, and river fish still form a feast. Even as wetlands shrank and the land lifted into modern shapes, pockets of this watery world remained. Enough habitat endured to keep a giant fed.

There is another clue in how anacondas live. They are generalists. When one prey fades, another fills the gap. That flexibility likely helped them through times that wiped out more demanding species.

Temperature alone does not explain anaconda size. Wetlands shrank, climates cooled, and competition from new predators grew. Yet these snakes did not shrink with the world around them.

One reason may lie in how anacondas hunt. A larger body handles larger meals and longer fasts. In a swamp that swells one year and dries the next, that matters. Another reason may be chance mixed with resilience. Those that survived the changes were already large and well suited for rivers and floodplains. Over time, that body type became the rule, not the exception.

The fossils also show neighbors that vanished, including massive crocodiles and turtles. Their loss hints that survival was not easy. The anaconda’s persistence stands out against that backdrop.

You share your world with a creature rooted in deep time. When you see a green anaconda slide through dark water, you are watching a body plan older than the Andes as we know them. The animal in front of you would not surprise the wetlands of 12 million years ago.

This study fills in a blind spot in the snake record. Until now, scientists did not know when anacondas became giants. With these bones, that chapter finally has a date.

The findings sharpen how scientists read climate change through animals. Not every species shrinks when the planet cools. Some hold steady through flexibility and diet. That lesson matters as habitats shift today.

For conservation, the study highlights the value of wetlands. Anacondas stayed large because suitable rivers and swamps survived. Protecting those places protects a creature that has outlasted ages.

For science education, the work offers a powerful narrative. Living anacondas are not relics in decline. They are proof that some designs endure. This can inspire young researchers to look to fossils not just for what vanished, but for what persists.

Research findings are available online in the Journal of Vertebrate Paleontology.

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