Why prehistoric insects grew to terrifying sizes
In short: Imagine a world where dragonflies were as large as hawks and centipedes stretched as long as cars. During the Carboniferous Period, Earth's atmosphere became a high-octane playground for invertebrates, creating a primeval jungle that feels more like a nightmare than our modern reality.
Roughly 350 million years ago, during the Carboniferous Period (approximately 359 to 299 million years ago), Earth was dominated by vast lowland swamp forests. These tropical jungles were hyper-efficient at photosynthesis, absorbing carbon dioxide and releasing massive amounts of oxygen. Consequently, oxygen concentrations hit a peak of 35%, exceeding current human safety limits by over one and a half times.
For decades, the prevailing theory was that this surplus of oxygen acted as a primary fuel source, allowing adult insects to grow to astonishing proportions while still meeting their metabolic energy needs. In this high-oxygen environment, we see evidence of aggressive centipedes the size of cars, scorpions as large as bull terriers, and predatory dragonflies comparable in size to modern seagulls or birds.
However, recent research by Wilco Verberk and David Bilton suggests a more complex survival mechanism. By studying stonefly larvae, researchers found that juvenile insects are often more sensitive to oxygen fluctuations than adults because they absorb gas directly through their skin with little control over the intake. This implies that these prehistoric bugs may have evolved their massive size not just as a benefit of high oxygen, but as a necessary defense against oxygen toxicity during their larval stages.
This shift in perspective suggests that insect gigantism was a sophisticated biological response to an atmospheric extreme. While the oxygen fueled their growth, it also posed a lethal risk to smaller, more vulnerable life forms. Understanding these ancient giants provides a unique window into how life adapts—or is forced to transform—when the very air becomes a powerful and potentially dangerous resource.
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Sources
- Summary by Intellect · source: The Terrifying World of Giant Insects: What Was Earth Like During the Carboniferous Period?
- Also: Why Giant Bugs Once Roamed the Earth
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