A widely accepted scientific explanation for why ancient insects grew to enormous sizes—some dragonflies reached two feet in wingspan—has been called into question by new research suggesting that changes in atmospheric oxygen levels alone cannot account for their evolutionary downsizing.
For decades, paleobiologists have pointed to declining atmospheric oxygen concentrations as the primary reason why modern insects remain relatively small compared to their prehistoric counterparts. The theory held that during the Carboniferous period roughly 300 million years ago, oxygen levels reached 35% of the atmosphere compared to today’s 21%, allowing insects to grow much larger.
However, recent findings published in a peer-reviewed study indicate that insect respiratory systems may have been more adaptable than previously understood. ‘The breathing capacity of these ancient insects could have compensated for lower atmospheric oxygen levels,’ according to researchers familiar with the study. ‘This suggests other evolutionary pressures were at play in determining insect body size.’
The research team analyzed fossilized tracheal systems—the breathing apparatus of insects—and found evidence that these structures could have evolved to become more efficient as oxygen levels declined. This challenges the assumption that reduced atmospheric oxygen would necessarily force insects to shrink in size.
Scientists note that the emergence of flying predators, particularly birds and bats, coincided with the decline of giant insects, suggesting that predation pressure rather than oxygen availability may have been the decisive evolutionary factor. ‘The timing is too coincidental to ignore,’ said one paleobiologist following the research.
The findings have significant implications for understanding how modern ecosystems might respond to changing atmospheric conditions and could reshape theories about the relationship between environmental factors and species evolution.