Insects are everywhere—in the air, on the ground, in the ground, and sometimes in your house and food. Yet there are none whatsoever in the known fossil record between 385 million and 325 million years ago. The earliest known insect fossil is a 385-million-year-old wingless creature that looks like a silverfish. But for the next 60 million years there is not so much as a single dragonfly, grasshopper or roach.

This so-called hexapod gap has long vexed paleontologists, given that insects today are found in almost every imaginable land habitat. One hypothesis suggests that chokingly low oxygen levels kept insect diversity from soaring during the gap and that these creatures proliferated only once the life-giving gas increased.

But advances in the understanding of atmospheric oxygen levels are challenging that idea, explains Sandra Schachat, a paleoentomologist at Stanford University, who led a recent study that modeled the gas's availability during the hexapod gap. Atmospheric oxygen at the time was much higher than once believed, according to the research, which was published in January in the Proceedings of the Royal Society B.

A fungus gnat in amber. Credit: Natural History Museum, London Alamy

The disagreement between Schachat's findings and earlier research stems from the fact that her team used more recent atmospheric data that nowadays can be gathered cheaply and efficiently. “If these results are confirmed, we could dismiss low oxygen levels as a possibility” for explaining the gap, says Jesus Lozano Fernandez, a paleobiologist at the University of Bristol in England, who was not involved in the new work.

Schachat and her team combed through fossil information from a public paleontology database and realized there was something special about many of the insect fossils that came after the gap: they had wings. This was likely the trait that helped hexapod diversity take off; winged insects can zip away from predators and get at otherwise unreachable foods such as leaves and other insects. “The gap is simply the tail end of a larger interval in which insects are very rare on the landscape because wings had not yet originated,” Schachat says.

The mystery now bugging Schachat is how insect wings evolved at all; the earliest flying insects found after the gap seem to have already been very diverse. “The two very first winged insects that we have in the fossil record—they're about as different from each other as you could imagine,” she says. The origins of wings, then, must lie within the gap itself. Lurking somewhere in it, there may be undiscovered fossils that could reveal how insects became the first animals to take to the skies.