The world of insects is full of beautiful and dazzling species. Silverfish are not one of them. The insects look like tiny fishing sinkers, with their teardrop proportions and leaden gray complexion. They live in basements and moldy rooms, where they nibble on film and book bindings. And unlike most insects, they don’t have wings, but weave their way through life on their bellies.
But these domestic scavengers are a glimpse of history before mammals or dinosaurs – a glimpse of the mysterious first insect, which paleontologists say looked like a silverfish. This primordial creature couldn’t fly either. Flight transformed insects, launching them on a trajectory to abundance: today, in almost every corner of the Earth, some 10 quintillion insects hunt, pollinate and digest. When insects first existed, however, such vastness was not the case. The wings – and the absence of wings – may explain why.
The preserved insects are dispersed in time and space, with less than 10,000 species described in the scientific literature. Although fossil ants alone outnumber fossil dinosaurs, insects nesting in rock and amber represent only a fraction of the estimated 5.5 million living species. “The trend we see in the fossil record is that insects are really rare,” says Jessica Ware, a curator at the American Museum of Natural History in New York who studies the evolution of dragonflies.
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Complicating matters further is that the oldest possible insect fossils are crushed and small, inviting interpretation but no clear answers. Ware says it’s not unusual for separate members of his lab to come to different conclusions about the same body part; what might look like a fossilized insect wing under a microscope to Ware might appear like a leg to another researcher, she says.
The first proposed insects date back to the Devonian, the geological period that began 420 million years ago. In 2004, two entomologists argued that a 400-million-year-old pair of jaws barely a tenth of a millimeter long must belong to the oldest known insect. They also claimed that the mandible so closely resembled that of a mayfly that this insect must have had wings as well. But more than a decade later, another duo of entomologists reanalyzed the jaws and retorted that the specimen had no insect attributes – it was probably another long-legged invertebrate, a thousand- paws, they said.
The status of this animal has not yet been definitively resolved. Sandra R. Schachat, a paleoentomologist and researcher at the University of Hawaii, thinks the idea of a 400-million-year-old flying insect is hard to support without any fossil wings to point to. A compound eye fragment discovered on the other side of the Atlantic more clearly resembles an insect, she says.
The remains were found in Gilboa, New York, among the fossilized tree trunks of the world’s first known forest. The eyeball, millions of years younger than the Scottish jawbone, “really seems to belong to archaeognathsays Schachat, referring to the order biologists use to classify hairtails, wingless relatives of silverfish that still exist today.
But for tens of millions of years after that eyeball, there’s nothing in the fossil record of insects. Schachat and other entomologists call it the “Hexapod Gap”, which refers to the six legs – “hexa” plus “pod” – of insects and their close cousins. This gap lasted about 385 million to 325 million years ago, until, on the more recent side, buggy parts were again sprinkled in the fossil record.
In an article published in 2018 in the journal Proceedings of the Royal Society B, Schachat and his colleagues examined several theories as to why ancient insects had a phase of disappearance. Maybe something in the environment changed and only a few insects survived in this 60 million year old empty space. Or maybe the animals weren’t really absent, only their fossils were.
Ultimately, the team found that environmental causes didn’t seem to explain the missing bugs. They estimated the amounts of oxygen in the Devonian air from preserved chemical traces and determined that there should have been an atmosphere healthy enough for ancient insects to breathe.
Additionally, this era contains fossils of spider-like animals, centipedes, and centipedes, all the size of insects and made of similar things. “We see a lot more arachnid and centipede fragments than possible insects,” Schachat says. This is a sign that the right types of sediment existed to preserve small invertebrates.
Instead, Schachat and his colleagues say the discrepancy reflects the fact that insects were much rarer back then. What made the difference in allowing insects to take over the modern world, they concluded, were wings, particularly the fact that the Hexapod Gap reflected a time when insects had not yet evolved them. “The fossil record can accurately record the transformative impact of the evolution of insect flight,” they wrote in the study.
Before the wings, insects were confined to crawling, as silverfish still do, or otherwise depended on the prehistoric equivalent of hitchhiking in luggage. Then insect wings developed – the earliest fossil evidence of these dates back 324 million years, just after the Hexapod Gap ended. And suddenly the sky was theirs.
Aside from a few hundred known species of bristletails, silverfish, and their relatives the firebrats, nearly all insects have wings or, in the case of groups like fleas, have lost these appendages from flying ancestors over the course of of their evolutionary history. “Once winged insects appear in the fossil record, they suddenly make up the vast majority of what we see,” Schachat says.
Sporting wings spiced up the appearance and behavior of insects, changing the way they grabbed food, mate, and escaped predators. Flying insects can be bumblebees or darts, small or large creatures. Some species have grown to gigantic proportions: 300-million-year-old insects called griffinflies had wingspans that spanned two feet.
“Having wings allows you to open up your niche space,” Ware explains. “There was all the atmosphere that was not used.”
Insects beat bats, birds, and pterosaurs through the air for hundreds of millions of years and thrived there. “There are very, very good reasons to believe that wings facilitated insect diversity and abundance,” Schachat says. They diversified into new species, not only taking to the skies, but also burrowing into the ground and swimming in fresh water as they traveled to new areas and claimed new ecological roles. Inhabiting so many corners of the planet, insects have shaped life.
[Related: The land of lost fireflies is probably a humble New Jersey bog]
For such influential organizations, insects remain full of secrets. How they learned to fly in the first place is an open question. There are no discoveries yet of an ancient insect with intermediate wing-like structures, Schachat says. Perhaps one of the first species experimented with gliding; modern arboreal bristletails have been observed sliding towards tree trunks. Perhaps, according to another hypothesis, the wings of insects began as gills.
Tracing insects back to their roots isn’t just an academic exercise, Ware says, it’s essential. These animals are so essential to agriculture and human nutrition that if all the insects were eliminated, our species would die in about three months, she points out.
“To understand their evolution is to underestimate 400 million years of life on Earth,” says Ware. “It’s the closest thing to a time machine.” We live among the two extremes of insect evolution – the animals that buzz and flit above us, and the primitive silverfish at our feet, heirs of an earthly lineage.