The fossil proof on insect evolution and wings

The insect world teems with beautiful and dazzling species. Silverfish are not one of them. The insects resemble tiny fishing lead with their teardrop-shaped proportions and leaden-grey complexion. They stay in basements and musty rooms, where they nibble on sheds and book covers. And unlike most insects, they don’t have wings, but rather crawl through life on their bellies.

But these household scavengers are glimpses of pre-mammal or dinosaur history — glimpses of the mysterious first insect that paleontologists believe looked like something like a silverfish. This primal creature could not fly either. Flight transformed insects and sent them on a trajectory to plenty: Today, about 10 quintillion insects hunt, pollinate, and digest in almost every corner of the globe. However, when insects first existed, such a vastness was not the case. Wings – and winglessness – may explain why.

Extant insects are found scattered across time and space, with fewer than 10,000 species described in the scientific literature. Although fossil ants alone outnumber fossil dinosaurs, the insects hidden in rocks and amber represent only a fraction of the estimated 5.5 million living species. “The trend that we’re seeing across the fossil record is that there are few insects,” says Jessica Ware, a curator at the American Museum of Natural History in New York, who studies how dragonflies evolved.

[Related: A close look at amber fossils that have stuck through the ages]

To make matters worse, the oldest possible insect fossils are squished and small, which invites interpretation but doesn’t provide clear answers. Ware says it’s not uncommon for different members of her lab to come to different conclusions about the same body part. what might look like a fossilized insect wing to Ware under a microscope might appear like a leg to another researcher, she says.

The first proposed insects can be traced 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 just a tenth of a millimeter long must have belonged to the oldest known insect. They also claimed that the lower jaw was so similar to that of a mayfly that this insect must have had wings too. But more than a decade later, another duo of entomologists reanalyzed the jaws and countered that the specimen had no insect attributes — it was likely another long-legged, ground-lying invertebrate, a centipede, they said.

The status of this animal has yet to be finally clarified. Paleo-entomologist and University of Hawaii research associate Sandra R. Schachat believes the idea of ​​a 400-million-year-old flying insect without fossil wings to point to is difficult to support. A fragment of the compound eye uncovered across the Atlantic is more clearly insect-like, she says.

The remains were found in Gilboa, New York, among the fossilized logs of the world’s first known forest. The eyeball, several million years younger than the Scots pine, “really looks like it should belong to Archaeognatha,” says Schachat, referring to the order biologists use to classify bristletails, wingless relatives of silverfish, that still exist today.

Paleoentomologists say that Earth’s first insect probably resembled such little silverfish. deposit photos

But for tens of millions of years after that eyeball, there’s nothing in the fossil record of insects. Schachat and other entomologists call this the “hexapod gap,” which refers to the six legs — “hexa” plus “pod” — of insects and their close relatives. This gap lasted about 385 million to 325 million years ago, until faulty bits are again scattered through the fossil record on its newer side.

In a 2018 article published in the journal Proceedings of the Royal Society B, Schachat and her colleagues explored several theories as to why ancient insects went through a period of disappearance. Perhaps something has changed in the environment, and few insects have survived in this 60-million-year void. Or maybe the animals weren’t missing at all, just their fossils.

In the end, the team found that environmental causes didn’t seem to explain the missing bugs. They estimated the oxygen levels in Devonian air from preserved chemical traces and determined that there should be a healthy enough atmosphere for ancient insects to breathe.

In addition, this era contains fossils of arachnids, centipedes, and millipede-like animals, all about the size of insects and made of similar material. “We see a lot more fragments of arachnids and centipedes than of possible insects,” says Schachat. This is a sign that the right types of sediments existed to support small invertebrates.

Instead, Schachat and her colleagues say this gap reflects insects being much rarer back then. What made the difference for insects to take over the modern world, they concluded, were wings — specifically that the hexapod gap reflects a time when insects hadn’t evolved them yet. “The fossil record can accurately record the transformative effects of the evolution of insect flight,” they write in the study.

Before the wings, insects were restricted to crawling, as silverfish still do, or otherwise relied on the prehistoric equivalent of hitchhiking with luggage. Then insect wings evolved — the first fossil evidence of them dates from 324 million years ago, just after the end of the Hexapod Gap. And suddenly the sky was theirs to inhabit.

Apart from a few hundred known species of bristletails, silverfish, and their relatives the lionfish, almost all insects have wings — or, in the case of groups like fleas, lost these appendages from flying ancestors in their evolutionary history. “Once winged insects show up in the fossil record, they’re suddenly the vast majority of what we see,” says Schachat.

Then insect wings developed, and suddenly the sky was theirs for life.

Sporting wings beefed up the way insects looked and behaved—changing how they caught food, how they mate, and how they evaded predators. Flying insects can be bumblebees or darters, small or large creatures. Some species grew to gigantic proportions: 300-million-year-old insects called griffon flies had wingspans of over two feet.

“If you have wings, you can open up your niche space,” says Ware. “There was the entire atmosphere that was not used.”

Insects beat bats, birds and pterosaurs in the air by hundreds of millions of years — and thrived there. “There is very, very good reason to believe that wings promoted insect diversity and abundance,” says Schachat. They branched out into new species, not only taking to the skies but also burrowing into the ground and swimming through freshwater as they traveled to new territories and claimed new ecological roles. Inhabiting so many corners of the planet, insects shaped life.

[Related: The land of lost fireflies is probably a humble New Jersey bog]

For such influential organisms, insects remain full of mysteries. How they even learned to fly is an open question. There are still no discoveries of an ancient insect with intermediate wing-like structures, says Schachat. Perhaps an early species experimented with soaring; Modern arboreal bristletails have been observed gliding onto tree trunks. Perhaps, according to another hypothesis, insect wings began as gills.

Tracing insects back to their roots isn’t just an academic exercise, Ware says — it’s essential. These animals are so important to agriculture and human nutrition that if every insect were wiped out, our species would become extinct in about three months, she points out.

“Understanding their evolution means underestimating 400 million years of life on Earth,” explains Ware. “This is the closest thing to a time machine.” We live between the two extremes of insect evolution—the animals that buzz and flutter overhead and the primitive silverfish at our feet, inheritors of an earthbound lineage.