To say that there has been an incredible amount of diversity in the forms of life on our planet during the course of its existence would be more than an understatement. Scientists estimate that the species extant on our planet today represent less than 1% of the incredible range of organisms that once thrived here, and which have now faded into extinction.
Because of this, the evolution of life here on Earth is arguably among the most remarkable phenomena known to exist in all of nature, and one which continues to surprise us and our understanding of biology. Within the complex maze of constant changes and adaptations that Earth species have undergone, a number of mysteries have endured over the years which still evade conclusive understanding. Among these are questions about the earliest fossil insects, and the peculiar origins of their wings.
From within Devonian cherts unearthed in Scotland, some of the earliest known fossil insects represent a wingless variety of collembolans, which are essentially ancient predecessors to modern springtails. These primitive insects are named for their springlike tail which allows them to leap out of the way of potential harm when they are disturbed, and varieties of them have been recovered from such chert deposits dated to around 402 million years ago.
Sample of Rhynie chert from Scotland, the likes of which contains some of the oldest ancestors of living insects (Public Domain).
These ancient springtails stand as both a testament to this simple, yet obviously successful evolutionary form, as varieties of springtails still exist today. However, biologists stop short of calling these creatures insects, since they lack both wings and the proper mouthparts for such designation, instead classifying them as hexapods. Nonetheless, it appears that modern varieties of springtails and their insect cousins share common ancestry with the hundreds of million-year-old bugs that turn up fossilized in chert deposits today.
Herein lies the true mystery, since there remains a surprising gap in the fossil record between these early springtails, and the first winged insects that only begin to appear in the Upper Carboniferous period some 100 million years later. At some point during this lengthy period, insects began to evolve the ability to fly, but why is there so little evidence of it in the fossil record?
Some scientists have theorized that the earliest forms insect wings might have taken hadn’t actually been designed for flight, but instead growths that aided in cooling the bodies of ancient bugs. From here, random mutations over time would have led to the musculature required to move these portions of early insect bodies, after which they eventually became adapted for flight as is commonly observed with insects today, a feature useful not only in locomotion, but allowing the similar protection mechanism of the springtail’s unique leaping activities.
Modern springtail (collembolans) as seen under a microscope (Wikimedia Commons 3.0).
While the controversy over the origins of insects remains, a step toward discovering one of the long-sought representatives from the “missing” 100-million-year period did finally turn up in 2012, as detailed in a paper that appeared in the journal Nature.
“Although it can hardly be described as well preserved, the fossil shows a six-legged thorax, long single-branched antennae, triangular jaws and a 10-segmented abdomen,” wrote William Shear of the fossil discovery. Insects are, as Shear noted, the only known arthropods to have ever existed with this combination of traits, and therefore it would seem that the specimen in question had been just that. While the creature differed enough from insects known to us since around 250-million-years ago that there had been a few questions, the authors of the Nature paper nonetheless concluded that they were probably looking at one of the earliest ancestors of modern insects.
Dubbed Strudiella devonica, this specimen was recovered from samples extracted from a quarry in Belgium, and dated to approximately 370 million years ago. While perhaps one of the most significant discoveries of its kind, it only further emphasizes how strange the absence of anything definitively attributable to insects remains during their “missing” period in Earth’s ancient history.
There have been some promising discoveries found in Scottish chert deposits that resemble jaws of a possible insect, as well as in fossils recovered from New York that appear to represent portions of creatures that resemble modern insect eyes. As Shear noted in 2012, “these fragments more or less complete the picture of all that is known of insects at this crucial time in Earth’s history.”
To be recognized as one of the oldest and most successful forms life has ever taken on Earth, it is indeed mysterious that so little is known of the origins of insects. This, along with their early absence in the fossil record, endures as one of the most perplexing mysteries that nature and evolution presently keep from us.
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