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The evolutionary history of insects is unclear, but recent genetic studies suggest they detached from crustaceans about 410 million years ago. The earliest known specimen is the Devonian fossil Rhyniognatha hirsti, which suggests it had already developed flight. The origins of insect flight are still a mystery, but one researcher suggests it evolved from skimming on water. Insects were the only animals to colonize the land for millions of years, but larger tetrapods eventually joined them. The next major milestones in insect history occurred throughout the Mesozoic, when most modern groups as we know them evolved.
The evolutionary history of insects, like that of many other invertebrate groups, is poorly understood. For many decades they were thought to have branched off from centipedes and millipedes, which are known to have colonized the earth as early as 428 million years ago, during the Silurian period. But recent genetic studies suggest that insects are more likely to have detached from crustaceans about 410 million years ago. The circumstances of this evolutionary change are subject to debate and much less clear than the evolution of lobe-finned fishes into primitive tetrapods.
The earliest known specimen in the history of insects is the Devonian fossil Rhyniognatha hirsti, dated between 396 and 407 million years ago. It was found in the Rhynie Chert Formation, a well-preserved Devonian ecosystem that includes some of the earliest land plants with vascular tissue and among the earliest and best preserved terrestrial arthropod fossils. The jaws of this insect suggest that it had already developed flight, hiding the origins of insect flight and other important aspects of insect history in mystery.
When and how exactly flight entered the history of insects is poorly understood. One researcher, Jim Marden, presented a model in which insect flight evolved from evolutionarily adaptive intermediate stages involving skimming on water. He points to sandflies, a group of living insects that use their wings to skim the surface of the water. Species have been found employing water skimming varieties that have less and less effective contact with water, each step providing substantial gains in speed and thus ability to avoid predators and seek out food sources.
There are several known groups of hexapods (six-legged invertebrates) that are evolutionarily basal to insects and would have split from them before about 400 million years ago when the first fossil insects appear. These include the abundant springtails as well as the lesser known proturans and dipluri. Springtails, proturans, and diplures are all thought to have evolved their hexapod form of locomotion independently of each other, but only the insects gained the ability to fly.
For tens of millions of years, insects and other small invertebrates were the only animals to colonize the land, which at the time was covered by low-lying plants no taller than life. As plants grew and a lineage of fish evolved into the first amphibians, the insects were joined by larger tetrapods, which would have consumed them in large numbers to survive. However, thanks to the high oxygen levels of the Carboniferous period, about 320 million years ago, some insects grew to enormous sizes, such as the griffon vulture Meganeura, which had a wingspan of two feet. But when oxygen levels dropped, these insects promptly died due to an inability to circulate sufficient oxygen through their bodies.
The next major milestones in insect history occurred throughout the Mesozoic, when most modern groups as we know them evolved. Around 120 million years ago, flowering plants evolved, and cooperation between insects (especially bees) and these newcomers resulted in a mutually beneficial evolutionary relationship. As a result, flowering plants are now the dominant terrestrial flora.
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