Insects make up more than half of known animal species, making them essential to any study of the history of life. Yet our knowledge of their evolution has been hampered because they don’t fossilize well. So two remarkable deposits that have yielded an astonishing diversity of specimens have overturned theories on the rise of the largest group of insects in the world today, the Holometabola, along with the freshwater ecosystems in which many of them lived.

Modern Holometabola include ants, bees, beetles, and butterflies. Their domination of the planet's biodiversity exemplified in the famous quote; God must be "inordinately fond of beetles", was thought to date back to the Jurassic, based on both fossil evidence and molecular clocks of species separation. However, studies of two deposits from north-western and central China show Holometabola were thriving and already exceptionally diverse 237-238 million years ago during the Triassic.

Among the species from these locations described in Science Advances are the earliest definite examples of caddisflies, water boatmen, scorpion flies, and a wide array of beetles – apparently favored by God even then. The discoveries are particularly rich in forms adapted to live around water showing that even then insects made up an important component of freshwater ecosystems.

More than 800 fossilized insects have been collected from the Karamay and Tongchuan Formations, the largest haul ever found from the Triassic, and a testimony to the importance of these sites. At Tongchuan 28 families of insects and 11 orders were represented in the finds, with 14 families and two-thirds of the individual specimens belonging to the Holometabola, indicating their dominance goes back much longer than suspected. The Karamay collection included 10 families and six orders.

At the end of the Permian era, the Earth suffered the greatest mass extinction in its history. The few species that survived faced much less competition than at other times, making space for the first dinosaurs and mammal-forerunners, as well as major expansions in new types of plants. It seems insects were doing something similar, something we had previously missed due to a lack of fossil evidence.

The timing of these deposits almost perfectly matches the start of a great diversification of plant species, which probably created many new niches into which insects could spread. The authors speculate that the protective pupal stage, rapid development, large population sizes, and limited competition between adults and offspring that characterize the modern descendants of the species found in these sites would have protected these species against environmental variability. This would have allowed these insects to thrive during an era marked by some of the hottest temperatures in Earth's history, and some wild swings in environmental conditions.