The most recent understanding of the evolution of insects is based on studies of the following branches of science: molecular biology, insect morphology, paleontology, insect taxonomy, evolution, embryology, bioinformatics and scientific computing. The study of insect fossils is known as paleoentomology. It is estimated that the class of insects originated on Earth about 480 million years ago, in the Ordovician, at about the same time terrestrial plants appeared. Insects are thought to have evolved from a group of crustaceans. The first insects were landbound, but about 400 million years ago in the Devonian period one lineage of insects evolved flight, the first animals to do so. The oldest insect fossil has been proposed to be Rhyniognatha hirsti, estimated to be 400 million years old, but the insect identity of the fossil has been contested. Global climate conditions changed several times during the history of Earth, and along with it the diversity of insects. The Pterygotes (winged insects) underwent a major radiation in the Carboniferous (358 to 299 million years ago) while the Endopterygota (insects that go through different life stages with metamorphosis) underwent another major radiation in the Permian (299 to 252 million years ago).
Most extant orders of insects developed during the Permian period. Many of the early groups became extinct during the mass extinction at the Permo-Triassic boundary, the largest extinction event in the history of the Earth, around 252 million years ago. The survivors of this event evolved in the Triassic (252 to 201 million years ago) to what are essentially the modern insect orders that persist to this day. Most modern insect families appeared in the Jurassic (201 to 145 million years ago).
In an important example of co-evolution, a number of highly successful insect groups — especially the Hymenoptera (wasps, bees and ants) and Lepidoptera (butterflies) as well as many types of Diptera (flies) and Coleoptera (beetles) — evolved in conjunction with flowering plants during the Cretaceous (145 to 66 million years ago).
Many modern insect genera developed during the Cenozoic that began about 66 million years ago; insects from this period onward frequently became preserved in amber, often in perfect condition. Such specimens are easily compared with modern species, and most of them are members of extant genera.
Fossils Preservation
Due to their external skeleton, the fossil history of insects is not entirely dependent on lagerstätte type preservation as for many soft-bodied organisms. However, with their small size and light build, insects have not left a particularly robust fossil record. Other than insects preserved in amber, most finds are terrestrial or near terrestrial sources and only preserved under very special conditions such as at the edge of freshwater lakes. While some 1/3 of known non-insect species are extinct fossils, due to the paucity of their fossil record, only 1/100 of known insects are extinct fossils.
Insect fossils are often three dimensional preservations of the original fossil. Loose wings are a common type of fossil as the wings do not readily decay or digest, and are often left behind by predators. Fossilization will often preserve their outer appearance, contrary to vertebrate fossils whom are mostly preserved just as bony remains (or inorganic casts thereof). Due to their size, vertebrate fossils with the external aspect similarly preserved are rare, and most known cases are subfossils. Fossils of insects, when preserved, are often preserved as three-dimensional, permineralized, and charcoalified replicas; and as inclusions in amber and even within some minerals. Sometimes even their colour and patterning is still discernible. Preservation in amber is, however, limited since copious resin production by trees only evolved in the Mesozoic.
There is also abundant fossil evidence for the behavior of extinct insects, including feeding damage on fossil vegetation and in wood, fecal pellets, and nests in fossil soils. Such preservation is rare in vertebrates, and is mostly confined to footprints and coprolites.
Freshwater and marine insect fossils The common denominator among most deposits of fossil insects and terrestrial plants is the lake environment. Those insects that became preserved were either living in the fossil lake (autochthonous) or carried into it from surrounding habitats by winds, stream currents, or their own flight (allochthonous). Drowning and dying insects not eaten by fish and other predators settle to the bottom, where they may be preserved in the lake's sediments, called lacustrine, under appropriate conditions. Even amber, or fossil resin from trees, requires a watery environment that is lacustrine or brackish in order to be preserved. Without protection in anoxic sediments, amber would gradually disintegrate; it is never found buried in fossil soils. Various factors contribute greatly to what kinds of insects become preserved and how well, if indeed at all, including lake depth, temperature, and alkalinity; type of sediments; whether the lake was surrounded by forest or vast and featureless salt pans; and if it was choked in anoxia or highly oxygenated.
There are some major exceptions to the lacustrine theme of fossil insects, the most famous being the Late Jurassic limestones from Solnhofen and Eichstätt, Germany, which are marine. These deposits are famous for pterosaurs and the bird-like Archaeopteryx. The limestones were formed by a very fine mud of calcite that settled within stagnant, hypersaline bays isolated from inland seas. Most organisms in these limestones, including rare insects, were preserved intact, sometimes with feathers and outlines of soft wing membranes, indicating that there was very little decay. The insects, however, are like casts or molds, having relief but little detail. In some cases iron oxides precipitated around wing veins, revealing better detail.
Compressions, impressions and mineralization
There are many different ways insects can be fossilized and preserved including compressions and impressions, concretions, mineral replication, charcoalified (fusainized) remains, and their trace remains. Compressions and impressions are the most extensive types of insect fossils, occurring in rocks from the Carboniferous to the Holocene. Impressions are like a cast or mold of a fossil insect, showing its form and even some relief, like pleating in the wings, but usually little or no color from the cuticle. Compressions preserve remains of the cuticle, so color distinguishes structure. In exceptional situations, microscopic features such as microtrichia on sclerites and wing membranes are even visible, but preservation of this scale also requires a matrix of exceptionally fine grain, such as in micritic muds and volcanic tuffs. Because arthropod sclerites are held together by membranes, which readily decompose, many fossil arthropods are known only by isolated sclerites. Far more desirable are complete fossils. Concretions are stones with a fossil at the core whose chemical composition differs from that of the surrounding matrix, usually formed as a result of mineral precipitation from decaying organisms.
A major source (lagerstätte) is the Mazon Creek fossil beds of the Late Carboniferous in Illinois; these are composed of shales and coal seams yielding oblong concretions. Within most concretions is a mold of an animal or plant.
When an insect is partly or wholly replaced by minerals, usually completely articulated and with three-dimensional fidelity, is called mineral replication. This is also called petrifaction, as in petrified wood. Insects preserved this way are often, but not always, preserved as concretions, or within nodules of minerals that formed around the insect as its nucleus. Such deposits generally form where the sediments and water are laden with minerals, and where there is also quick mineralization of the carcass by coats of bacteria.
Evolutionary history
The insect fossil record extends back some 400 million years to the lower Devonian, while the Pterygotes (winged insects) underwent a major radiation in the Carboniferous. The Endopterygota underwent another major radiation in the Permian. Survivors of the mass extinction at the P-T boundary evolved in the Triassic to what are essentially the modern Insecta orders that persist to modern times. Most modern insect families appeared in the Jurassic, and further diversification probably in genera occurred in the Cretaceous. By the Tertiary, there existed many of what are still modern genera; hence, most insects in amber are, indeed, members of extant genera. Insects diversified in only about 100 million years into essentially modern forms. The evolution of insects is closely related to the evolution of flowering plants. Insect adaptations include feeding on flowers and related structures, with some 20% of extant insects depending on flowers, nectar or pollen for their food source. This symbiotic relationship is even more paramount in evolution considering that more than 2/3 of flowering plants are insect pollinated. Insects, particularly mosquitoes and flies, are also vectors of many pathogens that may even have been responsible for the decimation or extinction of some mammalian species.
Before the Devonian
Molecular analysis by Gaunt & Miles 2002 suggests that the hexapods diverged from their sister group, the Anostraca (fairy shrimps), at around the start of the Silurian period - coinciding with the appearance of vascular plants on land.
Misof et al. suggest that insects could have appeared much earlier, in the Early Ordovician or even Cambrian. According to this version, the early radiation of insects occurred no later than in marine or coastal environments. However, the authors emphasize that due to the lack of insect fossils of the Cambrian to the Silurian, this version remains highly controversial.
Devonian
The Devonian (419 to 359 million years ago) was a relatively warm period, and probably lacked any glaciers. Fossils that were considered as Devonian insects, such as Rhyniognatha hirsti or Strudiella devonica were later reconsidered that their affinities as insects are insufficient. But based on phylogenic study, the first insects probably appeared earlier, in the Silurian period, from stem-group crustaceans like Tanazios dokeronThe origin and evolution of Anthropods, Graham E. Budd & Maximilian J. Telford; Nature 2009 that had lost the second antenna. The first winged insect likely evolved in the Devonian given the appearance of large numbers of insects with wings in the Carboniferous. Based on the discovery of geolologically younger & better preserved relatives, Leverhulmia was re-stablished in 2026 as the earliest unequivocal stem-group insects in the fossil record.
Carboniferous
The Carboniferous () is famous for its wet, warm climates and extensive swamps of mosses, ferns, horsetails, and calamites. Glaciations in Gondwana, triggered by Gondwana's southward movement, continued into the Permian and because of the lack of clear markers and breaks, the deposits of this glacial period are often referred to as Permo-Carboniferous in age. The cooling and drying of the climate led to the Carboniferous rainforest collapse (CRC). Tropical rain forests fragmented and then were eventually devastated by climate change.
The 2026 discovery of Chosha praecursor, an amphibious stem-insect from the Late Mississippian Tesnus Formation of Texas suggested the revision of previously dubious referred taxa, as well unnamed specimens from Mazon Creek fossil beds.