The Devonian Biosphere
Complex Ecosystems and Symbiotic Relationships
The Origin of Insects
Insects, belonging to the class Insecta within the phylum Arthropoda, are the most diverse group of animals on Earth. With over 1 million described species and estimates of several million more yet undiscovered, they occupy nearly every terrestrial and freshwater ecosystem. Their evolutionary roots stretch deep into the Paleozoic Era. Fossil evidence places the first true insects in the Early Devonian (~410 million years ago), while molecular data suggest they may have originated even earlier, during the Ordovician (~480 Ma).
The earliest insects were small, wingless forms (apterygotes), resembling modern silverfish. Other hexapod relatives, such as springtails (Collembola), share a common ancestor with insects but are not considered true insects. These early lineages were adapted to humid environments such as soil, moss, and leaf litter, where they fed on detritus and plant material.
One of the key adaptations of insects was the development of a tracheal respiratory system. Tracheae allow oxygen to diffuse directly to tissues and support the high metabolic demands of small, active terrestrial animals. While different arthropod groups (such as arachnids and myriapods) evolved tracheae independently, insects share a common tracheate ancestor. Variations in atmospheric oxygen levels over geological time strongly influenced insect body size and physiology; for example, giant dragonflies with wingspans over 70 cm appeared in the high-oxygen Carboniferous atmosphere.
A major evolutionary breakthrough was the origin of wings, which appeared in the Late Devonian (~380–360 Ma). This innovation enabled insects to disperse widely, escape predators, and exploit new habitats. Winged insects (pterygotes) underwent rapid diversification during the Carboniferous and Permian, leading to the establishment of most modern orders.
Further specialization occurred with the evolution of diverse mouthparts, allowing insects to feed on an extraordinary range of resources. While early insects consumed detritus and plants, later groups evolved predatory, piercing-sucking, and chewing adaptations. The evolution of nectar-feeding occurred much later, in the Cretaceous, alongside the rise of flowering plants (angiosperms). This triggered a powerful episode of coevolution, driving explosive diversification in both insects (particularly bees, butterflies, and beetles) and plants.
Insects also developed survival strategies such as burrowing into soil, wood, or plants, enabling them to evade predators and exploit hidden food sources. These innovations—combined with flight, metamorphosis, and ecological flexibility—made insects one of the most successful and widespread groups of animals in Earth’s history. Today, they remain a cornerstone of terrestrial ecosystems, shaping pollination, decomposition, and food webs worldwide.
Early Plant and Insect Symbiosis
The earliest interactions between plants and insects began as simple herbivory during the Devonian and Carboniferous (~400–300 million years ago). Fossil leaves, stems, and coprolites from these periods show evidence of feeding damage, mining, and gall-like structures, demonstrating that insects were already consuming spores and plant tissues from ferns, horsetails, and seed ferns.
By the Permian and Mesozoic eras, some insects had moved beyond herbivory and developed pollination-like relationships with certain gymnosperms. Fossil evidence and comparative studies suggest that beetles, thrips, and scorpionflies were early pollinators of cycads, bennettitaleans, and gnetophytes. These interactions predated flowering plants, but were less specialized than the pollination systems that evolved later.
The most profound shift occurred in the Early Cretaceous (~125 million years ago) with the rise of angiosperms. Flowering plants evolved nectar, colorful petals, and scents to attract pollinators. In parallel, insects such as bees, butterflies, and wasps diversified rapidly, developing specialized mouthparts and body structures suited for pollen and nectar feeding. This marked the beginning of the highly specialized plant–pollinator coevolution that dominates modern ecosystems.
Beyond pollination, plants and insects also evolved other mutualisms. Some plants developed extrafloral nectaries to attract ants for defense against herbivores, while others provided hollow stems or thorns as shelter for protective insect partners. These associations deepened the ecological interdependence between the two groups.
In summary, insect–plant interactions began with simple feeding on non-flowering plants, expanded into early pollination with gymnosperms, and culminated in the intricate and diverse coevolutionary relationships between insects and flowering plants that shape today’s terrestrial ecosystems.
The Late Devonian extinction
Around 375 to 360 million years ago, Earth experienced one of its most catastrophic mass extinction events—the Late Devonian extinction. This period marked a profound loss in biodiversity, particularly within marine ecosystems, where approximately 75% of all species vanished. The aftermath of this extinction dramatically reshaped the trajectory of life on Earth.
Several hypotheses have been proposed to explain the causes of the Late Devonian extinction. One widely accepted theory suggests that significant tectonic movements led to a substantial drop in sea levels, resulting in the destruction of shallow marine habitats. The loss of these vital environments likely contributed to the extinction of many marine species that relied on them for survival.
Additionally, prolonged volcanic activity during this period may have released large amounts of CO2 into the atmosphere, leading to global warming and ocean acidification. These environmental changes would have created inhospitable conditions for many marine organisms, further exacerbating the extinction event.
Another hypothesis posits that a cataclysmic event, such as an asteroid impact or a gamma-ray burst, could have triggered the extinction. However, this idea is less widely supported in the scientific community.
There is also evidence to suggest that the Late Devonian period experienced widespread anoxic events, where oxygen levels in the oceans dropped dramatically. This severe reduction in oxygen would have made survival difficult for many marine species, leading to widespread die-offs.
Despite its devastating impact, the Late Devonian extinction set the stage for the evolution and diversification of new species in the subsequent Carboniferous period. The extinction cleared ecological niches, allowing new forms of life to emerge and flourish, shaping the future of Earth's biodiversity.
The Origin of Amphibians
Amphibians, including frogs, salamanders, and caecilians, represent a significant evolutionary step as some of the earliest vertebrates to venture onto land. Their origins can be traced back to a group known as labyrinthodonts, which emerged around 365 million years ago during the Late Devonian period. These ancient amphibians developed key traits such as strong bony jaws and limbs capable of crawling or hopping, allowing them to thrive in both aquatic and terrestrial environments.
The transition from gills to lungs in amphibians is a particularly fascinating aspect of their evolution. In their larval stage, modern amphibians, like frogs and salamanders, primarily use gills to extract oxygen from water, much like their aquatic ancestors. As they mature and undergo metamorphosis, these amphibians develop lungs, enabling them to breathe air and survive on land. This shift from gill to lung respiration is a crucial adaptation for life on land and is accompanied by the development of a more complex circulatory system.
In addition to lungs, many amphibians also rely on cutaneous respiration, or breathing through their skin. Their moist, vascularized skin allows for direct gas exchange with the environment, which is particularly important when lung breathing is not efficient, such as during hibernation or in certain aquatic conditions.
Throughout the Carboniferous period, amphibians diversified significantly, giving rise to tetrapods—a group of four-limbed vertebrates that include not only modern amphibians but also reptiles, birds, and mammals. These early tetrapods were characterized by strong backbones and limbs, which facilitated efficient movement on land.
By the Permian period, approximately 290 million years ago, a group of large amphibians known as temnospondyls had evolved. These amphibians were among the largest ever to exist, with adaptations such as robust bodies and burrowing behaviors that helped them survive in various environments.
The Triassic period saw the emergence of the first frogs and salamanders, some of which eventually returned to aquatic environments. Over time, these amphibians developed a range of adaptations, including lungs and other traits that allowed them to breathe air and continue their evolutionary journey both in water and on land.