300 million years ago, giant insects like dragonflies the size of hawks and centipedes larger than humans roamed the Earth. This period, known as the Carboniferous, was characterized by high oxygen levels (35% compared to today's 21%), which allowed these creatures to grow to immense sizes. These high oxygen levels also made the air extremely flammable, leading to frequent forest fires. Despite these conditions, giant insects thrived. However, over time, these colossal arthropods either shrank significantly in size or disappeared.

The earliest giant insect fossils, such as the Meganeura, were discovered in France in 1880. Meganeura, with a wingspan of over 25 inches, was the largest known flying insect. It was a tireless predator, agile in flight, and had no airborne competitors as birds and flying reptiles did not yet exist. Its huge appetite required a high-energy metabolism. Scientists believe its size was directly linked to the high oxygen levels, as insects breathe through a system of tubes (trachea) that becomes less efficient with increasing size in lower oxygen environments. An experiment in 2007, using synchrotron x-rays, proved this link by showing how tracheal tubes take up a larger fraction of the body in larger insects, limiting their maximum size.

The transitional period of the Permian epoch, marked by a decrease in oxygen levels from 35% to 23%, brought about the demise of Arthropleura, a massive centipede-like herbivore that could reach 10 feet in length. While it had physical advantages like articulated legs and a protective shell against predators like Ichthyostega, its extinction was primarily caused by decreasing food supplies. As the Carboniferous rainforests dried out and vegetation changed due to climate change and the emergence of wood-decaying fungi, Arthropleura lost its primary food source, leading to its disappearance around 280 million years ago.

Surprisingly, new Meganeura fossils from the late Permian period were found in France and the United States, indicating their survival despite decreasing oxygen levels. Professor Michael Engles suggests that the movement of their wings provided a unique advantage for Meganeura. The contraction of flight muscles helped to ventilate their bodies, bringing oxygen to metabolically active tissues, a mechanism that wingless arthropods lacked. This 'full-body ventilation' allowed Meganeura to continue dominating the skies. However, their eventual extinction would be influenced by new airborne predators.

Around 250 million years ago, during the late Permian, early vertebrates began to take to the skies. Jean-Sébastien Steyer discovered the fossil of Coelurosauravus, a gliding reptile about 250 million years old. This small reptile, though unable to fly by flapping, used its retractable wings to glide and catch insect prey. While it couldn't challenge the giant Meganeura, it competed for smaller prey, contributing to the overall decline of giant insects. Later, 230 million years ago during the Triassic period, pterosaurs, such as Batrachognathus, developed flapping flight and became more agile hunters, directly competing with and preying on insects. These flying vertebrates exerted increasing predatory pressure, especially on the less maneuverable large insects.

Matthew Clapham's research reveals that insect size no longer correlated solely with oxygen levels after the Jurassic period, around 150 million years ago. This coincides with the evolution of Archaeopteryx, the first bird. The emergence of birds, which were initially skilled insect eaters and more agile fliers, created both predation pressure and competition for food sources, contributing to the decrease in insect size. Furthermore, the appearance of flowering plants (angiosperms) like Archaeofructus 125 million years ago significantly altered aquatic ecosystems. The decaying leaves of these plants consumed oxygen in the water, impacting the larval stage of dragonflies and leading to the extinction of the last large-sized insects. Thus, a combination of atmospheric changes, new predators (flying reptiles and birds), and ecological shifts caused by flowering plants led to the demise of giant insects.

For a long time, Archaeopteryx, discovered in 19th-century Germany, was considered the earliest known bird. However, discoveries in China since 1996 have revolutionized our understanding. Thousands of feathered dinosaur fossils, like Sinosauropteryx, confirmed that many dinosaurs were clad in feathers, suggesting a direct evolutionary link to birds. Shuishen, a paleontologist, identified melanosomes in Sinosauropteryx, revealing its color pattern. Furthermore, the discovery of large feathered dinosaurs like Yutyrannus, living in a temperate climate with harsh winters, proved that feathers initially served as insulation. Later, courtship displays in species like Caudipteryx likely drove the evolution of more complex feathers.

The discovery of Microraptor, a dinosaur with asymmetrical feathers on all four limbs, provided crucial evidence for the origin of flight. This 'four-winged' dinosaur likely used its feathers for gliding from trees, a different hypothesis from the ground-up theory of flight. The analysis of its skeleton and feather structure suggests that flight evolved gradually through trial and error. Confuciusornis, an early bird from the same period, further demonstrates this transition with its narrow feathers and adaptations for perching and flapping flight. New discoveries, such as Anchiornis, even older than Archaeopteryx, confirm that feathered dinosaurs predated the earliest known birds, strengthening the link between dinosaurs and birds.

The discovery of Kulindadromeus in Siberia, a feathered herbivorous dinosaur, was a revolutionary finding. This showed that feathers were not exclusive to carnivorous theropods, the lineage ancestral to birds, but appeared on a common ancestor of both groups. This implies that feathers could have been present across a much wider range of dinosaurs, possibly as early as the Triassic period 220 million years ago, suggesting that feathers might be a fundamental dinosaurian characteristic. This discovery has broadened the understanding of feather evolution and its potential functions beyond flight, including insulation and display.

Contrary to earlier beliefs that early mammals were all mouse-sized and only flourished after dinosaur extinction, discoveries in China revealed a surprising diversity. Repenomamus, a wolf-sized mammal, was even found to prey on young feathered dinosaurs. These early mammals, characterized by fur, distinct ears, and varied teeth, were already adapting to their environment. Thrinaxodon, a mammalian reptile from the Triassic, burrowed and estivated to survive harsh climates, demonstrating early adaptive strategies that contributed to mammalian survival. Advanced X-ray technology allowed analysis of ancient burrows, revealing how these ancestors coped with extreme conditions, such as droughts.

The study of isolated mammal teeth from the Triassic period showed complex dentition, indicating diverse diets and hinting at the ecological niches filled by early mammals. The discovery of fossilized hair in Amber further confirmed that warm-blooded mammals already had modern hair to protect them from the climate. Fossils like Eomaia scansoria, discovered in China, provided evidence of early placental mammals, capable of tree-climbing and insectivorous diets. These findings challenged traditional views. The molecular clock method, analyzing genetic mutations in modern mammals, suggests that placental mammals diverged much earlier, around 160 million years ago, a timeline that paleontologists had struggled to confirm with fossil evidence until newer discoveries like Juramaia sinensis, the oldest known placental mammal from the Jurassic period, started to bridge the gap between genetic and fossil evidence.

Early mammals developed crucial adaptations that helped them survive alongside dinosaurs and ultimately thrive after their extinction. Lactation, first seen in a type of mammal like Yanjiaitherium, provided a survival advantage for young in times of scarcity. The evolution of ear bones, as seen in the Leonoidea fossil, improved hearing, allowing mammals to detect predators and prey more effectively, transforming a jawbone structure into the sophisticated inner ear. Furthermore, the diversification of teeth, as observed in multituberculates by Gregory Wilson using CT scans, enabled them to exploit new food sources like flowering plants, leading to a wider range of species and body sizes. These advanced features, present even in the late Jurassic, equipped mammals for their future success.