Chapter 24: Mass Extinctions and Adaptive Radiations

The most intensely studied mass extinction occurred at the Cretaceous-Tertiary (K-T) boundary 65 million years ago, famously wiping out most non-avian dinosaurs and half of all species. The widely accepted Collision Theory posits that a massive extraterrestrial impact initiated this event, evidenced by worldwide layers of the rare element iridium and shocked quartz, and the discovery of the immense Chicxulub crater off the Yucatan Peninsula. However, climate stress from intense volcanic activity, such as the outpouring of the Deccan Traps lava flows, also contributed significantly to environmental collapse. Extinction dramatically reshaped life on Earth, facilitating the widespread adaptive radiation of mammals, whose stem lineages survived the K-T event. Mammalian diversification was enhanced by the opening of vacated ecological niches and vast geographic changes resulting from tectonic activity and the breakup of the supercontinent Pangaea. The chapter also details the successful evolutionary paths of other surviving groups, specifically highlighting that birds—which are defined as living avian dinosaurs—persisted and radiated rapidly, alongside the extinct flying reptiles known as pterosaurs, demonstrating independent adaptations for sustained flight. Finally, the remarkable adaptive radiation of insects is covered, linked to their modular body plan and coevolution with flowering plants, leading to complex social organizations characterized by caste systems and division of labor that often function as highly integrated superorganisms. The text underscores the critical evolutionary lesson that dominance is fleeting and success is unpredictable in the long term, emphasizing the interconnectedness of coevolution and coextinction events throughout Earth’s history, including the far more severe Permian-Triassic extinction event.