Some scientists believe that current theories about the extinction of dinosaurs emphasize external factors, such as asteroid impacts, cosmic events, ozone layer depletion, large-scale volcanic eruptions, and drastic climate change. However, these external theories fail to address the fundamental question: why did many other species that coexisted with dinosaurs survive? Their ecosystems and food chains before the mass extinction were not significantly different from those of the dinosaurs, yet they not only escaped the catastrophe but also evolved into the diverse and vibrant world we know today. This leads some scientists to consider that the more direct cause of dinosaur extinction might be related to the dinosaurs themselves; for example, the enormous size and cold-blooded nature of most dinosaurs may have contributed to their extinction.
Among the many possible factors that could cause a large-scale extinction of a species family, a new, highly contagious disease is one that deserves particular attention. It is not uncommon for certain pathogens to cause complete crop failure in specific aquaculture industries, and the harm caused by human immunodeficiency is well-known. When wild animals encounter similar diseases, the catastrophic destruction of their populations is equally inevitable. From this perspective, it is possible that the extinction of dinosaurs was linked to an epidemic of highly contagious diseases. Moreover, since microorganisms are the main cause of infectious disease outbreaks, and changes in the ecological environment are most likely to cause their mutations, even if the extinction of dinosaurs was caused by a catastrophe, the mechanism is highly likely to be the failure of the dinosaurs' immune system due to mutations in pathogens.
The interactions between organisms, leading to life and death, have always been a crucial factor in the evolutionary history of species change. Faced with the threat of pathogens, an animal's ability to fight infection is vital to its survival. Modern biology has proven that the anti-infection function of vertebrates consists of two main parts: first, the natural defense system (immunologically termed non-specific immunity), including external and internal barriers formed by skin, mucous membranes, and blood, as well as various phagocytes and factors in body fluids capable of killing pathogens. This natural defense system is very ancient in origin. Second, the specific immune system, mainly composed of immune-active cells capable of recognizing specific antigens. This specific immune system is a new functional system that emerged after animals evolved into vertebrates, reaching a relatively complete level only in mammals and birds.
While we cannot directly understand the function of the immune system in dinosaurs, scientists can still speculate about it through comparative biology. The immune systems of modern animals like turtles and lizards, which belong to the reptile family like dinosaurs, are quite underdeveloped. They lack complete lymphatic vessels and lymph nodes, and their only immune organ, the thymus, is poorly developed, generally lacking clear differentiation between the cortex and medulla. Birds are similar to reptiles in this lack of thymus tissue differentiation, which is further evidence of their close kinship. However, birds have highly developed other immune organs, and their immune function is far superior to that of reptiles. The lack of antibody diversity in reptile body fluids makes them ill-equipped to provide specific immunity against a wide variety of pathogens. Furthermore, the cold-blooded nature of reptiles limits the proliferation rate of immune cells, making it difficult for them to adapt to the invasion of different pathogens. The immune systems of modern cold-blooded vertebrates are relatively limited in their ability to fight infection. They generally rely primarily on natural defense systems to non-specifically resist pathogen invasion, with some enhancements. For example, sharks have squalamine, and frogs have magnum opus, highly effective defensive peptides that can disrupt bacterial structures, thus compensating for immune system deficiencies and ensuring the continuation of the species. However, these enhancements still cannot compare to the sophisticated, specific immune systems found in mammals.
As a typical example of extinct reptiles, dinosaurs certainly did not have a very perfect immune system. If their natural defense system was not very strong, the consequences of encountering pathogens that their defense system could not control would have been unimaginable.
Therefore, some scientists speculate that at the end of the Cretaceous period, due to some reason (possibly a catastrophic event), a variety of new pathogens capable of breaching the general animal defense systems suddenly evolved on Earth, causing a pandemic. Many animals with compromised immune systems, such as dinosaurs, were unable to resist and went extinct. As a result, only some cold-blooded animals with enhanced natural defense systems survived the catastrophe, while warm-blooded birds and mammals, possessing sophisticated and efficient immune systems, were more likely to avoid this fate and thus emerged as the dominant animals in the subsequent Cenozoic era.
