The Cretaceous-Paleogene (K-Pg) extinction event 66 million years ago profoundly reshaped life on Earth. It marked the end of the Mesozoic Era, leading to the disappearance of approximately three-quarters of plant and animal species, including all non-avian dinosaurs. This catastrophic event, primarily attributed to a massive asteroid impact in the Yucatán Peninsula, created a lingering impact winter that disrupted photosynthesis and global food chains. This pivotal moment prompts an alternate history: what if this extinction never occurred, and how would Earth’s ecosystems and life have unfolded?
A Different Planet
Without the K-Pg impact, Earth’s physical environment would have continued its Late Cretaceous trajectory, characterized by warmer temperatures and higher sea levels. The absence of the impact winter and atmospheric disturbances would mean a more stable, albeit still slowly cooling, global climate.
Plant life would have continued its evolution without sudden ecological collapse. Flowering plants (angiosperms), which had rapidly diversified and become a prominent part of the flora by the end of the Cretaceous, would have continued to spread. Conifers, cycads, ferns, and ginkgoes would also persist. The consistent presence of large herbivorous dinosaurs would have maintained significant grazing and browsing pressures, influencing plant evolution and ecosystem structure over millions of years.
Dinosaur Evolution and Diversification
In a world where the K-Pg extinction did not occur, dinosaurs would have continued their evolutionary journey for another 66 million years. Late Cretaceous dinosaur diversity was high, with various groups like hadrosaurs, ceratopsians, and tyrannosaurs thriving. This ongoing diversification would likely have seen new forms and adaptations emerge to exploit various ecological niches.
Theropods, including large predators like tyrannosaurs and smaller, agile dromaeosaurs, might have evolved more specialized hunting strategies, leading to new forms adapted for pursuit, ambush, or cooperative hunting. Some lineages could have developed enhanced sensory capabilities or greater intelligence, similar to modern birds. Sauropods, the giant long-necked herbivores, might have continued their large size or diversified into new, smaller forms to exploit different plant resources or semi-aquatic environments. Ornithischians, such as armored ankylosaurs and horned ceratopsians, could have further refined their defensive mechanisms or developed novel feeding adaptations in response to evolving plant life.
The Mammalian Predicament
Mammals, which had already appeared in the Mesozoic, remained largely small and occupied restricted ecological niches in a dinosaur-dominated world. This pattern would likely have persisted. Most Mesozoic mammals were comparable in size to shrews, mice, or rats, with only a few reaching the size of a fox.
These mammals would have continued to adapt to avoid dinosaurian predators, potentially remaining nocturnal, burrowing, or evolving for arboreal life. Their body size would have remained constrained by competitive pressure and direct predation from dinosaurs, preventing the widespread emergence of larger and diverse mammalian forms seen after the K-Pg extinction. The rich array of large herbivores and carnivores that define modern mammalian fauna would likely not have evolved in such a scenario.
The Question of Human Existence
The evolution of humans, Homo sapiens, is intrinsically linked to ecological opportunities that arose after the K-Pg extinction. The environmental and ecological pressures that fostered bipedalism, large brains, and tool-making in hominids would have been profoundly different. Our ancestors thrived in environments where competition from large reptiles was absent.
The absence of dinosaurs allowed mammals to diversify, filling vacated large terrestrial niches. This mammalian radiation led to the evolution of primates and hominids. Bipedalism, a defining human trait, offered advantages in open landscapes, improving energy efficiency and temperature regulation. However, the expansion of open grassland environments, which provided the backdrop for early hominid evolution, might have been different or non-existent with continuous dinosaurian herbivore presence.
In a dinosaur-dominated world, any primate lineage would face predatory pressure from large theropods and resource competition. The development of a large, energy-intensive brain, a hallmark of human evolution, would require abundant and accessible food resources, which might be scarce where dinosaurs occupied many primary ecological roles. It is highly improbable that Homo sapiens, or any similarly intelligent, large-brained, bipedal hominid, would have evolved under persistent dinosaurian ecological dominance. If intelligence emerged, it might be in a small, secretive, or arboreal lineage, constantly avoiding the planet’s primary inhabitants.