A world untouched by the Chicxulub impact, the asteroid that struck Earth 66 million years ago, assumes the immense release of energy, atmospheric dust, and global wildfires that defined the end-Cretaceous extinction never occurred. The impact event initiated a biological collapse, eradicating approximately 75% of all species. Removing this cataclysmic moment means the history of life on Earth diverges, maintaining the ecological structure that had been stable for tens of millions of years and avoiding the profound environmental shock that reset global ecosystems.
Continued Dinosaur Dominance
The late Cretaceous period saw non-avian dinosaurs at the height of their evolutionary success, which would have continued unimpeded. Large-bodied herbivores like the horned ceratopsians, such as Triceratops, and the vast, duck-billed hadrosaurs were thriving across North America and Asia. These species were ecologically dominant, efficiently partitioning resources. Large predator niches would remain firmly occupied by massive theropods like Tyrannosaurus rex and its global relatives.
Smaller, agile theropods, including dromaeosaurs and troodontids, would continue their diversification, maintaining their roles as mid-sized and small terrestrial predators. The absence of the extinction event means the massive ecological vacuum later filled by mammals would never materialize. Consequently, large-bodied herbivore and carnivore niches would remain firmly under the control of the great reptiles. Subsequent epochs would be characterized by the steady evolution of new dinosaur species, not their replacement by a different class of animal.
The Suppressed Role of Mammals
In this dinosaur-dominated world, the evolutionary trajectory of mammals would be one of long-term stasis, constrained by intense ecological pressure. Late Cretaceous mammals were already diversifying, but they were predominantly small, usually weighing less than a kilogram. They were largely restricted to nocturnal, burrowing, and insectivorous or omnivorous lifestyles, surviving in the margins of the dinosaur empire. This body size limit was a direct consequence of competition, as any mammal attempting to grow larger would face immediate pressure from established dinosaurian competitors.
Larger niches, such as those for grazers and pursuit predators, would remain inaccessible to mammals for millions of years. Mammalian evolution would likely focus on specialized adaptations for resource exploitation within small-bodied guilds, rather than explosive diversification or gigantism. Their warm-blooded physiology, which proved beneficial for surviving the post-impact climate crisis, would offer little advantage in a stable, warm world mastered by reptiles. Mammals would remain a successful but ecologically minor component of the global fauna.
Reshaped Global Ecosystems
The structure of this alternative Earth’s ecosystems would be fundamentally organized around large reptilian megafauna, creating trophic webs distinct from the Cenozoic era. On land, niches currently occupied by large mammalian herbivores, such as elephants, rhinos, and bison, would instead be filled by descendants of hadrosaurs and ceratopsians. These megafaunal reptiles would require different metabolic and reproductive strategies than their mammalian counterparts, influencing the pace of nutrient cycling and seed dispersal across continents.
In the skies, the dominant aerial niches would continue to be partitioned between birds and pterosaurs. Birds would be prevented from achieving their massive post-extinction radiation into all flying niches by the continued presence of specialized, often gigantic, pterosaurs like the azhdarchids. Meanwhile, the marine realm would retain its Mesozoic character, with massive marine reptiles such as mosasaurs and plesiosaurs continuing their dominance as apex predators. The rise of large marine mammals, like whales and seals, would be suppressed by these enduring reptilian rivals.
The plant world would follow a different path, marked by an uninterrupted rise of flowering plants, or angiosperms. While angiosperms were already spreading before the impact, the mass extinction caused a temporary, global retreat, resulting in a “fern spike.” In this scenario, the transition to angiosperm-dominated forests would proceed gradually and continuously, leading to diverse plant communities without the sudden evolutionary bottleneck that shaped modern flora. The continued co-evolution between large dinosaur herbivores and specific plant groups would further sculpt the landscape, leading to unique biomes unlike any found today.
The Question of Sapience
The concept of sapience, or human-level intelligence, is driven by specific selective pressures, such as complex social dynamics, tool manipulation, and communication. In a world defined by dinosaur dominance, the evolutionary path toward sapience would be open to different lineages. One possibility lies with the smaller, more agile theropods, such as the troodontids, which possessed large relative brain sizes and stereoscopic vision.
If selective pressures for greater dexterity and problem-solving emerged, a lineage of advanced dinosaurs could conceivably develop technology-using intelligence. Alternatively, the highly intelligent corvids and parrots, who belong to the avian dinosaur lineage, might pursue this path. Even a specialized mammal, forced into complex, niche-exploiting behaviors, could eventually cross the threshold, though it would face far greater competitive hurdles than primates did in our own history. The evolution of sapience is not guaranteed, but the absence of the Chicxulub event would merely change the identity of the species that might potentially achieve it.