Nonavian dinosaurs have been extinct for 66 million years, but what would have happened if they’d survived?
What If the Dinosaur-Killing Asteroid Had Missed Earth?
Sixty-six million years ago, a catastrophic event reshaped life on Earth. A 9-mile-wide (15-kilometer) asteroid struck what is now Mexico, unleashing tsunamis, wildfires, and acid rain. The impact filled the atmosphere with debris, blocking out the sun and causing a mass extinction that wiped out 75% of all species—including the nonavian dinosaurs—within months.
But what if that asteroid had never hit? Would dinosaurs have continued to dominate the planet? Could they have survived ice ages and even evolved into intelligent beings?
Would Dinosaurs Still Rule?
Paleontologists suggest dinosaurs might have persisted as Earth’s dominant species. They had already thrived for 165 million years, surviving environmental changes such as fluctuating sea levels, temperature shifts, and volcanic eruptions.
“For those of us who collect fossils of the last dinosaurs, we can see quite clearly that everywhere in the world … there was a great diversity and abundance of dinosaurs,” said Steve Brusatte, a paleontology professor at the University of Edinburgh, in an email to Live Science. “Dinosaurs were strong, successful, still diverse, [and] still at the top of their game when the asteroid hit.”
Some scientists argue that dinosaurs were already declining, with extinctions outpacing the emergence of new species. However, Brusatte is unconvinced, believing they were still thriving when disaster struck.
Could Dinosaurs Have Survived an Ice Age?
Most dinosaurs lived in warm climates, but some, particularly feathered species, inhabited colder regions. Dinosaurs like Tyrannosaurus rex may have been warm-blooded, meaning they could regulate their body temperature rather than relying on external warmth.
“Many of them had feathers, [so] they could insulate themselves just like mammals can,” Brusatte noted. Dinosaurs could have adapted further, much like mammoths, which evolved woolly coats during the last Ice Age.
Could Dinosaurs Have Become Intelligent?
Could some dinosaurs have evolved into intelligent beings? In 1982, paleontologist Dale Russell speculated that troodontids—small, birdlike dinosaurs with large brains, stereoscopic vision, and opposable fingers—might have evolved into sentient creatures if they had survived. His concept, the “dinosauroid,” imagined a humanoid-like, green-skinned dinosaur.
However, most paleontologists reject this idea as too speculative. A 2023 study concluded that “neither Troodon nor any other dinosaur could have begun a primate-like lineage that evolved to a human level of intelligence.”
Still, modern birds, the closest living relatives of dinosaurs, demonstrate surprising intelligence. “The dinosaurs that live on today—birds—are really smart,” Brusatte said. “They actually have more neurons in their brains than mammals, on average.”
What Would Have Happened to Mammals?
If dinosaurs had survived, mammals likely would have remained small and insignificant. “I suspect that mammals would have mostly or entirely stayed small for many millions of years more,” Brusatte explained. The asteroid’s impact allowed mammals to diversify and grow larger, eventually giving rise to primates—and ultimately, humans.
Paleontologist Paul Sereno, a professor at the University of Chicago, agrees. “We evolved in a mammalian world impossible to have taken place with large nonavian dinosaurs running around,” he said. “We are not inevitable.”
A 2021 study found that the extinction of large dinosaurs and competing mammal species gave primates the opportunity to flourish. Had the asteroid missed, human evolution as we know it likely never would have occurred.
“History would have been totally different,” Brusatte said. “Our exact ancestors surely would have never had their chance to evolve.”