When an asteroid hit the Earth sixty-six million years ago, it wiped out nearly all living species, leaving only a few breeds of birds. But what if that defining moment had never happened? If dinosaurs had continued to live to this day, what would the world look like, and would mammals have evolved into intelligent human beings?

It is undeniably difficult to imagine highly intelligent raptors and triceratops inventing the lightbulb, becoming CEOs, or creating political and social societies.

However, the hypothetical scenario in which dinosaurs didn’t go extinct forces us to take a philosophical look at evolution – are humans here by dumb luck, or is the evolution of intelligent beings inevitable?

To tackle this question, let’s first explore whether dinosaurs could have grown into a high-thinking species. In the 1980s, paleontologist Dale Russell imagined a scenario in which a carnivorous dinosaur became intelligent with a large brain, opposable thumbs, and could walk upright.

This “dinosauroid” is not impossible but it is unlikely due to the biology of dinosaurs. Most importantly, an animal’s starting point determines its evolutionary endpoint. This phenomenon is evident in how dinosaurs’ bodies and brains developed during their time on Earth.

Beginning in the Jurassic Period, sauropod dinosaurs, Brontosaurus grew to be 33–55 tons and nearly 100 feet in length. This evolution occurred in multiple dinosaur groups during different time periods on different continents with a wide variety of climates. But other groups in these same areas didn’t become supergiants.

The massive dinosaurs were all sauropods, and the theory is that aspects of the sauropod anatomy – lungs, hollow bones with a high strength-to-weight ratio, metabolism, or a combination of features – allowed them to evolve into the biggest land animals ever.

While dinosaurs successfully evolved in body size, Jurassic groups like Allosaurus, Stegosaurus, and Brachiosaurus, had small brains. There are notable exceptions. By the late Cretaceous Period, tyrannosaurs and duckbills evolved larger brains, and some dinosaurs further adapted by becoming smaller, growing longer legs, and interacting strategically in social situations.

Despite these gains, the brains of evolving giant herbivores and carnivores remained small, suggesting that given millions more years on Earth, dinosaurs would probably never have become intellectual beings. And given their enormous size, mammals would likely never have displaced them.

On the other end of the spectrum, evolving mammals were constrained by their smaller size. However, mammals’ brains grew bigger with time, suggesting that the extinction of dinosaurs guaranteed that mammals would develop intelligence and dominance. But alas, primate evolutionary history is not that simple.

Primates in Africa did progress into big-brained apes and over a period of 7 million years, became modern humans. But primates in other parts of the world evolved differently. The monkeys that reached South America 35 million years ago just evolved into more monkey species. Three separate primate groups in North America eventually went extinct.

What about Africa – flora, fauna, geography, climate – allowed humans to flourish there? Despite extensive research, a definitive answer continues to elude us. As unsatisfying as it may be, evidence suggests that once all dinosaurs disappeared, human evolution was a product of luck and opportunity.

So when you