The dinosaur extinction disaster caused a significant shift in evolution, including the primitive primates that eventually evolved into modern humans.
66 million years ago, an asteroid struck Earth with the force of 10 million atomic bombs, altering the entire course of evolution.
The sky turned dark, and plants could no longer photosynthesize. Trees died, and herbivorous animals perished as well. The food chain collapsed. Over 90% of all species vanished. When the dust settled, all dinosaurs were wiped out, leaving only a few bird species to survive.
However, that disaster catalyzed evolution for all species. The surviving mammals had the opportunity to thrive, including primitive primates that later evolved into humans as we know them today.
What If Dinosaurs Had Continued to Live?
Imagine if that asteroid had veered off course and not collided with Earth. Would dinosaurs still be alive today? Would predatory birds plant their flags on the Moon? Would dinosaur scientists uncover the theory of relativity or discuss a hypothetical world where mammals completely dominated Earth?
This scenario resembles a science fiction story, but it raises profound philosophical questions about evolution. Is the presence of humans today a mere coincidence, or was the evolution of intelligent humans inevitable?
The human brain, tools, language, and social groups define us as the dominant species on this planet. Approximately 8 billion upright humans inhabit 7 continents. In terms of biomass, humans outnumber all wild animal species combined.
Dinosaurs and mammals through time. (Photo: Nick Longrich).
We have transformed half of Earth’s land area to sustain ourselves. One could argue that beings like humans must have evolved as a result.
In the 1980s, paleontologist Dale Russell from North Carolina State University proposed a thought experiment where a carnivorous dinosaur evolved into an intelligent creature capable of using tools. The descendant of this dinosaur would have a large brain, opposable digits, and walk upright on two legs. While this is impossible, it’s not entirely out of the realm of possibility.
The biological characteristics of a species limit its evolutionary direction. Your starting point will constrain your endpoint.
If you drop out of college, it’s nearly certain you won’t become a neurosurgeon, lawyer, or NASA rocket scientist, but you could become an artist, actor, or entrepreneur. The paths we take in life open some doors while closing others. This principle applies to the evolutionary process as well.
The Connection Among Giant Dinosaurs
Consider the sizes of various dinosaurs. Starting in the Jurassic period, the long-necked dinosaurs, such as Brontosaurus and its relatives, evolved into colossal creatures nearly 30 meters long and weighing about 30 to 50 tons—ten times the weight of an elephant and as long as a blue whale.
The biological characteristics of a species limit its evolutionary direction.
The same occurred with other dinosaur families: Diplodocidae, Brachiosauridae, Turiasauridae, Mamenchisauridae, and Titanosauria.
This phenomenon happened across different continents, during various periods, and under varied climates, from deserts to tropical rainforests, yet other dinosaurs in similar environments did not evolve into such giants.
The common link among these evolved giants is that they are all sauropods. Something in the anatomy of sauropods—such as their lungs, hollow bones with a strength-to-weight ratio, metabolism, or all these factors—was key to unlocking their evolutionary potential.
These factors allowed them to grow to gigantic sizes in ways that other terrestrial animals, both preceding and succeeding them, did not.
Where Would Dinosaurs Have Evolved If They Hadn’t Gone Extinct?
Dinosaurs developed their bodies exceptionally well, but their brains did not grow significantly. By the end of the Cretaceous period, approximately 80 million years later, Tyrannosaurus Rex and duck-billed dinosaurs evolved further and had larger brains. However, a T. rex’s brain weighed only 400 grams, while a Raptor’s brain weighed just 15 grams, compared to the average human brain, which weighs 1.3 kilograms.
It seems they also had increasingly complex social lives. They began living in herds and developed complex horns for fighting and display. Nevertheless, most evolved repeatedly into giant herbivores and small-brained carnivores.
For about 100 million years in dinosaur history, it is evident they would have evolved differently had that asteroid not struck Earth. They could still exist as supergiant, long-necked herbivores and massive predators like the Tyrannosaurus Rex.
While their brains might have grown slightly larger, there is little evidence to suggest they would have evolved to become wise. Moreover, there is no indication that mammals could have replaced dinosaurs. Dinosaurs reigned as the dominant species in their environment until the asteroid impact.
Meanwhile, mammals faced different limitations. They never evolved into supergiant herbivores or carnivores, but they continuously developed larger brains. The brain has evolved in sperm whales, killer whales, baleen whales, elephants, leopards, seals, and primates.
The model of dinosaur descendants. (Photo: Dale Russell and Ron Seguin, Canadian Museum of Nature).
Today, some descendants of dinosaurs, namely birds like crows and parrots, have complex brains. These animals can use tools, communicate, and count. However, only mammals like primates, elephants, and dolphins have evolved with the largest brains and the most complex behaviors.
So, was the extinction of dinosaurs a guarantee for mammals to evolve into intelligent beings? Perhaps not.
Two Factors That Shaped Human Evolution
Your starting point may limit your endpoint, but it does not guarantee it. Steve Jobs, Bill Gates, and Mark Zuckerberg all dropped out of college, but if dropping out automatically made you a billionaire, then all dropouts would be wealthy. Even starting from the right place at the right time, you still need opportunity, effort, and a bit of luck.
The evolutionary history of primates suggests that human evolution was unavoidable. In Africa, primates evolved into hominids with large brains and, over 7 million years, gave rise to modern humans. However, elsewhere, the evolution of primates took different paths.
Brain size compared to body mass of dinosaurs, mammals, and birds. (Photo: Nick Longrich).
When monkeys migrated to South America 35 million years ago, they evolved into several species of monkeys. Primates reached North America at least three different times, 55 million, 50 million, and 20 million years ago. Yet, they did not evolve into a species capable of creating nuclear weapons and smartphones; for reasons we still do not understand, they went extinct.
In Africa, and only in Africa, primates evolved in a unique way. Something in the animal, plant, or geographical systems of Africa propelled the evolution of hominids towards land-based living, large bodies, and tool use.
Even after the complete disappearance of dinosaurs, the evolution of modern humans still required the precise combination of opportunity and luck.
