What if the dinosaurs didn’t go extinct?
Would intelligence and civilization have evolved if dinosaurs had continued to dominate the planet? Probably not.
WHAT IF, 66 million years ago, the asteroid hadn’t hit, and dinosaurs hadn’t gone extinct? Would evolution have proceeded along the same lines as ours, producing human-level intelligence, tool use, language, and civilization? Probably not. One reason is that different animals have different constraints: different evolutionary pathways are available, depending where you start. Something like humans might require starting with a mammalian species. And even given that starting point, it may still have needed a little luck.
66 million years ago, an asteroid hit the earth with the force of 100 billion H-bombs. Dust and soot were shot into the atmosphere, plunging the Earth into darkness. Photosynthesis stopped, plants died, and the animals that ate them. Perhaps 90% of all species vanished, including all dinosaurs except a few bird species. After the dust settled, mammals flourished— including the tiny, tree-shrew-like creatures that ultimately evolved into us.
But what if that extinction didn’t happen?
Imagine if dinosaurs have evolved into something intelligent like us. Imagine dinosaurs inventing agriculture, civilizations, mathematics, nuclear weapons, a space program. Imagine highly evolved raptors putting a flag on the moon. Or dinosaur scientists, discovering relativity. Maybe even dinosaur biologists, discussing alternate history, a hypothetical world in which— incredibly—mammals took over the Earth.
It might sound like bad science fiction, but this thought experiment gets at a fundamental problem, a philosophical question about evolution’s workings.
Does the universe have to be the way it is, or could it have turned out differently? Are we just the result of chance, and luck? Or are certain things- like intelligent tool-users- likely to evolve?
You could argue something like us was bound to evolve. We’re a remarkable design— whatever our flaws, there’s no denying humans are successful. There are 8 billion Homo sapiens on all 7 continents. In terms of biomass there are more humans than wild animals. We modified most of Earth’s land to feed ourselves, domesticated plants and animals to provide our food, and we’re altering the climate. Brains, language, thumbs, and large social groups made us Earth’s dominant species.
Evolution adapts species to their environments over time. Given enough time, you’d expect it would inevitably hit on such a successful design- or something broadly like it. Perhaps dinosaurs would too?
“Dinosauroids”
In the 1970s, paleontologist Dale Russell proposed a thought experiment in which a carnivorous dinosaur evolved into an intelligent tool user. This hypothetical “dinosauroid’ was a large-brained, upright biped with opposable thumbs, and looked like a space alien. But would dinosaurs really have evolved into something like us?
It's probably not impossible, but probably not likely, either. One reason is that the anatomy and biology of an organism limit its evolution. Your evolutionary starting point determines your possible endpoints. Where you start from limits where you can go; not all evolutionary pathways are open to all species. Sometimes, as they say, you just can’t get there from here.
Consider how our lives are constrained. If you drop out of college, you’ll probably never be a brain surgeon, lawyer, or NASA scientist, but you could be a great artist, novelist, musician, actor, or entrepreneur. On the other hand, if you devote all your time to med school, you’ll probably never be a rock star or tech multi-billionaire. The paths we take in life open certain doors, and close others.
The same is true of evolution. The problem with assuming that dinosaurs could evolve into something like a human is that not all evolutionary pathways are equally available to all species.
Take one aspect of dinosaurs- their remarkable size. Among all animals, something about dinosaur biology made them uniquely able to evolve large size, particularly the Sauropoda, the brontosaurs.
Starting in the Jurassic, these long-necked dinosaurs repeatedly evolved huge size, with different sauropod lineages growing up to 30 meters long and 50 tons in weight. Was it something about the environment? Probably not. These giants evolved on multiple continents. They lived in different environments with different food, from forests of conifers, cycads and ferns in the Jurassic, to flowering plants in the Cretaceous. They existed in the sweltering hot of the Cenomanian and the cool Maastrichtian.
The fact that supergiant dinosaurs evolved in different times, places, climates, environments- and that only one dinosaur lineage, the sauropods, did this— argues that something about sauropod biology- their bones, lungs, or growth patterns- made their body plan uniquely able to evolve large size. And similarly, predatory dinosaurs repeatedly evolved huge, 10+ meter, 5-ton species. Ceratosaurids, megalosaurids, allosaurids, carcharodontosaurids, neovenatorids, and finally tyrannosaurids became giant apex predators over the course of ~100 million years.
So dinosaurs did big bodies well. But big brains? Not so much. From the Jurassic to the late Cretaceous, dinosaurs increased their brain size only slightly. At the end of their reign, even the biggest-brained dinosaurs, like tyrannosaurs and duckbills, had smaller brains than similar-sized mammals.
Dinosaur evolution was hardly static . They diversified over time, moving into new niches. Small herbivores diversified, as did birds. Longer-legged forms appear, suggesting an arms race between fleet-footed predators and prey. Herds and elaborate horns suggest increasingly complex social lives. But overall they seem to repeating themselves: giant, small-brained herbivores and carnivores, like a Hollywood movie franchise that’s just run out of ideas. I’d argue there’s little about 100 million years of dinosaur evolution that hints they would have done anything radically different if the asteroid hadn’t intervened- or that mammals would displace them. If the dinosaurs had survived they’d probably be doing the same things they did in the Cretaceous, or even the Jurassic. Giant, long-necked herbivores. Huge, two-legged carnivores. A dinosaur-dominated present would look a lot like the past.
On the other hand, mammals never produced supergiant herbivores and carnivores, but repeatedly evolved giant brains. Massive brains evolved in whales, elephants, and primates. If you want to evolve a highly intelligent species, a mammal is a good place to start. So was the evolution of intelligence foreordained when the dinosaurs went extinct?
Well, maybe not.
Starting points limit endpoints, but they don’t guarantee the outcome either. Where do you start to become fabulously wealthy? Steve Jobs, Bill Gates, and Mark Zuckerberg all dropped out of college. So that’s one viable starting point to acquiring an inconceivable fortune. But if dropping out automatically made you a multibillionaire, every college dropout would be rich.
Even if you start in the right place, you need something else- a certain amount of opportunity, or luck, or most likely, both.
Consider our lineage, the primates. Clearly primates are capable of evolving into intelligent tool users; after all that’s where we come from. But not all primates did.
Yes, in Africa, primates evolved into big-brained apes, began using tools, and eventually evolved into humans. But that didn’t happen elsewhere. When primates reached South America, they just evolved into more monkeys- squirrel monkeys, spider monkeys, howler monkeys, tamarins, and marmosets- but nothing remotely human-like, or even ape-like. And primates reached North America multiple times, 55 million years ago, 50 million years ago, and 20 million years ago. But far from evolving into a sentient species making nukes and smartphones, these North American primate lineages simply went extinct.
One possible endpoint for primates was us, but not all primates are destined to evolve into something like us, or even to survive long-term. We’re just one species out of thousands that have existed over the past 55 million years, and by far the most unusual. The rest of the primate lineages are all either apes, monkeys, marmosets and lemurs— or they’re extinct.
Something about Africa— its unique flora, fauna, geography, climate, and history—pushed primate evolution in a totally different direction. That implies that even with the dinosaurs gone, even starting with mammals as a starting point, the evolution of human-like intelligence still needed something else- a rare combination of opportunity, and luck.