Sixty-six million years ago, an asteroid struck Earth with the force of 10 billion atomic bombs and changed the course of evolution. The clouded sky and the plants have stopped photosynthesis. The plants died, then the animals that fed on them. The food chain has collapsed. More 90% of all species faded away. When the dust settled, all the dinosaurs except a handful of birds had disappeared.

But this catastrophic event made human evolution possible. The surviving mammals thrived, including few proto-primates that would evolve within us.

Imagine the asteroid passed by and the dinosaurs survived. Imagine highly evolved raptors planting their flag on the moon. Dinosaur scientists, discovering relativity, or discussing a hypothetical world in which, incredibly, mammals took over the Earth.

It may sound like bad science fiction, but it raises deep, philosophical questions about evolution. Is humanity here by chance, or is the evolution of smart tool users inevitable?

Brains, tools, language and large social groups make us the dominant species on the planet. 8 billion ago Homo sapiens on seven continents. By weight, there are more humans than all wild animals.

We have modified half of the terrestrial earth to feed us. You could say that creatures like humans were destined to evolve.

In the 1980s, the paleontologist Dale Russell proposed a thought experiment in which a carnivorous dinosaur became a smart tool user. This “dinosauroid” had a large brain with opposable thumbs and walked upright.

It’s not impossible but it’s unlikely. An animal’s biology constrains the direction of its evolution. Your starting point limits your ending points.

If you drop out of college, you probably won’t be a NASA neurosurgeon, lawyer, or rocket scientist. But you could be an artist, an actor or an entrepreneur. The paths we take in life open some doors and close others. This is also true in evolution.

Consider the size of the dinosaurs. From the Jurassic, the sauropod dinosaurs, Brontosaurus and relatives evolved into giants of 30 to 50 tons up to 30 meters long – ten times the weight of an elephant and as long as a blue whale. This has happened in several groups, including DiplodocidaeBrachiosaurids, Turiasaurids, Mamenchisaurids and Titanosaur.

This happened on different continents, at different times and in different climates, from deserts to tropical forests. But the other dinosaurs living in these environments did not become supergiants.

The common thread connecting these animals was that they were sauropods. Something about the anatomy of sauropods – lungshollow bone with a high strength-to-weight ratiometabolism or all these things – have released their evolutionary potential. This allowed them to grow in a way that no land animal had before, nor has since.

Similarly, carnivorous dinosaurs repeatedly developed huge ten-meter, multi-ton predators. More than 100 million years, megalosaurids, allosaurids, carcharodontosaurids, neovenatorides and finally tyrannosaurs evolved giant apex predators.

Dinosaurs did big bodies well. big brains not really. Dinosaurs showed a weak trend of increasing brain size over time. Jurassic dinosaurs love Allosaurus, stegosaurus and Brachiosaurus had a small brain.

At the end of the Cretaceous, 80 million years later, tyrannosaurs and duckbills had developed larger brains. But despite its size, the T. rex brain still weighed only 400 grams. A Velociraptor brain weighed 15 grams. The average human brain weighs 1.3 kg.

Dinosaurs entered new niches over time. Small herbivores have become more common and birds have diversified. Long-legged forms evolved later, suggesting an arms race between float-footed predators and their prey.

Dinosaurs seem to have had increasingly complex social lives. They began to live in herds and evolved elaborate horns for combat and display. Yet dinosaurs mostly seem to repeat themselves, evolving into giant herbivores and carnivores with small brains.

There is little about 100 million years of dinosaur history to hint that they would have done something radically different had the asteroid not intervened. We would probably still have those supergiant, long-necked herbivores and huge tyrannosaur-like predators.

They may have evolved with slightly larger brains, but there is little evidence that they would have become geniuses. Nor is it likely that mammals would have moved them. The dinosaurs monopolized their environment until the very end when the asteroid hit.

Mammals, on the other hand, had different constraints. They never evolved into supergiant herbivores and carnivores. But they repeatedly developed big brains. Massive brains (as big or bigger than ours) have evolved in killer whales, sperm whales, baleen whales, elephants, leopard seals and monkeys.

Today, some descendants of dinosaurs – birds like crows and parrots – have complex brains. They can use tools, talk and count. But it’s mammals like monkeys, elephants and dolphins that have developed the biggest brains and the most complex behaviors.

So did eliminating dinosaurs guarantee that mammals would develop intelligence?

Well, maybe not.

Start points can limit end points, but they don’t guarantee them either. Steve Jobs, Bill Gates and Mark Zuckerberg all dropped out of college. But if dropping out automatically made you a multibillionaire, every dropout would be rich. Even starting in the right place, you need opportunity and luck.

The history of primate evolution suggests that our evolution was anything but inevitable. In Africa, primates evolved into big-brained apes and over time 7 million yearsproduct modern humans. But elsewhere, the evolution of primates has taken very different paths.

When the monkeys have reached South America 35 million years ago they just evolved into more ape species. And primates have reached North America at least three times, 55 million years ago, 50 million years agoand 20 million years ago. Yet they haven’t evolved into a species that makes nuclear weapons and smartphones. Instead, for reasons we don’t understand, they disappeared.

In Africa, and in Africa alone, the evolution of primates has taken a unique direction. Something about the fauna, flora or geography of Africa led the evolution of apes: terrestrial, large, big brain, using tools primates. Even with the dinosaurs gone, our evolution needed a good combination of opportunity and luck.