Sixty-six million years ago, an asteroid struck Earth with the force of 10 billion atomic bombs and changed the course of evolution. The sky darkened and plants stopped photosynthesizing. The plants died, and then the animals that fed on them died. The food chain collapsed. More than 90% of all species have disappeared. When the dust settled, all of the dinosaurs were extinct except for a handful of birds.
But this cataclysmic event made human evolution possible. The remaining mammals thrived, including the small primitive primates that would evolve into us.
Imagine the asteroid missed, and the dinosaurs survived. Imagine highly evolved birds of prey flying their flag on the moon. Dinosaurs, discovering relativity, or discussing a hypothetical world, where mammals incredibly dominated the Earth.
This may sound like bad science fiction, but it raises some deep philosophical questions about evolution. Is humanity only here by chance, or is the evolution of intelligent tool users inevitable?
Brains, tools, language, and large social groups make us the dominant species on planet Earth. There are 8 billion sane people on seven continents. By weight, there are more humans than all land animals.
We have modified half of the land area to feed ourselves. You could argue that creatures like humans had to evolve.
In the 1980s, paleontologist Dale Russell proposed a thought experiment in which a carnivorous dinosaur evolved into a gadget user. This “tyrannosaurus” had large brains with opposite thumbs and walked upright.
This is not impossible but unlikely. An animal’s biology determines the direction of its development. Your starting point limits your end points.
If you dropped out of college, you probably wouldn’t be a brain surgeon, or a lawyer, or a rocket scientist at NASA. But you may be an artist, an actor or an entrepreneur. The paths we take in life open some doors and close others. This is also true of evolution.
Consider the size of dinosaurs. Starting with Jurassic and sauropod dinosaurs, Brontosaurus and its relatives evolved into 30-50-ton giants reaching 30 meters in length – ten times the weight of an elephant and the length of a blue whale. This occurred in multiple groups, including Diplodosidae, Brachiosaurids, Triassiaurids, Mammansaurids and Titanosaurids.
This happened on different continents, at different times and in different climates, from deserts to rainforests. But the other dinosaurs that lived in these environments didn’t become super giants.
The common thread connecting these animals was that they were sauropods. Something about sauropod anatomy—lungs, hollow bones with a high strength-to-weight ratio, metabolism, or all of those things—unleashed their evolutionary potential. He allowed them to grow in a way never before seen in a wild animal, or not since.
Likewise, carnivorous dinosaurs have repeatedly evolved ten-meter-long, multi-ton megapredators. Over the course of 100 million years, megalosaurs, allosaurs, carcharodontosaurs, neovenatorians and finally dinosaurs evolved giant apex predators.
Dinosaurs did well with large bodies. Big brains not so much. Dinosaurs showed a weak trend toward increasing brain size over time. Jurassic dinosaurs like Allosaurus, Stegosaurus, and Brachiosaurus had small brains.
By the late Cretaceous period, 80 million years later, dinosaurs and duckbills had evolved larger brains. But despite its size, the T. rex’s brain still weighs only 400 grams. Velociraptor’s brain weighed 15 grams. The average human brain weighs 1.3 kilograms.
Dinosaurs entered new domains over time. Small herbivores became more common and birds diversified. Long-legged forms evolved later, suggesting an arms race between fleet-footed predators and their prey.
Dinosaurs seem to have an increasingly complex social life. They began living in herds and developed elaborate antlers for fighting and displaying. However, dinosaurs seem to mostly repeat themselves, evolving into giant herbivores and small-brained carnivores.
There are nearly 100 million years of dinosaur history to suggest that they would have done anything radically different if the asteroid hadn’t intervened. We probably still have those giant, long-necked herbivores and tyrannosaurus-like predators.
They may have evolved slightly larger brains, but there is little evidence that they evolved into geniuses. It is also unlikely that mammals displaced them. Dinosaurs monopolized their habitats until the end, when the asteroid struck.
Meanwhile, mammals had various limitations. They never evolved giant herbivores and carnivores. But they have developed big brains over and over again. Megabrains (larger or larger than our own) evolved in orcas, sperm whales, baleen whales, elephants, cheetah seals, and monkeys.
Today, a few of the descendants of dinosaurs — birds like crows and parrots — have complex brains. They can use tools, speak and count. But it is mammals such as monkeys, elephants, and dolphins that have evolved the largest brains and the most complex behaviours.
Does the elimination of dinosaurs guarantee the evolution of mammalian intelligence?
Well, maybe not.
Starting points may limit endpoints, but they do not guarantee them either. Steve Jobs, Bill Gates, and Mark Zuckerberg have all dropped out of college. But if dropping out automatically made you a billionaire, every college dropout would be rich. Even if you start in the right place, you need chances and luck.
The evolutionary history of monkeys indicates that our evolution was not deterministic at all. In Africa, primates evolved into large-brained apes, producing modern humans over the course of 7 million years. But primate evolution elsewhere has taken very different paths.
When monkeys arrived in South America 35 million years ago, they had just evolved into more species of monkey. Primates arrived in North America at least three separate times, 55 million years ago, 50 million years ago, and 20 million years ago. However, they did not evolve into a species that makes nuclear weapons and smartphones. Instead, for reasons we don’t understand, they went extinct.
In Africa, and only Africa, primate evolution has taken a unique direction. Something about the animals, plants, and geography of Africa drove the evolution of apes: terrestrial, large-bodied, big-brained, tool-wielding primates. Even with the dinosaurs gone, our evolution needed the right mix of chance and luck.
Introduction to the conversation
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