Sixty-six million years ago, an asteroid hit Earth with the force of 10 billion atomic bombs and changed the course of evolution.
The sky darkened and plants stopped photosynthesising. The plants died, then the animals that fed on them. The food chain collapsed. More than 90 percent of all species disappeared. When the dust settled, all but a handful of dinosaurs were extinct.
But this catastrophic event made human evolution possible. The surviving mammals thrived, including tiny proto-primates that would evolve inside us.
Imagine if the asteroid had missed and dinosaurs survived. Imagine highly evolved birds of prey planting their flag on the moon. Dinosaur scientists, discovering relativity, or discussing a hypothetical world where, incredibly, mammals have taken over Earth.
This may sound like bad science fiction, but it touches on some deep, philosophical questions about evolution. Is humanity here just by chance, or is the evolution of intelligent tool users inevitable?
Brains, tools, language and large social groups make us the dominant species on the planet. There are 8 billion homo sapiens on seven continents. By weight, humans outnumber all wild animals.
We have adapted half of Earth’s land to feed ourselves. You could argue that creatures like humans would inevitably evolve.
In the 1980s, paleontologist Dale Russell proposed a thought experiment in which a carnivorous dinosaur evolved into an intelligent tool user. This “dinosaur” had a large brain with opposable thumbs and walked upright.
It’s not impossible, but it’s unlikely. The biology of an animal determines the direction of its evolution. Your starting point limits your ending points.
If you drop out of college, you probably won’t be a brain surgeon, lawyer, or NASA rocket scientist. But maybe you are an artist, actor or entrepreneur. The paths we take in life open some doors and close others. That is also the case in evolution.
Consider the size of dinosaurs. Starting in the Jurassic, sauropod dinosaurs, Brontosaurus and relatives evolved into giants 30-50 tons up to 30 meters long – ten times the weight of an elephant and as long as a blue whale.
This happened in multiple groups, including Diplodocidae, Brachiosauridae, Turiasauridae, Mamenchisauridae, and Titanosauria.
This happened on different continents, at different times and in different climates, from deserts to rainforests. But other dinosaurs that lived in these environments didn’t become supergiants.
The common thread that connected these animals was that they were sauropods. Something about the anatomy of sauropods — lungs, hollow bones with a high strength-to-weight ratio, metabolism, or all of these things — unlocked their evolutionary potential. It allowed them to grow up in a way that no land animal has ever had before or since.
Likewise, the carnivorous dinosaurs repeatedly evolved huge ten-meter, multi-ton predators. Over 100 million years, megalosaurids, allosaurids, carcharodontosaurids, neovenatorids, and finally tyrannosaurs evolved giant apex predators.
Dinosaurs did well with large bodies. Big brain not so much. Dinosaurs showed a weak trend towards larger brain size over time. Jurassic dinosaurs like Allosaurus, stegosaurus, and Brachiosaurus had small brains.
By the end of the Cretaceous, 80 million years later, tyrannosaurs and duckbills had evolved 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 kilograms.
Dinosaurs have invaded new niches over time. Small herbivores became more common and birds diversified. Long-legged forms evolved later, suggesting an arms race between swift-footed predators and their prey.
Dinosaurs appear to have had increasingly complex social lives. They began to live in herds and developed elaborate horns for fighting and display. Yet dinosaurs seem to mostly repeat themselves, evolving giant herbivores and small-brained carnivores.
There is little across 100 million years of dinosaur history to indicate that they would have done anything radically different had the asteroid not intervened. We’d probably still have those super-giant, long-necked herbivores and huge tyrannosaur-like predators.
They may have developed slightly larger brains, but there is little evidence that they would have evolved into geniuses. Nor is it likely that mammals would have driven them out. Dinosaurs monopolized their environment until the very end, when the asteroid hit.
Mammals, meanwhile, had other limitations. They never evolved supergiant herbivores and carnivores. But they repeatedly developed big brains. Huge brains (as big or bigger than ours) evolved in killer whales, sperm whales, baleen whales, elephants, leopard seals and monkeys.
Today, a few descendants of dinosaurs — birds such as crows and parrots — have complex brains. They can use tools, talk and count. But it’s mammals like monkeys, elephants and dolphins that evolved the largest brains and most complex behaviors.
So did eliminating the dinosaurs guarantee that mammals would develop intelligence?
Well, maybe not.
Start points may 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 multi-billionaire, every high school dropout would be rich. Even if you start in the right place, you need opportunity and luck.
The evolutionary history of primates suggests that our evolution was anything but inevitable. In Africa, primates evolved into big-brained apes and gave birth to modern humans in over 7 million years. Elsewhere, however, primate evolution followed very different paths.
When monkeys reached South America 35 million years ago, they simply evolved into more monkey species. And primates reached North America at least three times, 55 million years ago, 50 million years ago and 20 million years ago.
Yet they did not evolve into a species that makes nuclear weapons and smartphones. Instead, they went extinct for reasons we don’t understand.
In Africa, and only in Africa, primate evolution took a unique direction. Something in the fauna, flora or geography of Africa drove the evolution of apes: terrestrial primates with large bodies, large brains and tool-using primates.
Even with the dinosaurs gone, our evolution needed the right combination of chance and luck.
Nicholas R. LongrichSenior Lecturer in Paleontology and Evolutionary Biology, University of Bath
This article is republished from The Conversation under a Creative Commons license. Read the original article.