Surprising discovery: what dinosaurs would look like if they had never gone extinct

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 the plants stopped photosynthesizing. The plants died, then the animals that fed on them. The food chain has collapsed. Over 90% of all species have disappeared. When the dust settled, all dinosaurs except a handful of birds went extinct.

But this catastrophic event made human evolution possible. Surviving mammals flourished, including small proto-primates that would evolve into us.

Imagine the asteroid missed 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 dominated the Earth.

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, there are more humans than all wild animals.

In the 1980s, paleontologist Dale Russell proposed a thought experiment in which a carnivorous dinosaur evolved into an intelligent 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 end points.

Consider the size of dinosaurs. Starting in the Jurassic, sauropod dinosaurs, brontosaurs and relatives evolved into 30 to 50 ton giants up to 30 meters long – ten times the weight of an elephant and as long as a blue whale.

This happened in several groups, including Diplodocidae, Brachiosauridae, Turiasauridae, Mamenchisauridae and Titanosauridae.

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

The common thread that linked these animals together was that they were sauropods. Something about sauropod anatomy — lungs, hollow bones with a high strength-to-weight ratio, metabolism or all those things — unlocked their evolutionary potential. It allowed them to grow in a way that no land animal had done before, or since.

In the same way, carnivorous dinosaurs repeatedly evolved huge, ten-meter and several-ton predators. More than 100 million years ago, megalosaurids, allosaurids, carcharodontosaurids, neovenatorids, and finally tyrannosaurids evolved as giant predators.

Diagram of brain size versus body mass for dinosaurs, mammals and birds. Dinosaurs handled large bodies well. Big brains not so much. Dinosaurs showed a weak tendency to increase brain size over time. Jurassic dinosaurs like Allosaurus, Stegosaurus and Brachiosaurus had small brains.

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

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

Dinosaurs seem to have had increasingly complex social lives. They began living in herds and developed elaborate horns for fighting and display. However, dinosaurs seem to repeat themselves, evolving giant herbivores and carnivores with small brains.

There’s just about 100 million years of dinosaur history to suggest that they would have done something radically different if the asteroid hadn’t intervened. We’d probably still have those long-necked supergiant herbivores and huge tyrannosaur-like predators.

They may have evolved slightly larger brains, but there is little evidence that they evolved into geniuses. Nor is it likely that mammals displaced them. Dinosaurs monopolized their environments until the end when the asteroid hit them.

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.

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