What would dinosaurs look like today if they never went extinct? : ScienceAlert

What would dinosaurs look like today if they never went extinct? : ScienceAlert

Sixty-six million years ago, an asteroid hit the earth with the force of 10 billion nuclear bombs and changed the course of evolution.

The skies darkened and plants stopped photosynthesis. The plants died, then the animals that fed on them. The food chain collapsed. About 90 percent of all species disappeared. When the dust settled, all of them dinosaurs except for a handful of birds was extinct.

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

Imagine if the asteroid had missed and dinosaurs survived. Imagine highly evolved birds of prey planting their flag the moon. dinosaur scientists, discovering relativity, or discussing a hypothetical world in which, 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 A wise man on seven continents. By weight there are more people than all wild animals.

We have modified half of the Earth’s surface to feed ourselves. You could say creatures were just like humans bound to evolve.

In the eighties, paleontologist Dal 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.

Dinosaur model
Dinosaur model. (Dale Russell & Ron Seguin/Canadian Museum of Nature via Naish & Tattersdill, Canadian Journal of Earth Sciences2021)

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.

Giant dinosaurs and mammals by time chart
Giant dinosaurs and mammals through time. (Nick Longrich)

Consider the size of dinosaurs. Starting in the Jurassic, sauropod dinosaurs, Brontosaurus and relatives grown into giants of 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 Diplodocidaebrachiosaurs, 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 – lungshollow bones with a high strength-to-weight ratiometabolism, or all 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. More than 100 million years, megalosaurids, allosaurids, carcharodontosaurids, neovenatorides, and finally tyrannosaurs evolved giant apex predators.

Diagram of brain size versus body weight for dinosaurs, mammals and birds
Brain size versus body weight for dinosaurs, mammals and birds. (Nick Longrich)

Dinosaurs did well with large bodies. Big brains 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 duckbill 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 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 moved in herds and evolved expanded 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 large 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 — do complex brain. They can use toolstalk and count. But it is mammals such as monkeys, elephants and dolphins that developed 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 monkeys and, beyond 7 million yearsproduced modern man. Elsewhere, however, primate evolution followed very different paths.

When monkeys reached South America 35 million years ago they just evolved into more monkey species. And primates reached North America at least three different times, 55 million years ago, 50 million years agoand 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 about the fauna, flora or geography of Africa drove the evolution of apes: terrestrial, large body, big brain, using tools primates.

Even with the dinosaurs gone, our evolution needed the right combination of chance and luck.The conversation

Nicholas R. Longrichassociate professor of paleontology and evolutionary biology, University of Bath

This article has been republished from The conversation under a Creative Commons license. Read the original article.

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