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 atomic bombs and changed the course of evolution.

The skies darkened and the plants stopped photosynthesizing. The plants died, then the animals that fed on them. The food chain has collapsed. Over 90 percent of all species have disappeared. By the time the dust settled, all but a handful of birds of the dinosaurs were extinct.

But this cataclysmic event made human evolution possible. The surviving mammals thrived, including the tiny proto-primates that would eventually evolve into us.

Imagine the asteroid missed and the 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 conquered the Earth.

That might sound like bad science fiction, but it gets to 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. That’s 8 billion homo sapiens on seven continents. By weight, there are more humans than all wild animals.

We have modified half of the Earth to feed us. It could be argued that creatures like humans were meant to evolve.

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.

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. An animal’s biology constrains the direction of its evolution. Your starting point limits your endpoints.

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

Giant dinosaurs and mammals through the timeline
Giant dinosaurs and mammals through time. (Nick Longrich)

Consider the size of the dinosaurs. Since the Jurassic, the sauropod dinosaurs, Brontosaurus and relatives evolved into 30-50 ton giants up to 30 meters long, sometimes the weight of an elephant and as long as a blue whale.

This happened in multiple groups, including Diplodocid, 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 connecting these animals was that they were sauropods. Something in sauropod anatomy — lungs, hollow bones with a high strength-to-weight ratio, metabolism, or all of those things — unlocked their evolutionary potential. He let them grow in a way that no land animal had ever done before, or has done since.

Similarly, carnivorous dinosaurs have repeatedly evolved huge predators of ten meters and many tons. Over the course of 100 million years, megalosaurids, allosaurids, carcharodontosaurids, neovenatorids, and finally tyrannosaurs evolved into gigantic apex predators.

Brain size versus body mass diagram for dinosaurs, mammals, and birds
Brain size versus body mass for dinosaurs, mammals, and birds. (Nick Longrich)

Dinosaurs did well with big bodies. Big brains not so much. Dinosaurs showed a weak trend of increasing brain size over time. Jurassic dinosaur like Allosaurus, stegosaurus, And Brachiosaurus he had a small brain.

By the Late 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 Velociraptors brain weighed 15 grams. The average human brain weighs 1.3 kilograms.

Dinosaurs have entered 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.

It seems that dinosaurs had increasingly complex social lives. They began living 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 about 100 million years of dinosaur history to suggest that they would have done anything 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 would have evolved genes. Nor is it likely that mammals would have moved them. Dinosaurs monopolized their environments until finally when the asteroid hit.

Mammals, meanwhile, had different constraints. They never evolved supergiant herbivores and carnivores. But they have repeatedly developed large brains. Massive brains (as big or bigger than ours) evolved into orcas, sperm whales, 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 is mammals such as monkeys, elephants and dolphins that have developed the largest brains and the most complex behaviors.

So did eliminating the dinosaurs guarantee that mammals would develop intelligence?

Well, maybe not.

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

The evolutionary history of primates suggests that our evolution was far from inevitable. In Africa, primates evolved into large-brained apes and, over 7 million years, produced modern humans. But elsewhere primate evolution has taken very different paths.

When apes reached South America 35 million years ago, they had just evolved into other ape species. And primates reached North America at least three separate times, 55 million years ago, 50 million years ago and 20 million years ago.

Yet they haven’t evolved into a species that produces nuclear weapons and smartphones. Instead, for reasons we don’t understand, they became extinct.

In Africa, and in Africa alone, primate evolution has taken a unique direction. Something in the fauna, flora or geography of Africa drove the evolution of apes: large-bodied, big-brained, tool-using terrestrial primates.

Even without dinosaurs, our evolution needed the right combination of opportunity and luck.The conversation

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.

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