I never learned much about evolution, so please do explain it like I’m 5
In: Biology
Others have answered correctly that the evolutionary pressures change over time so exact species don’t evolve twice (though you do get similar parallel evolution in some cases Carcinification).
Just to add: dinosaurs technically are not extinct as scientific consensus is that dinosaurs evolved into birds, meaning birds are dinosaurs. If we want to get pedantic (and it’s Reddit, so we do) the asteroid 65 million years ago ended the “non-avian dinosaurs”, but some dinosaurs survived and still thrive in the present-day, specifically the avian dinosaurs, or birds.
Lots of answers for evolution already, but do you know why everything was BIG? There was way more oxygen on earth due to way more plants. More oxygen allows for creatures to grow larger. That’s why there were giant bugs flying all around. Giant bugs can make good food for giant dinosaurs.
The short answer is that evolution is a blind process; there is no plan or agent at work. Organisms change randomly over time, and some of those random changes happen to be advantageous. One way that a change might be good for an organism is if there is an empty niche.
For example, many animals might compete for limited food sources. Let’s say we have a bunch of animals all eating grass, if one of those animals happens to be a little taller, maybe it can be more successful by eating leaves. That animal adapts to fill the niche of a browser. That’s one theory for how giraffes and some long necked dinosaurs evolved.
When the meteor hit 65 million years ago, most large animals got wiped out. What was left were many small animals. There were some dinosaurs we call birds today. There were small mammals resembling rats, shrews and opossums. And there were lots of reptiles.
Of those groups, some birds did evolve to resemble non-avian dinosaurs (ostriches and the extinct moa, for example), but birds have evolved themselves into a corner and cannot easily adapt from their optimal strategy of being small, light flying creatures.
Reptiles like lizards and crocodiles resemble the ancestors of dinosaurs. They could have conceivably evolved to fill the niches left by the dinosaurs. After adapting, their descendents might well have resembled true dinosaurs through convergent evolution, but that didn’t happen. The reason why is because they were out competed by mammals. Mammals reproduce more quickly and more actively consume resources compared to reptiles. They were able to adapt faster and fill the empty niches before the reptiles could.
Dinosaurs didn’t re-evolve, but mammals evolved to take their place and fill the same ecological roles.
Some dinosaurs survived (birds) and they did start becoming large again until they were outcompeted by mammals who were now evolving on equal footing, and better able to adapt to the new climate of Earth
To that point mammals did start evolving into megafauna, but a certain species playing with new rules (humans and tools) paired with rapidly changing climate again made being huge not a good trait to have.
Another factor is time. It took dinosaurs almost 200 million years to reach their peak of size (Jurassic and Cretaceous periods), whereas current animals have only had ~66 million years. They may need another 80 million for the environment to be suitable for huge animals again.
TL;DR – being huge requires a lot of specific conditions that all happen randomly to occur at the same time.
They kind of came back via a different route, it’s crazy when I look at a pigeon and think “Yeah, your many times great great grandaddy woulda eaten me”
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