I never learned much about evolution, so please do explain it like I’m 5
In: Biology
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.
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