Mass extinctions are called mass extinction for a reason. It wipes out most of the species on Earth. About 76% of all species for that extinction, to be exact.
Everything that’s alive today is a descendant from one of the 24% of species that survived. Those survivors would largely be smaller organisms that didn’t need as many resources to survive. The large dinosaurs did not fit that description, but the smaller ones did and became our modern birds.
There have been 5 mass extinction events in Earth’s history, and it only takes a few species to survive to repopulate the Earth. The 2nd one, the Permian-Triassic extinction or “The Great Dying” wiped out 96% of all species, and the dinosaurs then evolved from there to rule the planet for the next few million years. Even then, Triassic, Jurassic, and Cretaceous dinosaurs all look very different because they are all millions of years apart from each other, with many smaller extinctions in between.
Want something that will really blow your mind? Grass likely didn’t evolve until after dinosaurs went extinct.
It very nearly did. Even in the groups that survived, like mammals and birds, most species died out. And even in the species that survived, most individuals would have died out. Quite a lot of plant species would have lost all adult individuals, and only survived thanks to buried seeds.
Remember, to make it through, only one lineage has to survive. A lot of survivors probably just got really lucky.
Also, while asteroid being the Dinosaur killer is a popular theory, a more recent alternative theory which seems plausible is that Dinosaur were already dying off due to the Deccan Traps (in modern day India) volcanoes spewing lava and greenhouse gasses and alot of sun blocking debris for a few hundred thousand years before the asteroid hit.
While the asteroid hit might have been the final nail in coffin, it’s possible about 75% of animal life had already died due to the mass volcanic event.
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