I never learned much about evolution, so please do explain it like I’m 5
In: Biology
What we commonly think of “dinosaurs” weren’t all around at the same time. They evolved and went extinct over many many millions of years with the conditions on earth changing throughout.
The ones that were not fit in the new environment went extinct, the ones that were fit enough to remain suddenly didn’t need to compete with the ones that died, so their evolutionary trajectory branched off.
Some dinosaurs do still exist though! Certain species of crocodile and shark are (apparently) largely unchanged over these geological timescales. Birds are some of the closest living relatives of the dinosaurs.
Evolution is not a process with an optimal “end point” — it’s not efficient or even optimal, it’s just the conditions of the earth effectively doing “selective” breeding over and over for millions of years.
So the asteroid that killed most of the dinosaurs (birds are dinosaurs so some survived!) made almost every animal larger than 25kg extinct.
So what you had left was a bunch of small mammals and a bunch of small flying dinosaurs (birds) left.
So dinosaurs might have evolved back from birds, but it’s thought that mammals took the big animal spots instead because they give birth to live young, which could be much bigger than the young of egg laying animals.
To grow big an animal from an egg has to compete with a load of small animals, then a load of medium sized animals, then a load of big animals. For mammals they can start reasonably big already (think like a baby elephant, who are born bigger than many adult humans)
All this “random evolution” and can of paint thrown at a wall explanations seem to forget Carcinification and similar phenomena.
With the same environment, evolution (which isn’t RANDOM at all btw), tends to find the same best solutions.
The answer to OP is that the environment changed so much, so fast, and for such a long period of time, that the solutions found by dinosaurs evolution were not good anymore.
Lack of food and light favoured smaller, warm-blooded animals, for an extended period of time. Eventually the filled the gaps left by dinosaurs and even if conditions turned back to the original, food abundance and light, which favoured the dinosaurs, the survivors of what they once were couldn’t compete with other animals that had taken their ecosystem niches anymore.
Also, the Dinosaurs were larger reptiles. What mostly survived that extinction event were small mammals.
They key for the answer is in the extintion. Meteor have not killed dinosaurs. It killed giant flora. That had immence effect to oxigen level in atmosphere. Dinosaurs suffocated and that happend decades after impact.
Thus dinosaurs could not return by any means* as there were no longer conditions where they could live.
Note: birds **are** dinosaurs, crocodiles **are not**
Although the species we recognized as dinosaurs went extinct, their descendants did not. The children of the dinosaurs are called birds. When the world changed, the dinosaurs had to change with it. Dinosaurs changed into birds over millions of generations, with each child of each dinosaur looking a tiny bit more like a bird. Well, not really each child, but the children that did look a tiny bit more like a bird tended to survive and have children of their own, while the children that looked more like a dinosaur tended not to survive and have children, because the world was not a good place for dinosaurs to live, and it was a good place for birds to live. So birds were more likely to survive than dinosaurs. And now we have birds, but no more dinosaurs.
To add to other comments about the randomness of evolution;
4 billion years ago – First life forms
240 million years ago – Dinosaurs appeared
65 million years ago – Dinosaurs went extinct.
Evolution takes time.
They actually kind of did. Birds are a group of theropod dinosaurs that survived the extinction of all other dinosaurs. And while all of the surviving birds were small, some of them rather quickly again evolved into large, flightless forms that were among the dominant animals of their ecosystems. Now however, they also had to compete against the mammals, which were also evolving very rapidly following the extinction of the non-avian dinosaurs. This fierce competition from mammals might have hindered birds from evolving to become as huge as the largest theropods again.
Still, huge flightless birds would continue survive – including meat eating ones who were apex predators of their ecosystems. Some would survive until only a few tens of thousands of years ago, Late enough that even early humans encountered them. The giant carnivorous birds survived especially long in places with few large mammalian predators, but in the end though they all seem to have disappeared largely due to increased competition from carnivorous mammals, perhaps in combination with disadvantageous environmental changes.
And even though the giant flightless carnivorous birds are now all gone, we do of course still have other giant flightless birds around – like ostriches, emus, cassowaries and rheas. And until very recently the even larger moas and elephant birds, which were sadly driven to extinction by humans just a few hundred years ago. All of these belong to the bird order called ratites, who are de facto rather large, flightless theropod dinosaurs.
The avian dinosaurs did, look up *Phorusrhacids*, those things would have given most mid-sized dinosaurs a hard time. oxygen levels changed and prevented land-dwelling things getting quite as big
No one can truly know why. Evolution is a relatively random process based on external pressures. Evolution will try to make changes to the genome based on the environment. Sometimes those changes go well, sometimes they go bad. The bad ones are discarded, the good ones are kept. That’s the general idea.
Not all the dinosaurs went extinct, though most did. All living creatures are descended from animals that survived the asteroid impact. Funnily enough, chickens seem to be descended from the Tyrannosaurus Rex. The big apex predator got nuked into a chicken.
The reason why dinosaurs didn’t “come back”, as we like to think of it, is partly because of something called convergent evolution. Convergent evolution is when two unrelated species evolve into the same creature (traits, looks etc.). A famous example of this are the evolution of wings in bats and birds. Bats and birds are unrelated to each other, but they both evolved wings because the environment they evolved in helped push in a direction where wings are very useful for the survival of the species. The likelihood of convergent evolution, at least to the degree of the Tyrannosaurus Rex coming back, is astronomically unlikely, largely because the asteroid impact changed the environment to such a degree where the same pressures didn’t exist anymore. The dinosaurs that survived both the impact and the massive environmental changes (check out nuclear winter) ended up dying out or migrated to other areas where they inevitably died out or evolution continued in a different environment that had different requirements and needs to survive.
Convergent evolution is usually only observed in small parts and traits, like the example of wings in both bats and birds, but entire species is practically impossible. Not entirely impossible, just so unlikely that it’s *practically* impossible.
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