I never learned much about evolution, so please do explain it like I’m 5
In: Biology
evolution is unlikely to work the same way a second time, due to changed conditions, climate, other competition from different species etc.
It would be like throwing a can of paint at the wall and getting an interesting pattern, then throwing a different can of paint on a wall that had been largely demolished and expecting the same pattern.
The “game had changed”
The conditions on earth were very different; this made previously good traits (size) not as good
Also note that after the extinction event; not all life died. Life on earth didn’t reset back to zero. The animals that survived proved their evolutionary benefits and were able to continue procreating
They went extinct because they were not fit to live in the changed environment.
By the time the environment “recovered”, other animals had already taken their place. Mammals who were fit to survive the fallout thrived in the absence of the large dinosaurs, and so did smaller flying dinosaurs (also known as “birds”).
In very simple ELI5 terms: Dinosaurs got up from their chair, and mammals sat down on it. Then there was nowhere left for dinosaurs to sit and they were “out”.
Besides, there wouldn’t have been enough time for dinosaurs to evolve “from scratch” anyways. Dinosaurs took about 350 million years to evolve the first time. They have only been extinct for about 66 million years.
Evolution takes random* turns around a maze. A replay won’t take the same random turns because it’s supposed to be random.
* Not perfectly random, just close enough.
Well, the reason the dinosaurs went extinct in the first place is because they couldn’t survive in the new environment.
The world that the saw the evolution of dinosaurs no longer exists, so the evolutionary pressures that caused them to evolve in the first place simply aren’t around anymore. This includes the ancestors that dinos evolved from in the first place. There’s also no real reason to believe that the same ecosphere – even if it was recreated after the meteor impact – would produce the dinos again, remember that evolution simply allows favourable genes to spread through a population, not that some specific set of genes (and species) will eventually evolve.
This answer is pretty unsatisfying – it really boils down to “because they didn’t”, but you might be interested in [carcinisation](https://en.wikipedia.org/wiki/Carcinisation), crab-like creatures have evolved several times all around the world, in different settings!
Perhaps then the real reason that dinosaurs could not evolve again is they are simply less cool than the crab.
Evolution is, ultimately, random. The populations you get are random, and the traits they develop are random. You can frame evolution as an “optimizing force” because statistically, creatures with bad traits die off faster than creatures with good traits, letting the good traits spread around the population, but evolution is entirely blind and deaf to any end goal, and there is no slot called “dinosaur” it would be trying to fill.
Whatever works, works. Last time, dinosaurs happened to fit the niches. When the environment drastically changed from the meteor impact, most of the large ones couldn’t adapt and died off, which led to mammals eventually taking over most of their niches.
Also, all dinosaurs didn’t die off. Smaller species are generally more tolerant to changes in the environment, so they survived and continued to adapt just like mammals did which is why we have birds. The only reason you don’t call a seagull a “dinosaur” is convention.
Things go extinct usually because their niche disappears. The environment can no longer support them after changing in some way, or other competition / circumstances make their survival strategy completely non-viable. Or, something out competes or overtakes them for their niche, or targets them specifically. So, the species disappears.
If things were to change again to make the species potentially viable, and the niche reappeared (or something very similar to it), it IS possible for something like them to evolve to re-fill that niche, but it could be a completely different species. Each species that arises comes from a species that already exists, by slowly diverging in their traits to better suit the niche. If we have no dinosaurs now, it’s unlikely to get dinosaurs again, unless for some reason their very specific characteristics are better suited than those of modern predators. And, since it’d likely be a lot longer of a stretch to go from birds or similar species to undergo enough change to become “dinosaurs” as we know them compared to just… having large canines / felines / herbivores, those niches are likely to remain dominated by modern species.
After all, it takes time to get good at fulfilling a niche. It works better if the niche evolves with your species, or said niche is empty–if it’s already filled, anything newly adapting to this niche is at a massive disadvantage often compared to species ALREADY very well suited to it.
I can answer this (check my username).
Evolution is two things: random mutations in the gene code, coupled with heritability.
The classic example is land tortoises on the Galápagos Islands. A breeding pair reached one island and created a population.
The population spread, and some tortoises arrived on islands that required different specialisation. Luckily, not every tortoise was the same, and those that had mutations to ***fit*** their environment better survived than those that didn’t.
This is what is meant by survival of the fittest.
So now we have tortoises that are larger, smaller, longer or shorter necks, beak shape, and so on between the islands.
Now you may say that they’re still tortoises. Which is true, but it’s still evolution.
Now let’s say instead of tortoises, it was ratites, a kind of bird. As Gondwana split apart, the common ancestor of ratites had its population split between different islands, which moved so far apart for so long that no interbreeding was possible. And now we have Kiwi in New Zealand, Ostrich in Africa, Emu in Australia, and the Rhea in South America.
Back to dinosaurs, or any creature really.
A parent population of dinosaurs have offspring, some are naturally and randomly different, and if they fit their environment better they survive more. The post-Cretaceous period was very different to before, and the gene pool was both different and due to random mutations, would not be repeated. Hence, new animals and plants.
Evolution is blind. It operates one generation at a time. It is random. And all it does is select the life form that, of the population, fits the environment. If no life form does, nothing reproduces. If it fits poorly, it reproduces poorly. If it fits well, it reproduces well. The environment is constantly changing, the gene pool of available genes is constantly changing, and new mutations are constantly introduced. Some help in the moment, most do not. But evolution does not care. It just ***is***.
Further reading: The Selfish Gene, Richard Dawkins.
In a way they did.
The catastrophe wiped out all land animals larger than a medium sized dog, (and many of those that were smaller too.)
All the species on land that survived were small and often capable of burrowing.
Over time many of these small animal species evolved to be bigger now that there were empty places for bigger animals to fill.
Shortly after the event, some flightless birds evolved to be rather big and carnivorous. Those creatures looked and acted like the recently extinct two legged carnivorous dinosaurs they were related to. Evolution had taken birds and taken them back to their theropod predator roots.
These creatures didn’t quite reach T-Rex size and had useless tiny wings instead of useless tiny arms, but the resemblances was there.
However before too long mammals started to take over that niche. The mammal predators that followed may have filled the same empty niches but acted and looked quite differently.
The absence of large plant eaters meant that predators could not get as big as before and circumstances didn’t push land based plant eaters to evolve to such great size again. Although some mammals like the Paraceratherium did end up getting quite big.
Of course in the water whales ended up breaking records and grew to sizes that reptiles and fish never did.
Generally your answer might simply be that evolution does not have an aim. Similar starting points and similar circumstances may result in similar outcomes, which is why so many animals evolved to look like crabs and why so many mammals ended up with sabreteeth independently, but there is nothing about the size and shape of extinct dinosaurs that made them inevitable to evolve again.
There were some attempts to return to large bipedal raptors, but they were not the most successful.
Terror birds went exiting due to competition with mammals. Fundamentally, mammals are better at living on the ground game.
If you transported some dinosaurs into the modern earth, they would get killed by moder predators. Just look at what cats do to birds in Australia. Or why there are no penguins in the arctic.
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