While going through my logs last month I came across a rather interesting search query that somebody used to find my blog.

why didn't dinosaurs evolve again after the big bang

After a little chuckle I carried on, however I did jot it down for future use.

I suppose fundamentally it represents the terrible truth that ignorance dominates in our society. Nothing demonstrates this more than Sherri Shephard:

We knew the shape and size of the Earth 2200 years ago, it is disappointing to see not everyone is aware of it today.

I suppose I could just ridicule this person, but the fact they ask a question at least show they're interested in learning. Unlike Shephard, who during her whole life apparently never wondered what the shape of the world was, or was ever shown a map or globe of the Earth.

The fundamental answer for why dinosaurs didn't evolve *again* after the Big Bang was because the Big Bang predates the dinosaurs by about 13.6 billion years. There was no Big Bang after when the dinosaurs were about.

The question I suspect he or she was meaning was why didn't dinosaurs evolve again after going extinct?

There are a few reasons why this hasn't happened yet and why it probably won't happen again in the future.

Firstly the conditions present today are not like those of the Triassic, Jurassic or Cretaceous periods, the Earth is much cooler now, and because of this reptiles are small, the only large land animals are mammals because they are warm blooded. Because of this, it would be difficult for reptiles to compete with mammals as we're "dug-in" and would be difficult to be ousted. Like how mammals were small while reptiles dominated the Earth. You need an extinction event to shake things up. The impact that ended the Cretaceous period lowered global temperatures, this hurt reptiles badly but gave mammals are head start in the new environment. Today global warming could give reptiles a boost at the expense of many mammal species.

But even assuming the Earth of the future closely resembled that of a hundred million years ago, the probability of species resembling dinosaur species is extremely slim, we're talking trillions to one in probability this is because of how complicated the genetic code is. The change of the same mutations and then the same selections happening to create a species of dinosaur is incredibly remote. We may well end up with large reptiles again on the Earth. But realistically they will never be the same as species of dinosaur that have gone extinct. That genetic code is lost, and the chance of it arising again is too unlikely for it to happen.