There are certain readily-filled niches in ecosystems. Even when the slate is wiped clean by some mass-extinction event, the evolutionary process means that animals from wholly different orders are plastic enough to re-fill those niches quickly.
Thus, if terrestrial ecosystems have generally had room enough to tolerate thriving populations of arboreal animals, flying animals, burrowing animals, fast moving carnivores who prey on large lumbering herbivores, carrion eaters in their wake, semi-aquatic animals, and so on, then we have found that such niches are invariably filled.
Velociraptors, Moas, Tigers and Marsupial Lions have occupied one such niche (predatory carnivores) by turns. Apatosaurs, Elephants, Diprotodons another (large herbivores). Pterodactyla, Archaeopteryx, Modern birds, and bats still another, and so on.
When a living can be had as an occupant in one of those niches, it seems applicants have always queued up, regardless of whether they have cold blood, feathers or pouches. These niches must represent enduring evolutionary "sweet spots", since they are filled over and over.
Obviously, intelligence confers a huge survival advantage. It enhances the ability for creatures to plan, and to act in concert through the use of language. Although many other animals are social species, an intelligent individual's ability to survive and reproduce is further multiplied through greater co-operation with the whole. The aggregation of learned survival strategies suddenly can be passed down the generations via a means better than mimicry or instinct. An animal can only mimic what it has seen, but language means the memes for, for example, an improved hunting method, or of rendering a food otherwise poisonous fit for consumption, can be passed across continents and down the centuries by stories, and eventually, writing. Intelligence means an unprecedented ability for a creature change its environment to suit itself, rather than need to continually adapt to suit the environment.
So if nature has continually repeated herself through the repetition of forms and characteristics advantageous to exploit a niche, and intelligence is manifestly such a characteristic, why is there no indication that intelligent life or civilisation has ever appeared before in the half-billion years that have elapsed since complex life arose?
I just through I'd throw that out there. It's a question I've turned over periodically. If anyone knows if any of the major writers like Dawkins have addressed this question, please point us in the right direction in the comments.
(The picture: Doctor Who's answer to this question. Intelligent life did arise before; the Silurians!
I'm not sure I subscribe to this theory, but my 7YO son may well).