I'd say that only one species can develop in a system. The speed of evolution is so variable, that races won't civilise at the same time. This means that the fastest race will dominate the whole system before other races can evolve to the point where they're 'intelligent'.

For example, suppose Mars was capable of supporting life, and they received spores from Earth during the dinosaur period. Those spores would start to evolve, but chances are, humans evolved millions of years faster. This means, we're going to Mars this century and colonising it first.

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Live long enough to see Space!

We can get to space as a species. The above link is information about life-extension (living longer) so that you can personally see a space-based civilization.

The MPrize (a prize to encourage life extension research now contains over 3 million dollars.