What evidence have we that they would not? Life on earth certainly expands as rapidly as the environment can support, or even faster than it can, in many cases.

What evidence have we that they would not? Life on earth certainly expands as rapidly as the environment can support, or even faster than it can, in many cases. Also "rapid" is a relative term, right? If we humans start expanding to other planets even within 10,000 years that would be quite rapid relative to the length of time it took for us to evolve.