So far as we can tell from the pattern on Earth, very primitive life formed quickly but the big step was to advance to life with nucleated cells. After that advance, the growth of brains appears to have been slow but steady, not even significantly interrupted by mass extinctions. Thus, it seems likely that any planet that gets to nucleated cells should advance at some reasonable pace to large-brained life. But large-brained life does not mean human beings necessarily. For example, if there were another K/T-like extinction event, it seems likely that dolphins would be more likely to survive than humans, since living in the oceans would tend to protect them from the immediate consequences. Their encephalization level is about 70% of that for humans, so we might see a continuation of the growth of "brain size" with no real change at the time of the extinction, just as has been true before, but with no humans surviving. Even more intriguing whether we can associate large brains with intelligence. No question, large brains are a requirement for intelligence -- but it does not necessarily follow that any large-brained animal will have the kind of versatile, thinking brain that we have.

We have already seen how many conditions are required for a planet to be as suitable as the earth for complex life to evolve on it; now we see that there are also a number of imponderables regarding the possibility the truly intelligent life would evolve on such a planet.