When I first studied human evolution it was believed that the start of tool use lead to a steady increase in brain power through a positive feedback loop. Human evolution was thought to be a continuous ramp up from apelike status. Under such an assumption it seemed obvious human level intelligence or higher should be common once chance set another species on the same path. The fewer low probability events needed for something to happen the more likely it is to happen (Pt=P1*P2*P3....).

Since then further evidence has shown that the pace of our evolution was not steady. It was often stagnant and perilously close to extinction for a long time. Simply starting on the path towards humans was no gaurantee we would get where we are today. The numerous dead end hominoids are prime examples.

Regards,
Mark