There is no garauntee mamals would have evolved to humans with even less significant changes in the evolutionary environment. Apes did not evolve in the new world. Homo habilis and erectus evolved in Africa, but nothing similar evolved in Europe or Asia from Ape stock. Homo erectus survived until 50,000 years ago in Asia despite evolution towards brainier neanderthals and modern man almost 200,000 years earlier in Africa and Europe. The genus Homo's walking ape predecessor Australopithicus had many species who went exrinct. Even our ancestors in that genus were rare and could have easily gone extinct with a 100,000 year type impact in the area. Dolphins and elephants evolved brain sizes near their current levels millions of years before Apes started to walk on 2 legs let alone increase brain size yet stalled out and never evolved advanced tool technology just like Homo erectus stalled out in Asia.

2 less mass extinctions and mammals would have ruled uninterupted from their mammal like reptile ancestors (Dimetrodon etc). It's not the mass extinctions that seem so important to me however, but the far more subtle factors that we have yet to determine which pushed our ancestors out of long periods of slow or stagnant evolution over millions of years into rapid increases in brain size over a few hundred thousand years at 2 widely spaced periods (Australopithicus to H. erectus and H.e. to H.s.s). I think the answer lies in the fact that dolphins and elephants reached a near top of the pyramid position due to a combination of physical prowess and large brains while humans with their relatively puny physical attributes had to develope poweful tool technology to reach a similar apex. I also think they had to reach that apex or go extinct during each period of rapid evolution.

Regards,
Mark