Prof. Jonathan Losos recently gave a talk (summarised in this article by Cameron Hill) and stated that it is arrogant to assume that humanity is the pinnacle of evolution, and somehow inevitable over a long enough evolutionary time scale. Which I completely agree with.
But the assertion that, if the Cretaceous-Paleogene (K-Pg) mass extinction hadn't happened, meaning that non-avian dinosaurs weren't wiped out 65 million years ago, that they would still be ruling today? I'm not so sure.
What if certain groups of mammals had opportunities to out-compete non-avian dinosaurs? (Yes, those opportunities usually come around when mass extinctions occur and niches open up for the taking). However, there is evidence that non-avian dinosaurs were already on the decline before the K-Pg mass extinction event (see here, here, here, and here). Perhaps new niches would have opened up for mammal groups even without an asteroid crashing landing in the Yucatán Peninsula. Or any other group of animals, for that matter. New research proposes that it was the avian dinosaurs (birds) ability to eat seeds that allowed them to survive the K-Pg extinction. If the non-avian dinosaurs were declining anyway, would avian dinosaurs still have taken this seed-y niche?
It comes down to how we identify faunal turnover in the fossil record, and how we understand the causes for changes in the 'dominant' clade. Mass extinctions are commonly invoked to mark the end of one clade's reign and the start of another - from synapsids (often known as 'mammal-like reptiles') dominating the Permian then suffering mass extinction at the Permian-Triassic boundary, after which pseudosuchian diversity increased throughout the Triassic until the Triassic-Jurassic extinction event. Then, dinosauria took over newly created niches and ruled the rest of the Mesozoic until the K-Pg event, where synapsids (in the form of mammals) once again took the reins.
Skeletons of the synapsid ('mammal-like reptile') Dimetrodon. Often incorrectly labelled as a dinosaur by toy manufacturers!
Photograph by Willem van Valkenburg from Delft, Netherlands (IMG_5100.jpg) [CC BY 2.0 (http://creativecommons.org /licenses/by/2.0)], via Wikimedia Commons.
Skeleton of the pseudosuchian (the group to which modern crocodiles belong) Prestosuchus chiniquensis (AMNH 3856).
Photograph by Vince Smith from London, United Kingdom (Prestosuchus chiniquensis Uploaded by FunkMonk) [CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons.
One thing is for sure: I one-hundred percent agree with Prof. Losos that humanity, or even bipedalism in any animal, is not inevitable in Earth's evolutionary history.
Image by the Canadian Museum of Nature, Ottowa, Canada.