So what if the deadly Chicxulub meteor had missed Earth entirely, and dinosaurs hadn't been largely wiped out 65 million years ago? It was only after that impact called an end to dinosaurs' planet-wide party -- which had been taking place for millions upon millions of years -- that the way was paved for mammals to diversify and thrive in the void. Before that, mammals had never really moved beyond the ranks of rodentlike creatures, but once the dinosaurs died off, many species gradually evolved characteristics that were more closely and consistently aligned with what we consider staple mammalian features [source: Smithsonian National Museum of Natural History]. Gradually, primates developed, though it would still be many millions of years before anything really resembling a human was on the scene.
It's impossible to say for sure whether people would now be pounding the pavement if dinosaurs hadn't fallen prey to that unexpected cosmically conceived shakedown. It's entirely feasible that if that catastrophe hadn't occurred, dinosaurs would still be alive today, and we mammals would still be small, burrowing creatures, quaking in fear of the terrible lizards that stalked our land.
On the other hand, it was another extinction event -- the one about 250 million years ago that marked the line between the Permian and Triassic geological periods -- which allowed the dinosaurs their shot in the spotlight in the first place [source: Choi]. So perhaps if Chicxulub hadn't struck, a different calamity would have caused the curtain to fall on the time of the dinosaurs, allowing mammals the chance to jump to the forefront. Or perhaps the twists and turns of evolution would have followed an entirely different course. Who can say?
Because it's also important to remember that even seemingly simple things like slight alterations in climate or atmospheric composition can trigger small but drastic changes in living conditions, and while some species adapt, others languish. So perhaps an ecological balance would have eventually been struck between mammals and giant reptiles, and we might have evolved in tandem with them, without their untimely extinction. What that alter reality would have looked like, however, remains shrouded in mystery.