Out of all the animals on Earth that we know of, we consider ourselves the most intelligent of the bunch, no matter how much we deride and belittle our fellow humans. However, what was the innocuous event that enabled the rise of complex brains in the millions of years that life has existed?
For that answer, we need to look back to 195 million years ago, at the Hadrocodium species, what scientists suppose as the first mammal. Measuring at an adorable inch long and weighing just two grams, this small shrewlike mammal struggled for existence among the giant thunder lizards (dinosaurs). This creature needed to have some sort of adaptation that enabled it to compete with not just dinosaurs but Lissamphibians (early amphibians) and sphenodonts (lizard ancestors).
These early mammals evolved extremely sensitive and pointed noses that allowed them to scour the night for bugs and avoid their reptilean predators at all times. While the skulls of these mammals can barely fit on the average fingernail, their brain size to body volume ratio greatly exceeded that of the fauna around at the time.
Using endocasts (inner cast of hollow objects: in this case, the shrew-like skull), Dr Timothy Rowe observed " enlarged olfactory bulbs, neocortex, pyriform cortex, and cerebellum" in the brain of the Hadocodium. In essence, the Hadrocodium's nose required a great deal of processing power, encouraging the selection of larger and more complex structures of the brain.
It would not be an exaggeration to say that the usefulness of the nose paved the way for selection for larger brains, which in turn, eventually paved the path for humanity's birth.
Rowe, T., Macrini, T.E., and Luo, Z.-X. 2011. Fossil evidence on origin of the mammalian brain (Science, 332, 955-957):
Vastag, Brian. "Mammals Win by a Nose: Sense of Smell Drove Evolution of Big Brains."
Washington Post. The Washington Post, 20 May 2011. Web. 30 Mar. 2012. <http://www.washingtonpost.com/national/mammals-win-by-a-nose-sense-of-smell-drove-evolution-of-big-brains/2011/05/19/AFIh1H7G_story.html>.