It sure looks dramatic when we compress the vastness of 3.5 million years – about 110,000 generations – in a single picture, as shown here (click to enlarge). We see that our brain capacity, measured by “cranial volume” (upper part of the skull that houses our brain), has grown more than two-fold during this time.

What could have happened back then that triggered this growth? Or, in the language of Evolution, how did the environment change millions of years ago that favored the “trait” of large brain size?

To make a complex story overly simple, after the hominids decided to come down from treetops (favored hangout of their ancestor – apes) and live on ground, they faced serious danger from large land animals with sharp tooth and claw. Being no match physically, only those with superior alertness and skill – signs of a healthy brain – survived, and passed on this trait to their offsprings. As the younger generations perfected these survival skills, their senses and intelligence continued to improve, and brain got bigger.

Thus, the new threat on survival of hominids favored the trait of a bigger brain, which continued through the next hundreds of thousands of generations, and has now become the 3lb mass of ganglion inside our head.

Next up: Is our brain still evolving?