What does this passage tell us? It tells us why our brains______.

Human brains are about three times as large as those of our early ancestors that lived four million to two million years ago, and for years, (11)scientists have wondered how our brains got so big. (12) A new study suggests social competition could be behind the increase in brain size.
    Compared to almost all other animals, human brains are larger as a percentage of body weight. (13) And since the emergence of the first species about two million years ago, the human brain has doubled in size. And when compared to earlier ancestors, (14)such as those that lived four million to two million years ago, our brains are three times as large. For years, scientists have wondered what could account for this increase.
    The three major theories have focused on climate change, the demand of ecology, and social competition.