How mammals evolved big brains Brain proportions were driven by body size and cataclysmic events. Scientists have now pieced together a 150-million-year timeline to determine how mammals evolved big brains. An international team, led by Jeroen Smaers of Stony Brook University, US, compared the brain mass of 1400 living mammals and 107 fossils and compared them to body size to determine how the scale of the two has changed through time. The result? Brain size and body size didn’t evolve in a stable way. Instead, the researchers found that big-brained animals like humans, elephants and dolphins all evolved their brain-to-body-size proportions in different ways.