http://humanorigins.si.edu/human-characteristics/brains http://humanorigins.si.edu/human-characteristics/brains http://humanorigins.si.edu/human-characteristics/brains Smithsonian 2 percent rapidly tripled its original size http://humanorigins.si.edu/human-characteristics/brains increased in size disproportionately larger in size http://humanorigins.si.edu/human-characteristics/brains Smithsonian http://humanorigins.si.edu/human-characteristics/brains humanorigins.si.edu Smithsonian http://humanorigins.si.edu/human-characteristics/brains 2.98 pounds http://humanorigins.si.edu/human-characteristics/brains http://humanorigins.si.edu/human-characteristics/brains http://humanorigins.si.edu/human-characteristics/brains Smithsonian rapidly tripled its original size brain size humanorigins.si.edu rapidly tripled its original size available on the exhibit’s website 2.98 pounds brain size tripled anthropologists say anthropologists say