Australopithecines = 28 cubic inches of brain
Homo habilis = 40 cubic inches of brain
Homo erectus = 59 cubic inches of brain
Homo heidelbergensis = 73 cubic inches of brain
Neanderthals = 87 cubic inches of brain
Homo sapiens (some of us; ha!) = 90 cubic inches of brain
Don't ask me how they determined the brain size from the ancient fossils, as I'm assuming they made their conjectures by way of examining their craniums. Anyway, some folks say that the hominids rapidly started increasing their brain size when they switched over to a meat/high fat diet. Some think that the humanoids slowly got smarter and equipped with bigger brains as their social skills developed over time. The ancient alien theorists often claim that aliens from another world simply altered our DNA and sped up the process.
No matter what the reasons for the increased brain size were, we still have loads of people walking around with crazy amounts of empty space in their heads. LOL!
Semi-related Post: http://www.perpendicularity.org/death-of-neanderthals.html
Image Credit: This work is in the public domain in the United States because it is a work prepared by an officer or employee of the United States Government.
---End of Post "Hominid Brain Sizes"