Throughout history, the human brain has been remarkably good at dismissing itself. Everyone from ancient Egyptians to Aristotle has downplayed the role of the mysterious stuff between our ears. Famed anatomist Galen gave the brain credit as commander of movement and speech, but even he brushed aside the white and gray matter, figuring the fluid-filled ventricles inside the brain did most of the work.

Human brains are big…
The average adult brain weighs just under 3 pounds (between 1.3 and 1.4 kilograms). Some neurosurgeons describe the texture of a living brain as that of toothpaste, but according to neurosurgeon Katrina Firlik, a better analogy can be found in the local health-food store. “[The brain] doesn’t spread like toothpaste. It doesn’t adhere to your fingers the way toothpaste does,” Firlik writes in her memoir, “Another Day in the Frontal Lobe: A Brain Surgeon Exposes Life on the Inside” (Random House, 2006). “Tofu — the soft variety, if you know tofu — may be a more accurate comparison.”

If you aren’t charmed by that description, consider this: About 80 percent of the contents of your cranium is brain, while equal amounts of blood and cerebrospinal fluid, the clear liquid that buffers neural tissue, make up the rest. If you were to blend up all of that brain, blood and fluid, it would come to about 1.7 liters, or not quite enough to fill a 2-liter soda bottle.

…But they’re getting smaller
Don’t get too cocky about your soda-bottle-sized brain. Humans 5,000 years ago had brains that were even larger. “We do know from archaeological data that pretty much everywhere we can measure — Europe, China, South Africa, Australia — that brains have shrunk about 9 cubic inches (150 cubic centimeters), from an average of about 82 in3 (1,350 cm3). That’s roughly 10 percent,” University of Wisconsin at Madison paleoanthropologist John Hawks told LiveScience in 2009. Researchers don’t know why brains might be shrinking, but some theorize that they’re evolving to be more efficient.

Others think our skulls are getting smaller because our diets include more easily chewable foods and so large, strong jaws are no longer required. Whatever the reason, brain size doesn’t directly correlate with intellect, so there’s no evidence that ancient man was brainier than humans of today.
Our brains burn through energy
The modern brain is an energy hog. The organ accounts for about 2 percent of body weight, but it uses about 20 percent of the oxygen in our blood and 25 percent of the glucose (sugars) circulating in our bloodstream, according to the American College of Neuropsychopharmacology. These energy requirements have spurred a debate among anthropologists about what fueled the evolution of big brains in the first place. Many researchers credit meat, citing evidence of hunting in our early ancestors. But meat would have been an unreliable food source, say other scientists. A 2007 study published in the Proceedings of the National Academy of Science found that modern-day chimps know how to dig for calorie-rich tubers on the savanna. Perhaps our ancestors did the same, boosting their brainpower with veggies.
As for what motivated the brain to balloon in size, there are three major hypotheses: climate change, the demands of ecology, and social competition.