Why did human brains decrease in size?

The human brain has decreased in size over the past 30,000 years, according to a new study.The research, published in the journal Nature, looked at brain size using cranial capacity, which is the measure of the interior space in the skull. The study found that the average cranial capacity for modern humans is about 1,000 cubic centimeters (cc), while the average for our ancient ancestors was 1,100 cc. The study's lead author, Christopher Stringer of the Natural History Museum in London, said the findings suggest that the human brain is still evolving. "We're seeing the consequences of natural selection on the human brain today," Stringer told LiveScience. It's not clear why the brain has shrunk, but Stringer said one possibility is that it's an adaptation to our changing diet and lifestyle. "The human brain is a very metabolically active organ, and it uses a lot of energy," he said. "So one possibility is that as we've become more efficient in our use of energy, natural selection has favored those individuals whose brains were a little bit smaller and used less energy." It's also possible that the shrinking brain is simply a byproduct of other changes that have occurred